Syne nie ie tht if 444 t 3 ‘ ae ny vty 4 UN a teagan ea i, ise sift! in ~ st hint reali " nt his! | Hh i i tae Hee eae iy H i th 0 We i\ HAN an ERM Cy ai ai si = — = =< = = —— > = Soe = SS; +. ‘<2 334 pases" > = Sn = = SSS COPYRIGHT DEPOSIT yk 4 my ea ie iy 6 Puarr I.—Mutant Forms of Drosophila Melanogaster (Am pelophiia). 1. Wild Type. 2. Ebony. 3. Yellow. 4. White Eye. 5. Bar Eye. 7. Vestigial. 8. Buff Kye. 9, Cherry Eye. (Frem Drawing by BE. M. WALLACE.) 6. Eosin Miniature Black. GENETICS RELATION TO AGRICULTURE BY, ERNEST BROWN BABCOCK PROFESSOR OF GENETICS, UNIVERSITY OF CALIFORNIA AND ROY ELWOOD CLAUSEN ASSISTANT PROFESSOR OF GENETICS, UNIVERSITY OF CALIFORNIA First EpIt1ion McGRAW-HILL BOOK COMPANY, Inc. 239 WEST 39TH STREET. NEW YORK LONDON: HILL PUBLISHING CO., Lp. 6 & 8 BOUVERIE ST., E. C. 1918 Coryricut, 1918, BY THE McGraw-Hitt Book Company, Inc. APR 22 1918 ¢ is ©ci#494684 a oe Ye { ule te Aan PREFACE Of all the sciences that contribute to the great, tertiary composite which is known as agriculture none is more important economically than genetics. One may not overlook the fundamental relation borne by the primary sciences, mathematics, physics and chemistry, and by the second- ary sciences, botany, zoology, geology, meteorology and economics, to the production and distribution of raw materials. But we confidently assert that the science which underlies the improvement of plants and animals for agricultural purposes is destined to receive increasing atten- tion is agricultural education and in agricultural practice. Without doubt vast possibilities await realization through the more thorough and systematic development of our living economic resources. Such de- velopment is directly dependent upon the successful utilization of genetic principles in plant and animal breeding. The science of genetics is still very young, but it is firmly established and is developing rapidly. It claims the attention of the producer of today and invites the most serious study of the agriculturist of tomorrow. It lays claim also to the interest of the eugenist, the sociologist and the philanthropist and all students of biology. This text has been prepared in response to a real and widely recognized need. The experience of the authors in teaching the principles of breed- ing to undergraduate students has forced home the conviction that an adequate presentation in a single text of the facts and principles of genetics and their practical applications is a prime necessity. Those familiar with the literature of the subject will appreciate the magnitude of the task and, we trust, will be lenient in criticizing our choice of subject matter. It is impossible to include many things of mutual interest to genetics and agriculture if the work be limited to a single volume. We are keenly aware of many deficiencies and it is our desire to prepare a revised edition of the book in the near future. With this in view the suggestions of others are earnestly solicited. We take this opportunity to express gratitude to all who have rendered assistance, especially to those who have read portions of the manu- script or assisted in proof-reading and to all who donated or loaned photo- graphs or who assisted otherwise with the illustrations. The onus of the work has been lessened in no small degree by the interest and en- couragement of our colleagues. THE AUTHORS. BERKELEY, CALIFORNIA, Feb. 18, 1918. Vil : : ie Li mt ¥ i * he ae f ll 6 ery ' a f Lot | ’ i 4- , oe i ’ F 't *} » £7 ar; NG Z whe as } v, yh ee Ls ) in i . Ge . ales i 4 ’ i> o* a. x : ‘ * i =) ‘7 i Pe an. . | 1! . Sate al 4 { i . it . y a ae zt i th 1. { =< ’ i 4 ; * . f i ’ ‘ { ' i \ ca : I a4 , x 5 { ie ” f f a ‘ty : ik by’ Y ‘ 7 Py ¢\ ; 1 . ' ‘ , . i ni (ued uve ¥ ‘} . = ' p 4) ’ “ Pe As a™ an “4 a ine ‘ Ni 1 . { " uy, «fT ’ i P i A eat hay 7 y it i bi byt, aa! ~ SU hi 4 J ve d > Pe meia CONTENTS : PART I Fundamentals CHAPTER I THe Mertruops aNnD Score OF GENETICS Paar Introductory . 1 Genetics igizes : SOR ORT TT OM OT Tr an oP re 1 Content of genetics .. . TSR CT ee OT ee Th ee Variation and Heredity aleasl 3 The problems of genetics Mics weedy Ais. Peau cs ance Mad ce) 7a LIM cad OnE PATCEMIOLNOUA OM POTICLICS:,: oc 3. caleocietaecn A ated J) Ss ah Xl wtascdacngel Sate ele A FTELCOUISILCN LOTSRCMELICS «cones AF GL WRTEMEMBEES Mog so ua 4 ea ah Se oe AO eA PUCTUOD OMECMELICSS 2 6 bc See A Be ee tee ee CNeHCA MIGd NICU CUTC ark ters) tars Loh, be EE WASSER: eG on ol ee CHAPTER II VARIATION Introductory. .. . [11 Ra WR Dt re oe TER org tind ai See, || D Darwin and anno BE ee ee, Rae eo io, ee, ee ae Hae universality Ol variation. 3. .%eeese te |. us eM ee t Reem ge ee ihneay aris hion COMCeptalt 6 ct laa 8, eo le WincshicnitOunOiVATIVtONS 7). 3) ee ee ee ers soe ees cn Ee aes ae Dini nonrand ae welupmient. <0 '). ‘se utews fut foo") sag eee vole amen epee Mati InOAnG GilVITOMMmenG s<. {sae al Bs dus, “ie Wee) «een ceo, CHAPTER III THE STATISTICAL STUDY OF VARIATION Introductory. . . . he ey Ue ER eb, voy Jet ee og ek | 2 Causes of doing eae wa ke WMA ey elie, Fe meeetee en a ee ae ne ere ee Pe Mamioiartisncalirerularity . . «<\. 2) MMi rn Berek) Lea 132 iameor Geviations. from the average. <<. . Jo4 Mi saab sie wes Sv oa) 84 SLnemormus) curve.and its significamee . +)... 3 =. «el see 84 hequrements of biometrical study {492 wis i... Ss se oP es ee. 86 EROS GMICAIRDCIIOR: (0 cas. eA EP CR Oe oe es one we ee Oe Requisites to reliability . . . . SALE AS Lee ry hae, Babee a 9 6 Grouping variates—frequency tater Oh AOE TAI MAD Ys ei ce Qe a 88 Frequency graphs . . . A Oe ee ents) The mean, standard died ota coetietent ae panty fa) Sa Ana IL SD ERI ENTE ie Ag bec 0. vid cu em ues x BY I Sn oh ix 2 CONTENTS Multimodal curves . Correlation and the semelaien coemioientis Regression . Employment and eae of fe peice: all Ee ikaal . CHAPTER IV Tue PuysicaAL Basis oF HEREDITY Introductory. Heredity a ponseunicnee of fhe Benend eontnuiny ss ate. The chromosomes Somatic cell division The production of germ cells: Synapsis—its significance ; Independent distribution of haven sone: Number of chromosome combinations. Chromosomes and sex in Drosophila. Recapitulation of the mechanism of heredity CHAPTER V INDEPENDENT MENDELIAN INHERITANCE Introductory. ; : Mendelism eesentially Boe hell - Mendel and his discovery . The monohybrid . Mendelian forminolegy 3 ; The chromosome interpretation in a acai ee : Sex-linked inheritance. The mathematical adequacy of Mendelism: The dihybrid. A case in maize The chromosome ec areic ae. Mathematical consideration . A case in guinea pigs . A case in Drosophila . The trihybrid A case in Sdeacoay. Multi-factor hybrids Four-fold factor seumeniuign & in mice. Methods of dealing with genetic data . Validity of segregation ratios. CHAPTER VI LINKAGE RELATIONS IN MENDELISM Introductory. 5 Chromosome mines aad eactera PaGcE 48 49 55 56 57 57 57 59 60 63 63 65 65 67 68 68 68 68 71 73 74 77 81 81 33 83 85 87 90 90 94. 94 95 100 . 104 . 104 CONTENTS A case of partial linkage in maize. Chromosome interpretation of linkage. Linkage in Drosophila. The factor groups An illustrative case from the nate group Linkage in the second and third groups. Linear arrangement of factors Application in Drosophila . The mode of inheritance in crossing-over ; Experimental verification of theory of linear arr angement a factors’ Interference . Linkage eieunmhiena3 in labhes plants anil jnititels Mathematical relations in linkage of phenomena . CHAPTER VII THe NATURE AND EXPRESSION OF MENDELIAN FAcTORS Introductory . Conception of feabanees as Teen in hecsiosanie Factors the genetic representatives of characters . Manifold effects of factors . ; The variability of factor expressions Duplicate factors . Multiple factors CHAPTER VIII ALLELOMORPHIC RELATIONSHIPS IN MENDELISM Introductory . Character fllationshine i in Pisa Dominance defined . The extent of dominance ; Intermediate expression in the neeetin Variable character expression in the hybrid . Competitive action of factors Mosaic expression of a character . The presence and absence hypothesis . Multiple allelomorphism . In Drosophila . In other species Arguments in favor of the cone iceptiad of fiuliiple dlislomonphicin CHAPTER IX Types or Factor INTERACTIONS - Introductory . Aleurone color factors in maize. Comb characters in fowl. xi PaGr 105 109 » 110 110 113 113 115 116 121 eli 122 125 127 . 129 129 129 . 133 . 134 . 136 ; dar . 144 144 144 . 144 . 147 . 149 . 150 . 152 153 . 155 156 158 160 163 . 163 164 xi CONTENTS PAGE factor system in StOCKSs: o. '. 4 =. 6 » «+, oe ee Sie TP feces ree Mruncate winged Drosophila)... ). . . .. soe) SE etka recteet sr See tee The factor explanation of reversion. . . . » | aut Hugo A eho) ee Darwin’s hybridization experiments with nipeeee Sey Phas nee Peapecenes) Satta ee eaamemenlecall Bactor analysis of plumage color in pigeoms #), . -))205) Gage. ae eee CHAPTER X Factor RELATIONS IN QUANTITATIVE INHERITANCE Introductory. . . . BS 6 ob a So EE one ae Meaning of puanpiiative Falter tioma fields ae let eye tyallgeh oa seh Cane te ee ‘Lalltand idwart races... < . ...: AE eee eee ties Intersexual forms im-Lymantria .. 1. ke eG ANnalogouscase im tobacco:...... «1 6. oe eS @onclusion. so een CHAPTER XII Species HypripizATION Introductory. ... Belt be Ne oe ey el RRR 74) US) Genetic Commenced aah Ta ONDEaC differences . ee hee. Eee ZI Species ‘hybridgan Antirrhinum.. . 5... . . «2a eee Species |hybrids im ‘Cavia *.92 2.) 0... 0... 1 eo CONTENTS The forms of species hybrids The vigor of species hybrids . Sterility in species hybrids : Partially sterile hybrids of wheat ule rye . Partially sterile hybrids in Nicotiana . Species hybridization in @nothera . Conclusions . CHAPTER XOIt Pure LIngEs Introductory . : Discovery of pure ines ; : Conditions necessary for the pacientes of pure ines Isolation of pure lines from mixed populations . The effect of selection within pure lines . Significance of the pure line principle in proeding CHAPTER XIV “ MuvratTions Introductory . i Two classes of poet aions Chromosome aberrations Factor mutations. The nature and causes of factor erie: Factor mutations, both germinal and somatic . Vegetative mutations versus somatic segregation . ‘‘Mutations” in the evening primroses PART II Plant Breeding CHAPTER XV INTRODUCTION, HISTORICAL Introductory . The beginnings of miei pee eding Pioneers in plant breeding . More recent progress in plant breeding; Classification of methods ; Mass selection . Line selection Hybridization . Clonal selection Organization of plant Breeding ole Seed and plant introduction . ; Collections of plant breeding anteeal Research on plant groups . Xill PaGE . 227 . 230 . 234 . 236 . 238 . 244 . 248 . 250 . 250 . 255 . 256 . 257 . 259 . 263 . 263 . 263 . 263 . 266 . 268 . 272 . 276 . 287 . 287 . 288 . 291 . 291 . 293 . 294 . 298 . 299 . 299 . 299 . 300 XIV CONTENTS Organizations of plant breeders. Summary . CHAPTER XVI On VARIETIES IN PLANTS Introductory. ; : Extent of variety difier edt aon in pints ; The origin of domestic varieties of plants Origin of sweet pea varieties . Flower color in sweet peas. Form and size in sweet peas . Habit in sweet peas. f Hybridization and selection in Meine dvede pea Creation of varieties of the rose Origin of varieties in the Boston Fern . CHAPTER XVII THe CoMPOsITION OF PLANT POPULATIONS Introductory . : Reproduction in pleats : Plants normally self-fertilized Plants normally cross-fertilized Discussion Populations of plants nor ral po eraliceas Populations as affected by crossing . Summary . CHAPTER XGVill SELECTION Introductory . : d Selection methods in maize Serene Inbreeding in maize. The ear-to-row method . The Illinois Station experiments . The remnant system : Selection methods in plgcesvolimened mene The plant-to-row method . iba Ineffectiveness of continued selection “lia pure ibaes The practical importance of keeping varieties pure . CLAP TER xx HYBRIDIZATION Introductory . PAGE . 300 . 301 . 802 . 302. . 3802 . 303 . 304 . 304 . 308 . 309 . 310 . ol2~ Be ay ait eal “Gis _ 320 meet . 324 . 325 . 825 . 3825 Oe . 327 . 339 . 336 . 337 . 339 . 341 . 342 CONTENTS XV PAGE Berea Ser eerima tee SN.) wee. oe ES eM kg. a ie Bate Me General method ... . RN ae Sk aw Se eee Miethodromiybnmalzine maize. “6 Geel Pawb wees) Up teib a 4. ea ee Dieinod ony priaizings wheat. . . os fie eeeoeks oo wl ow. F owl URED Method of hybridizing alfalfa . . . . RR ANS hia Soke El LIM Cod DSR ASS Some of the difficulties attending ybaroniee rarer sete td Ocoee ann Sandionssavorablatomhybridizations <".) seme ae. 2 tte... eS or Species hybridization . . . . 15) ee eee TTS EL ATCT POO Svalof method of creating populanans sal) SOLAR ee Ma IS DN . 852 CHAPTER XX UtinizATION oF Hysrips IN PLANT BREEDING Introductory. .. . cabkowott Nate eRy «gel! } LD ate SUR 1 97) 20k OREO DP aos Purposes of hy bridiz ae 5 Vee Sens. as ty aaecg RE Aeees IS oo Increased production in F maize Pande RS, ae sf Bk Oo OD. (SCransing nore Strains OF WIOTy Pes *.-. ) ¢.) «Sy neh 1 epee ee, eos Biecwar inbreeding in Straims,oi maizeoy 2... 3) ee te ee i. Method of comparing yields. . . . . FLY scouted kctee SNE eee Ee pe O Crossing species, subspecies, varieties and ae Eh hat: en Scat oust OW) notte. Superior qualities of first generation hybrids. ............ . 359 Immediate effect of crossing on size of kernel . .......... .°. 360 Centrauzeq-scca.cotm production. -s2 Sf). 2.5...) ltyeaid ays eee? GeeeOn memetvod, cL progucine: hybrid-corm seed’. =.. . «1.246 tases a eet ) «Eyal Gp airs ee et eae Oe Application in vegetatively propagated plants .............. . 364 Rapid growing tinfber and ornamental trees. . ............. . . 365 CHAPTER XXI MovtatTions IN PLANT BREEDING Introductory. <.. . Soe, ge a ake Ge ee RCO Occurrence of a Snne PE 2... at rn Meee Amul name. ol! c Riana iLerOp plalltge i '..cl ~.) uranyl bS6 6 SEN A, oye ee tees i Ot EEC UG Or NU LanONS |.) sce a Psy Fy tyes owen eee eee won ipespn Ee TeOnaOr Amos... Ss eee @ ants Mee. 5 7 ell as, gehen ma nied ere CHAPTER XXII GRAFT-HYBRIDS AND OTHER CHIMERAS Introductory. . . Re eS 2 aa > et Be dt Pei ei! Definition of pate fipad ee sae sd ae OE a ee te CSE gel = ot Le Winkler’s tomato-nightshade crafi-ayprae: RAMS A Neh Ps YEH, ne Rae | I (| Baur’s investigation of a natural chimera ahsepeer eh hee Pee es, ae eee ed) OS Brea cEACHUMGTAS.. /<) «Tic. ahd Deusen, eee Sem ote a) ae Two categories of variegation .. . BP Ore ye Ce oe ae IS The physiological behavior of graft hy brids PO ae bh A) rua eee ee 2 Modification of one graft-symbiont by the other . . .. ........ . . 388 XvVl1 CONTENTS CHAPTER XXIII Bubp SELECTION Introductory . Efficacy and pereredb ett, of a pelecta Bud variation in plants . See Seen Bud selection in Coleus . Bud selection in horticultural prectice: Performance records as a basis for bud edlegiien : Bud mutations in Citrus Deciduous tree fruits . “Pedigreed’”’ nursery stock Bud selection in the potato Certified seed potatoes : Other crops in which bud salga na may y apply : Limitations of bud selection . CHAPTER XXIV BREEDING DISEASE RESISTANT PLANTS Introductory. : : The causes of plant canennes i : The nature of disease resistance in sini : Disease resistance in natural species Phylloxera-resistant grapes Endothia-resistant chestnuts Blight-resistant pears . Breeding disease resistant vameties by neato Creating rust-resistant wheats . : Inheritance of disease resistance in other sims Breeding disease resistant plants by selection CHAPTER XXV Piant BREEDING METHODS Introductory . ‘ ; Need of yates savings: : Pedigree culture . The Sval6f system . Variety tests—purposes; Hifmenlties cael. Establishing varietal types. Determining best varieties for given “heritage Strain tests—purpose; difficulties involved . Plant-to-row tests : Factors that affect experimental ness CHAPTER, XXVI GENERAL CONSIDERATIONS AND CONCLUSIONS Introductory . PAGE . 385 . 385 . 385 . 386 » 391 - oun . 392 . 393 . 394 . 394 = ooN . 398 . 399 . 400 . 400 . 401 . 401 . 402 . 405 . 407 . 408 . 411 . 413 . 416 . 419 . 419 . 419 . 425 . 427 . 427 . 428 . 432 saree . 433 CONTENTS The relation of science to plant breeding—historical review The future relation of genetics and plant breeding : : Planning breeding operations in the light of scientific Tnowledae ; PART III Animal Breeding CHAPTER XXVIII THe GENERAL Aspects or ANIMAL BREEDING Introductory . The history of anal preceding The animal breeding industry The art of breeding . The problems of animal breedines The service of genetics : The service of genetics in education. The personal equipment of the animal br eeder ! CHAPTER XXVIII VARIATION IN Domestic ANIMALS Introductory . The sources of Partin. : Selection as a cause of variation . Variation by modifiability . : Modifiability and breeding value . Modifiability and correlation. Variation by recombination . Mutation in domestic animals . CHAPTER XXL MENDELISM IN Domestic ANIMALS Introductory . : : Importance of Ewenimental feecnine: Mendelism in horses Mendelism in sheep. Mendelism in swine. Mendelism in poultry . CHAPTER XXX ACQUIRED CHARACTERS IN ANIMAL BREEDING Introductory. ; The problem ; The belief in the inheritance oe acai chanics The argument against the inheritance of acquired characters . XVI PAGE . 437 . 440 . 440 . 443 . 443 . 445 . 447 . . 447 . 448 . 450 . 450 . 453 . 453 . 454 . 454 . 456 . 459 . 459 . 462 . 465 . 465 . 465 . 475 . 476 . 476 . 480 . 480 . 482 . 484 XViil CONTENTS PAGE The soma and germ plasm—experimental investigations. . . . . . Pe |. SABE Mheusolationiof the werm splasmy, (240°) .< (0) De es eee Mle inadequacy of affirmativelevidence (.)).- 0.0. 2. 04,1: 1S NRA eee tee The transmission of functional modifications. .............. . 491 Parallel induction .. . nT eee ee fk CB yo SOY The adequacy of other Pion PPR cS eS ee EE TMeKCONCIUSIONT ee Bcc ws wok. santleteccte S| eens ecm OLE CHAPTER XXXII Tur SELECTION PROBLEM IN ANIMAL BREEDING Imtroductory. 2. - - EP anh coe eto ey lw GOS General views of selection cs Ak ew SS Mhe American standard bred horse . . . » = «s,s > + Helms oc uues een) Hecuncityain fowls... 9: =)... . . os ek 4, ES) eee a Bantam fowls . . . PP tre Soe oe wird toh. SQ Selection and preedine: methods NP oe ons es cy oe ce UD Sclectigmam@dic6S. - 6... oc 2 2 3. te ie ee Re ee race perenne eee O02 CHAPTER XXXII HyBriDIzATION IN ANIMAL BREEDING Introductory. . . eh AM ee ee ey gk og OS! Growing ppamieoiee o Pe imenzatien et rae taper Oy Ay eID Crain ey i es he te Go tl OS eal. ED Crossbreeding ... . Oe Nc. 3 Saige aii aly rece emmy oo ae Species hybridization among damaees animate ern Tce Pale oto ess CHAPTER XXXIII DISEASE AND RELATED PHENOMENA IN ANIMAL BREEDING Introductory. .. . ee a ee sl ne eee, The inheritance of digests oy Cae Ms a rete pry een Uk Sie my The inheritance of predisposition to idisenee » on REE ene onl pam Ye. ine Nene Mhetnheritance of defects:..: . «+... «Cea nee ane ie ice ee ie ee maT ce Derectsim domesticianimals.. . ... . .. . (Re eee eo immunity; to disease)). 2...) /.<. . . . (Se eee ees Breedine forimmiunity.. 6. : =: . . . ..s . eens. anemia CHAPTER XGGXivi Sex In ANIMALS Introductory. .. . ne nn ky SEMI GC The determination aR SOX Sk a... es ee Sox-determinationin mammals... . .. . ..\. +s (elaeeaneneaenenaes ae Soe enteden Sax-determination in birds... =. <<, 2 leh. %. = w.'s 80 ou eee ele MWe, BOXEEGtIO. .. sc +. ceccwen Gy Hon) awe he ow ve ces ig Sere Ooo CONTENTS Causes of unusual sex-ratios . ; Metabolic theories of sex- Cdetorminatin : Inheritance of unusual sex-ratios . Secondary sexual characters . The effects of castration . CHAPTER XXXV FERTILITY IN ANIMALS Introductory . . é Factors influencing fertility The Darwinian theory of fertility . Inbreeding not in itself harmful Fertility as related to Mendelian factors. ’ The chromosomes and fertility—Drosophila . Sterility in other animals Sterility in hybrids . Fertility as related to Ligier Fecundity in fowls . Conclusion CHAPTER XXXVI Some Be.iers or PRacticAL BREEDERS Introductory . Telegony Harmful effects e By bridization Infection of the male. Saturation . Maternal impression Prepotency The Mendelian pudeepretation The relative factor interpretation . The hereditary complex interpretation Greater prepotency in the male Conclusions . CHAPTER XXXVII MeruHops OF BREEDING Introductory . : Methods means to an anh Phenotypic selection Limitations of phenotypic pecleraen F Pedigree breeding ae Breeding systems based on blood relation tip, Inbreeding. Line breeding Out-breeding. ar Other systems of breeding . Genotypic selection . XIX PAGE 542 . 544 546 . 548 548 . d51 . dol . 552 . 593 . 504 . 555 . 555 . 556 . 599 . 909 . 563 SS OOd “mond yi . 578 . 580 . 580 581 . 582 583 . 583 . 584 YOX CONTENTS CHAPTER XXXVIII MetuHops or ConpucTING BREEDING INVESTIGATIONS Introductory . The need of staal: Judging the individual Pedigrees . : The coefficient of imeneadinns The coefficient of relationship Marking individuals Recording data. . Coéperative breeding . CHAPTER XXXIX CoNCLUDING REMARKS Introductory. ; : The present lack of detmiled lenearledee The need of research . Neca ws The service of genetics The need of other knowledge. GLOSSARY . . List oF LITERATURE CITED . INDEX Pace . d91 . Oo . 59 ». ego . 598 . 600 . 602 . 603 . 606 . 607 . 607 . 608 = 609 . GU . 615 . 622 . 648 GENETICS IN RELATION TO AGRICULTURE. PART I-FUNDAMENTALS CHAPTER I THE METHODS AND SCOPE OF GENETICS Soon after Mendel’s report of investigations in heredity had been rediscovered, it became evident to most biological investigators that a flood of light had been thrown upon the problem of heredity, and the related subjects of variation, development, and evolution. The need for a new term, therefore, to designate this interrelated portion of bio- logical science led Bateson to coin the word, genetics, from the Greek root, TEN, “become.” The derivation does not indicate, it must be admitted, very clearly the portion of biology to which the term genetics applies, but this vagueness has in it an element of desirability, for it is extremely difficult to define accurately the boundaries which delimit the province of genetics. Bateson himself has stated that genetics deals with the physiology of heredity and variation; and a favorite statement of authors has been that genetics is the science of the origin of individuals. But these statements—they can hardly be called definitions—must be qualified carefully in order that they may be understood. Accordingly it has seemed desirable to construct a definition of genetics in purely objective terms. The following definition is, therefore, proposed to ful- fill this need; it, too, will require some qualification: Genetics is the science which seeks to account for the resemblances and the differences which are exhibited among organisms related by descent. The Content of Genetics.—If genetics be defined in the above manner, it may be stated roughly that variation is that portion of genetics having to do with the differences beween organisms, whereas heredity has to do with the resemblances which they exhibit. But this statement does not define very accurately the exact meanings of the two terms; to do this it is necessary to consider certain fundamental facts. Organisms exhibit various degrees of difference and resemblance, and classification is made possible first, by resemblances between individuals and, second, by differences between groups of individuals. Further, the orderly interrelations which are exhibited by living beings in general has 1 2 GENETICS IN RELATION TO AGRICULTURE made it possible to group them into orders, families, genera, and species according to the degree of resemblance which exists among groups of individuals. But this is merely a view en masse of the differences be- tween organisms, for it is universally true that no two individuals are exactly alike. There are, therefore, for all practical purposes, two orders of difference between individuals; first, racial differences, those which separate groups of individuals, and second, individual differences, those which distinguish the individuals of a group from one another. Strictly, of course, there are all possible gradations from the one degree of dif- ference to the other, but conveniently it may be said that the former, the racial differences, are those which characterize different lines of descent, whereas the latter, the individual differences, distinguish indi- viduals within a given line of descent. The problem as to the origin of racial differences is a problem of evolution; the problem of the origin of individual differences is a problem of genetics, and we accordingly shall construct our definition of variation to apply to differences exhibited by individuals related by descent. Now all multicellular organisms which reproduce by sex exhibit the common characteristic of two distinct cycles of cellular development; gametogenesis, or development of the germ cells, and somatogenesis or development of the body. The resemblances which make it possible to group individuals into orders, families, genera, and species are the result of the fundamental relation which exists between these two cycles, for it is a commonplace fact that the germ cells of any species can reproduce individuals of the same and no other species. This rela- tion of germinal constitution to the development of the soma is specific for all classes and grades of characters, but the order of specificity may be either racial or individual, just as the order of difference between individuals is racial or individual. The term variation carries with it the idea of deviation from type, and obviously the above statements, brief as they are, of the cycles in individual development leave room for several possibilities of deviation from type. Thus, if we look at the matter from one point of view, the guiding hand in determining the characters of the individual is the specificity of the germinal substance. But every individual develops under a certain set of conditions, the environment, which is independent of the germinal substance; and these conditions have a certain, usually merely modifying, influence in the development of the individual. There is, therefore, a possibility for differences to arise in individuals independently of differences in the germinal substance, differences which are specifically attributable to diversities in the environment, and which may have no effect on the germinal substance itself, just as the degree of heat, for example, may cause a variation in the end products THE METHODS AND SCOPE OF GENETICS 3 which a given chemical system yields. Differences in development may, also, occur because of actual diversities in the germinal substance, and these may arise from the intermingling of different kinds of germinal substance, such as obviously takes place in sexual reproduction, a cause of variation which has been ably advocated by Weismann and styled by him amphimixis; or they may arise from actual changes in the germinal substance, distinct from the intermingling of germinal elements which already exist, a form of variation which has been proposed and elaborated by de Vries under the name of the mutation theory.: Accordingly the term variation in genetics is so defined that it includes differences in individuals related by descent, although many authors do not include within the term those differences which are due to environmental conditions of all categories. The following definition is framed in conformity to that already given for genetics. Variation is difference, whether in the expression of somatic characters or in the elements of germinal substance, among organisms related by descent. Heredity is commonly defined as the tendency of offspring to develop characters like unto those of their parents; according to Castle it is resemblance based upon descent. Thomson presents a very able dis- cussion of the concept, heredity, together with criticism of definitions which have been offered from time to time for the term. According to his definition, by heredity is meant nothing more nor less than organic or genetic relation between successive generations. The universal tendency of: organisms to produce similar organisms is the cause of the maintenance of organic groups and group relations. But experimental research has demonstrated that sometimes new com- binations of germinal substance produce characters which have not been exhibited by parents. It is necessary, therefore, to define heredity in such general terms that it will include those exceptional characters which have never been exhibited by any ancestor. Now regardless of any external difference which may be exhibited by an individual, its germinal constitution bears a perfectly definite relation to those of its parents. For that reason the following definition is stated in terms of elements of the germinal substance, rather than in terms of somatic characters. Heredity is germinal resemblance among organisms related by descent. Finally, with respect to the content of genetics, emphasis should be laid upon the importance of a consideration of the various phases of development. In development are included all those changes and cycles through which the individual passes in attaining the adult condition. Obviously there is much in development which cannot be treated at all in an elementary text-book of genetics, for particular cycles or phases of 4 GENETICS IN RELATION TO AGRICULTURE development are treated as separate sub-divisions of biology, such as embryology, cytology, experimental morphology, and like subjects. While obviously there is much in all of these subjects which is irrele- vant to a treatment of genetics, nevertheless, rightly interpreted, there is little which is essential to any one of them which does not bear some more or less intimate relations to those phenomena which belong more strictly in the province of genetics. The reason for this is very apparent, the development of the individual is a consequence of the elaboration of the hereditary material, it is the fulfillment of the possibilities wrapped up in the germ cell; how then can it fail to possess much that is of very great significance to genetics? Assuredly the further advancement of the science of genetics will focus more and more attention upon the prob- lems of growth and differentiation in the individual; for that reason these emphatic statements are made. The Problems of Genetics.—Obviously the problems of genetics are those which grow out of a study of resemblances and differences in individuals related by descent. Wilson has reduced the statement of the problems of inheritance and development to that oft-quoted question: “How do the adult characteristics lie latent in the egg; and how do they become patent as development proceeds?’ Pearl has voiced very much the same thought in his statement that the critical problem of inheritance is the problem of the cause; the material basis; and the maintenance of the somatogenic specificity of germinal substance. There are four general methods of attacking the problems of heredity; namely, the methods of observation, experimental breeding, cytology, and experimental morphology. Each of these methods has its specific advantage and particular value as well as its definite limitations. In the following discussion each method is considered briefly with respect to its relation to the development of the science of genetics. The Method of Observation.—The method of observation, or de- scription as it is often called, requires special treatment because it employs the inductive mode of reasoning. Briefly the essential steps involved in the application of inductive reasoning to the problems of genetics may be stated as follows. The first step is the observation of the re- semblances and differences between representative individuals of a given line of descent or, if problems of evolution are under consideration, of different lines of descent. The next step is a comparison of the ob- servations which have been made for the purpose of determining whether they show orderliness with respect to each other; in other words to de- termine whether they probably have a common causal basis. If they do show such orderliness, an attempt is made to formulate the principles or laws which govern them. Finally, the principles or laws thus for- mulated are applied to other instances not included in the original set of THE METHODS AND SCOPE OF GENETICS 5 observations in order to test their general validity. The weakness of the method in biology lies in the lack of rigid experimental control over the phenomena which are under observation, and also in the fact that often it is either very difficult or impossible to subject to experimental verification the principles or laws which have been thus formulated. For this reason, the method of observation as a means of formulating prin- ciples and laws must constantly be subjected to rigid scrutiny, lest unde- tected fallacies lead to the acceptance of conclusions which actually have no significance from a biological standpoint. But although the observational method has very definite limitations in the determination of genetic principles, nevertheless it has been the chief method of investigation in the formulation of some of the most stimu- lating theories of biological science. The marshalling of evidence by Darwin in support of the evolution theory depended almost entirely on an application of this mode of research to a vast array of more or less iso- lated cases. The mass of evidence, which he accumulated in order to demonstrate that natural selection by favoring the ‘‘survival of the fittest,’’ to use Spencer’s phrase, results in evolutionary progress in suc- ceeding generations, will ever stand as a monument to his masterly skill in observation and interpretation. In addition to its utilization in the development of the evolution theory, the observational method has been employed widely in the field more strictly included in genetics. Sir Francis Galton employed a refined type of the observational method in his study of heredity. His object was to establish a law of organic resemblance within a single species, distinctly a problem of genetics. In order to do this he employed a system of more exact observation based upon accurate determinations in a large number of instances and mathematical reduction of the data thus collected. This system has since undergone notable development, particularly at the hands of Karl Pearson, and, as biometry, it is often accorded recognition as a distinct branch of biology. As one of the re- sults of his studies, Galton announced the law of ancestral inheritance which states that on an average each parent contributes one-quarter or 0.52, each grandparent one-sixteenth or 0.54, and so on to the total heritage of the individual, which equals 1.0. The other notable result of these studies, the law of filial regression states essentially that on the average any deviation from racial type is transmitted to the offspring in a lessened degree, so that’, in general, offspring differ less from the type of the race than their parents; specifically they exhibit a deviation from the racial mean only two-thirds as great as the parents. Mere observation, be it ever so precise, is subject to very decided limitations when employed as a method of analyzing the general problems of evolution and heredity. To be convinced of this, one need only con- 6 GENETICS IN RELATION TO AGRICULTURE sider the opinions which have been entertained by those who have em- ployed this method in the solution of biological problems. Thus Darwin believed that minute continuous variations are transmitted and form a basis for evolution and that the more striking discontinuous var- iations are of little moment in the origin of species. These are beliefs which rigid experimental investigation has failed to establish, and which are, therefore, highly improbable. In fact it has been clearly demon- strated that minute differences between individuals are for the most part not transmitted; and that distinct new characters which appear suddenly are often heritable. Similarly, the inheritance of acquired characters, so readily accepted by men with minds as keen as those of Darwin and Spencer, has failed to receive confirmation when subjected to rigid experimental enquiry. Definite knowledge on points such as these is of tremendous importance in making for progress toward the solution of the general problems of genetics, but such progress is slow and uncertain by the employment of the observational method of attack alone. It is for this reason that the favor of geneticists has swung so strongly toward a more rigid method of experimentation. However, the observational method is not unique in possessing limita- tions. No single method is known invariably to give correctresults. It is necessary to combine all available methods in order to insure the most certain and rapid approximation to the truth. But the difficulty with the observational method, particularly that part of it known as biometry, has been in the manner of its employment in the elucidation of genetic phenomena. It has been employed, as Pearl points out, both as a method of research and as a method of stating the results of experience. The former manner of utilization is unquestionably of great value in genetic research, its particular value residing in the fact that it has substituted exact methods of expression for vague and indefinite statements. It has performed a service of tremendous value to biology in the introduction of the probable error concept as an index of the degree of reliance to be placed in the results of determinations arrived at by other methods. The latter manner of utilization, however, as a method of stating the results of experience, the employment of which is characteristic of the biometrical school, is subject to serious objections. However, it is worthy of note that the method of observation will ever remain a valuable aid to the extension of knowledge, particularly in directions in which, by their very nature, it is impossible to employ experimental methods of research. It is difficult to imagine, for instance, any notable advance in our knowledge of human heredity save by a proper employment of this method of investigation. The Method of Experimental Breeding.—The essential feature of all experimental breeding is the raising of pedigreed cultures of plants and THE METHODS AND SCOPE OF GENETICS 7 animals, for which reason it is sometimes called the pedigree method. The notable progress which has been made in genetics during the past few decades has come from the application of this mode of enquiry. It is the analytic method of the geneticist and it is often and not unjustly compared, both with respect to its utility and its limitations, to the test- tube method of analytical chemistry. From it have come many stimu- lating ideas of heredity and variation; the Mendelian theory of heredity; the closely related pure line theory of Johannsen; and the mutation theory of de Vries: few methods of research can boast a more honorable array of achievements. Of these achievements, the Mendelian theory is the accepted founda- tion of present ideas of heredity. For theapplication of Mendelian methods of analysis three essential breeding operations are necessary ; first,the raising of pedigreed strains of plants and animals to determine their behavior under controlled conditions; second, the hybridization of diverse races; and third, the intensive study of the hybrid progeny through successive generations. From this outline of the breeding methods which are employed, it may be concluded rightly that the Mendelian method, like the Galtonian, is essentially statistical. It differs radically, however, from the Galtonian method in that it substitutes the observation of con- trolled progenies for that of ancestral generations. Its particular ad- vantage lies in the fact that it is strictly verifiable. Moreover, it hashada different and more specific purpose in view, namely to state in definite terms how the particular individual will behave in heredity, rather than to arrive at a determination of average behavior in this respect. The important result of this method of analysis has been to demonstrate that the germinal material is made up of definite units or factors which stand in close relationship to particular characters of the soma, and to demon- strate how these elements of the germinal substance are transmitted from generation to generation. The two remaining products of the pedigree culture method, the pure line theory and the mutation theory, stand in close relationship to the Mendelian theory of heredity; because they may be interpreted in terms of the elements which constitute the germinal substance. Of these the pure line theory may be said to add another conception to those of the Mendelian theory, namely that elements of the germinal substance possess a high degree of stability. If this conception be accepted, it follows—and this is the central postulate of the pure line theory—that variation among individuals of like germinal constitution is a response to external or internal conditions which are not reflected in the germinal substance. Such variations, therefore, are of no consequence for the establishment of new hereditary characters. A large number of plants, among them barley, oats, rice, wheat, and practically all the legumes, 8 GENETICS IN RELATION TO AGRICULTURE are almost invariably self-fertilized. They consequently give rise auto- matically to populations which are composed entirely of pure lines. The pure line theory, therefore, has tremendous practical significance. The mutation theory adds yet another conception to those which have already been stated, namely that of occasional mutability of germinal elements. It is, therefore, directly contradictory to the pure line theory in its fundamental postulate; but the very great infrequency with which changes occur in germinal elements saves the pure line theory from inutility. Here the important result has been to establish firmly the occurrence of occasional, definite, discontinuous changes in germinal substance in consequence of which new characters are added to the - heritage of the race. Much of the variability in individual characters whieh is exhibited by plants and animals appears to have had its begin- ning in mutational changes in the germinal substance. The mutation theory, therefore, is another consequence of genetic investigations which has far-reaching practical consequences. Fruitful as have been the results of the method of experimental breed- ing in prosecuting genetic research, students and investigators should not delude themselves as to the nature of the knowledge which it has yielded. It cannot stand alone as a mode of investigation, for even the present illuminating conception as to the structure and operation of the hereditary mechanism has been almost as much the result of cytological as of breeding investigation. But taking this conception in its present form, tremendous as has been the advance of recent years, this sort of knowledge cannot represent the ultimate goal of genetic research. Mendelism has given us the plan of heredity—the more intimate and fundamental knowledge of the material which is employed in the elabora- tion of that plan remain the task of some other mode of research. The Method of Cytology.—The method of cytology in genetic re- search is concerned primarily with questions of cell mechanism. It may be said to be directed toward the solution of two distinct problems, first the behavior of the hereditary elements in somatogenesis, the building up of the body, and secondly in the determination of the nature and operation of the mechanism which distributes hereditary elements from parent to offspring. These are matters of fundamental importance in genetic enquiry; it is unfortunate that the methods of dealing with the problems here presented are necessarily static and so little under the control of the investigator. Nevertheless even with these handicaps, the contributions of cytology to genetic interpretation are by no means inconsiderable. The determination of the equivalent distribution of the hereditary elements in the cell divisions of somatogenesis and the prob- able fact that every ultimate cell in the body normally possesses all the hereditary elements of the initial fertilized egg-cell have been established THE METHODS AND SCOPE OF GENETICS 9 as nearly as may be by cytological research. Moreover, the separation of homologous contributions of the parents in the formation of germ cells and the union of two homologous sets of hereditary elements for the production of new individuals represent another phase of the problems which have been solved by cytological research. Although obviously the dangers of misinterpretation in dealing with fixed and stained preparations of cells or sections of cells are very great, a fact which is disclosed by the diverse interpretations which different investigators have given of the same phenomena and structures, never- theless the importance of this field of research should not be under- estimated on that account. There are several reasons for reposing confi- dence in the results of cell investigations, and these come from two sources; from the confirmations of the growing field of what may be called experimental cytology, the observation of cell phenomena directly in living cells, and from the broad general result of cytological research that the mechanism which has been discovered is by nature such an one as might be expected from a priori consideration of the results of Mendelian investigations. The close correspondence which exists between cell behavior as it is believed to exist from cell investigations and hereditary phenomena as they are known to exist from Mendelian investigations has given renewed confidence to students of heredity in the validity of their interpretive conclusions. The most important progress which has been made within the last decade in genetic science has been that of interpreting Mendelian phe- nomena of inheritance in terms of the behavior of the cell mechanism. Thus far this work has been carried to any degree of completeness in only one species, the common fruitfly, Drosophila ampelophila. In the extensive investigations which have been made with this species, Morgan and his associates have demonstrated how close a correlation exists all along the line between cell behavior and hereditary distribution of characters. Certain characters are distributed independently of each other, the pairs of chromosomes separate independently of each other in the formation of gametes; certain characters display irregularities in distribution and expression associated with differences in sex, the chromo- some content of the two sexes is demonstrably different; four sets of characters exist the members of which tend to remain together in trans- mission in the combinations in which they occurred originally , the entire chromatin material is contained in four pairs of chromosomes; and finally irregularities in character distribution have been discovered, the chromosome constitution and distribution in such cases are correspond- ingly irregular. These facts the student will be better fitted to appreciate later on; they are given here to show how the results of one method of investigation are supported and strengthened by those of other methods. 10 GENETICS IN RELATION TO AGRICULTURE The Method of Experimental Morphology.—Under the heading morphology, we include those particular phases of development which are designated by the terms, ontogeny and embryology. The method of experimental morphology has for its task the solution of the problem of the development of the individual as it is related to problems of variation and heredity. The aim of this method is to determine how the characters of the adult become patent as development proceeds, the broad question of the origin of complexities within organisms. In the Mendelian method, the formal relations which exist between hereditary elements are dealt with, particularly their relations in dis- tribution and recombination. ‘The characters of the adult organism are for the most part the basis of judgment. In spite of the general truth of this statement, however, Mendelian analysis has in many cases extended into the field of the physiological relations which exist between hereditary elements, not merely with regard to contrasted homologous hereditary determiners, but with regard to the physiological relations existing in development between entire sets of hereditary elements, and at times even between these and definite factors of environment. But for the most part the solution of such problems depends upon thorough experi- mental study of development in individuals of known genetic constitution. This portion of the problem remains almost untouched. If development be thought of as a series of successive physico-chemical reactions, the complexity of the problem may easily be judged. Certain of the simpler features of it, however, have been attacked and the results of these preliminary studies have indicated still other modes of approach, so that we may expect that when geneticists come to appreciate the light which may be thrown upon heredity by the experimental investigation of development, research in this field will be greatly stimulated. Already as Jennings has pointed out the main features of the process of develop- ment are clearly indicated; the hereditary elements of the chromosomes remain the same in each cell, the reactions and functions of any cell depend upon this chromatin system working in conjunction with the cytoplasmic matrix in which it is located. From this fact may be drawn the broad conclusion that differentiation within the individual depends upon cytoplasm differentiation. The difficulty of the question of how and why should not deter investigation. Prerequisites for Genetics.—The foregoing discussion of. modes of research in genetics should indicate something as to the nature of the working equipment necessary for a study of the science. Since genetics is a biological science, intelligent study of it presupposes a thorough grounding in general biology such as is given in foundation courses in botany and in zoology. Inasmuch as practically all domesticated plants and animals belong to the higher orders, particular attention should be THE METHODS AND SCOPE OF GENETICS LM given to the cycles of developments in these organisms, especially those phases which are comprised in development and reproduction. Of particular importance is a general knowledge of physiology, not so much on account of the direct utility which it has in the study of genetics as for the attitude toward life phenomena which it awakens in the student. Genetics, indeed, is essentially a sub-division of physiology in the broader sense. A knowledge of mathematics is a valuable asset because it is often necessary to subject the data of heredity and variation to mathe- matical treatment in order to interpret them properly. For the elemen- tary study of genetics, a knowledge of the methods of dealing with simpler algebraic problems is sufficient; for advanced study a knowledge of the differential and integral calculus is highly advantageous. Finally it may not be out of place to mention the fact that investigation in genetics is not confined to those who employ the English language. A reading knowledge of French and German is practically necessary for those who desire to pursue the subject very far. The Applications of Genetics.—Genetics has both scientific and prac- tical applications. As an example of its scientific applications, the part which it has played in shaping doctrines of evolution instantly comes to mind, for of necessity such doctrines must conform to the fundamental principles of genetics. The science of genetics and that of evolution are by their very natures constantly encroaching each upon the fields of research of the other. Thus experimental investigations of evolution are of vital interest to genetics, because they deal with the mode of origin of hereditary characters. Genetics, also, has its applications in branches of biology other than that of evolution, indeed throughout the entire realm of biology its influence is felt in shaping thought and direct- ing interpretation. There are few other sciences which possess so much of general interest as that of genetics. The practical applications of genetics are found in agriculture and in human affairs. Here genetics involves many things which are extra- biological. Thus in agriculture emphasis is placed upon the employment of the principles of genetics for the amelioration of plants and animals for man’s use. Breeding, then, may be defined as the art of improving plants and animals by hybridization and selection. To make effective progress along this line methods of testing given individuals or races, both with respect to fixity of type and comparative value, have been de- vised. The methods of attack are very much the same as those which are employed in the experimental study of heredity and evolution, the primary aim of which is merely to discover underlying principles. Eugen- ics is concerned with the principles of genetics in so far as they may be applied in the improvement of the human race; but it includes much that is sociological, rather than biological. The applications of the prin- 12 GENETICS IN RELATION TO AGRICULTURE ciples of genetics, therefore, are always subject to such modifications as may be determined by practical considerations. Genetics in Agriculture—Modern agriculturists, for the most part, appreciate fully the importance of producing only the best types of plants and animals; for in spite of the strange anomalies of economic conditions which at times appear to give actually a greater return for smaller total yields, the fact must remain that the larger view of the agriculturist’s place in society requires of him as of all its other members the fullest possible returns compatible with economic principles and the require- ments for a permanent agriculture. But although the desirability of high production and quality is very generally recognized, it is a fact that very often this ideal cannot be attained except by the most careful and intelligent efforts. This is more often the case with plants than with animals, for plants are on the whole less independent of environmental conditions and therefore more susceptible to differences in them. Pro- ducers of crops are always in need of varieties which are better adapted to local conditions, but except in rare cases they are not fitted to develop such varieties. Here genetics comes very definitely to the aid of the plant breeder for its principles provide a safe guide for him in attaining his ideal. Already breeders of plants have realized a great saving of time and expense as a result of the application of principles derived from scientific investigations in their work. The animal breeder on the other hand has faced a somewhat different problem. The far greater comparative value of the individual in live- stock operations has led in animal breeding to the establishment of pure breeds of domesticated animals of remarkable excellence. Long applica- tion of the method of trial and error has developed a body of empirical knowledge which has achieved results nothing short of the marvelous. But while the old empirical methods have served their purposes well, nevertheless they cannot from their very nature give complete satisfac- tion. Knowledge is only secure when it rests upon a firm foundation of principle, and however excellent have been the results of empirical breeding from a utilitarian standpoint, they have not led to the discovery of fundamental principles. The principles of genetics provide a consist- ent interpretation of the results of breeding methods. To the novice a knowledge of such principles is an abundant aid in interpreting and organ- izing details of experience; by its help he can progress more safely and more surely in determining the methods of procedure which are es- sential to the fullest success in his breeding operations. The real service of genetics to animal breeding lies in the promotion of clarity of thought, and that is a thing of no little value. Although genetics thus far has contributed but little toward improve- ment of the existing methods of animal breeding, it is not a dream im- THE METHODS AND SCOPE OF GENETICS 13 possible of realization that in the future its contributions in this direction will be of considerable importance. The science of genetics is still in its infancy, it is still in the formative period of its existence. It has not yet been possible with any degree of satisfaction to analyze the heredi- tary constitution of any farm animal, even to the incomplete extent which has been accomplished in some plants and in some of the smaller animals. Obviously we cannot apply even the general principles of genetics intelligently in animal breeding until we are more thoroughly conversant with the facts of character behavior and factor relationship. Such facts can only be determined by means of carefully planned experi- mental investigations. A few investigations have already resulted in important extensions of our knowledge in this respect, others now under way promise to extend this knowledge considerably further. Systematic crossbreeding of cattle and sheep for definite commercial purposes is of proven value. The method of breeding for high winter egg production in fowls has been determined. Investigation of the inheritance of high milk production in cattle is under way. Geneticists are also seeking to analyze the extensive data with respect to certain characters such as color, fecundity, and speed which have been recorded in herd books. Progress in such work with the larger domestic animals is necessarily ex- ceedingly slow, but this should not deter investigators from organizing carefully planned experiments to extend knowledge in this direction. It is only in this way that genetics can take its proper place in practical animal breeding. The progressive agriculturist can well afford to en- courage every proper effort having as its aim the collection of genetic data. CHAPTER II VARIATION Organic differences, their nature and causes, have furnished abundant material for speculative enquiry since time immemorial. The great sig- nificance of the fact of organic individuality was not fully grasped until Lamarck founded his theory of evolution which postulated the progressive, imperceptible change of one species into another. It remained for Darwin to scrutinize all phases of organic life, past and present, wild and domes- ticated, in his search for a guiding principle which should explain the course of evolution. Darwin’s hypothesis of natural selection assumes variability without enquiring into its causes, but this does not mean that Darwin was not concerned with the problem of causes. In both his ‘Origin of Species” and ‘‘ Variation in Animals and Plants under Domestication” the causes of variability are often referred to and he suggested among others, the kind and amount of food, climatic changes and hybridization. Our respect for the great naturalist’s keen percep- tion deepens when we realize that very little has been added as yet to our knowledge of the causes of variation. The Universality of Variation.—Individuality is common to all or- ganisms. No two trees, no two leaves, no two cells in a leaf are identical in every respect. Individuals sometimes appear exactly alike but even identical twins will be found to differ in some features. The shepherd knows his sheep individually and the orchardist his trees. Were there no differences in individuals there would be no changes in species and there could be no improvement of cultivated plants. ‘Variation is at once the hope and despair of the breeder,” the hope because without it no improvement would be possible, the despair because very often, when improvement has been made, variation results in a tendency to fall below the standard previously reached. In the sugar beet, for example, a high percentage of sugar has been maintained by continually testing and selecting the ‘‘mother” beets for the next crop of seed. How- ever, this necessity for continual selection does not exist in respect to all important field crops although they are subject to the general law of variation. That this must be so is clear when we realize that many natural species as well as cultivated varieties of plants are really mix- tures of sub-species, varieties, or races and that upon being isolated these distinct forms reproduce their own particular type. This is most easily demonstrated in plants normally self-fertilized; yet in all naturally 14 VARIATION 15 cross-fertilized plants and in higher animals this same endless diversity among individuals is even more marked. The Variation Concept.—As we have implied in the above remarks the term, variation, may be used in very different senses in referring to different phenomena. Thus variation within a species or variety means that the group in question is heterogeneous. Among individuals varia- tion may consist of differences between members of the same generation or between parents and offspring. Even when thus restricted, however, the term is apt to prove ambiguous. Hence it is necessary to give some thought to the sources, nature and causes of these individual differences in order that we may use clear cut expressions which shall always convey to one another a concept of the same particular sort of organic difference. Classification of Variations.—1. Heritability —Character differences either represent something specific in the germ or they are merely the effect. of external stimuli upon the individual soma. In the first case they are inherited, although they will not reappear necessarily in all later generations or in all the progeny. In the second case they will not be inherited. This is a fundamental distinction and may well serve as our primary basis of classification. . According to heritability variations are either germinal or somatic. Under germinal variations we recognize two sub-classes, combinations and mutations. Purely somatic variations will be referred to hereafter as modifications. Modifications are non-heritable differences between the individuals of a race caused by the unequal influence of different environmental factors. Such variations frequently approximate continuity and, when studied statistically, display the normal variability curve, which will be explained in the next chapter. Combinations are heritable differences between the individuals of a race or between the offspring of a pair of parents caused by segregation and recombination of hereditary units. They also frequently display the normal variability curve. Mutations are heritable differences between parents and offspring which do not depend upon segregation and recombination. These three categories, as Baur has shown, are not to be recognized and separated merely according to appearances. The cause of any individual differences can usually be established only by careful breeding experiments; but by this means the separation of the three categories is always possible as the boundaries between them are quitesharp. Modi- fications are somatic effects of environmental differences and should not be confused with germinal changes which. are sometimes induced by natural or artificial means and which result in the production of muta- tions. Within this first category must be included all place-effects in plants and somatic environmental effects in animals. Modifications 16 GENETICS IN RELATION TO AGRICULTURE comprise a large portion of what are commonly spoken of as fluctuations due to environment, but all cases of fluctuating variation are not modifica- tions inasmuch as variations due to combinations frequently display the normal variability curve also. Modifications are not heritable. The second category, variation by combination of hereditary units is often confused with modification, as already stated, because of the fact that variations caused by segregation and recombination when studied statis- tically often display the normal variability curve. This is especially apt to be the case in quantitative characters (those of size or weight) and segregation and recombination may be the cause of gradations in color intensity. In autogamous (self-fertilized) organisms hybridization between races is sufficiently rare to be negligible ‘n this connection, 7.e., in such species the fluctuating variations are caused by the environment. But in allogamous organisms (those in which two individuals are neces- sary to accomplish sexual reproduction) fluctuating variations may be caused either by the environment, by segregation and recombination of factors, or by both causes acting together. We shall take up the third category, mutations, in a later chapter. For the present it is sufficient to remember that mutations are no doubt the least frequent of the three classes, that easily distinguishable mutations are comparatively rare, but that there may also occur true mutations of such moderate extent, as compared with the population, that their existence would only be detected by breeding tests, since their progeny would exhibit a different range of fluctuation from that of the population. 2. Nature. We may next enquire into the nature of variation as it affects the organism. Upon this basis we may distinguish between four classes: morphological, physiological, psychological and ecological. Morphological variations are differences in size and form (Fig. 1). In general morphological variations have more significance for the biolo- gist than for the agriculturist. However in many products of the farm, size and conformation are of decided importance. Two sub-classes under morphological variations are meristic and homeotic variations. Meristic variations are differences in number of repeated parts such as the petals in a flower, the leaflets in a compound leaf or number of phalanges. Homeotic variations are differences caused by the replace- ment of one part by another, as the production of an antenna in place of an eye in an insect. Physiological variations are differences in quality and performance. Examples of qualitative variations are difference in degree of hardness of bone, flavor of meat, richness of milk, difference in normal color (Fig. 2), resistance to drouth, frost or alkali. Variations in performance constitute the most important group for the producer. Differences in performance are sometimes, though not necessarily, associated with VARIATION 17 certain details of structure. For example, note the prominent milk veins on the udder of Tilly Alcartra as shown in Fig. 231. Psychological variations are differences in mental traits. That mental and nervous conditions have very definite effects upon physical con- Fie. 1—Morphological variation in number, form and size of leaflets in the blue elderberry, Sambucus glauca. ditions is well known, but the problem of distinguishing between pur- poseful action and automatic response, between manifestations of reason and manifestations of instinct, is set for the students of animal behavior. While variations in mental characteristics have an important place in eugenics and merit the attention of livestock breeders, yet the inheritance 9 18 GENETICS IN RELATION TO AGRICULTURE of pyschological characters must be more extensively investigated before the subject can be considered with profit in a fundamental study of genetics. Ecological variations are those differences between individuals that result from their fixed relation to the environment. These differences are especially noticeable in plants and are known as place-effects or place variations. This category includes some of the phenomena of Fig. 2.—Substantive variation due to chlorophyll reduction in certain areas of the leaves of Eleagnus pungens. variation in crop yield and hence is of immediate significance to agricul- ture. Fig. 3 illustrates place-effects in a common weed. 3. According to differences between them there are two general classes of variations: first, the slight differences in every character which are always to beobserved even among individuals of identical heredity ; second, unusual, striking differences commonly known as sports. The first class are called normal, indefinite fluctuating or continuous variations and the second, abnormal, definite and discontinuous variations. It should be noted, however, that all discontinuous variations are not necessarily definite or even distinguishable. Continuous variations when examined statistically are found to conform to the law of statistical regularity. VARIATION 19 That is, if measured and plotted the graph will approximate the normal curve of variability (Chapter III). Continuous variations are either heritable (combinations) or non-heritable (modifications) and, as was stated above, the only certain method of determining the class in which a Fic. 3.—Place-effects in common mustard (Brassica campestris) due to soil differences (herbarium specimens). given case may fall is the breeding test. Discontinuous variations are essentially discrete differences whether they be large or small. They are also either heritable or non-heritable and there is no correlation between size and heritability. Thus the extremely large and small 20 GENETICS IN RELATION TO AGRICULTURE mustard plants shown in Fig. 3 considered by themselves are discontinu- ous variations, but they are almost certainly due entirely to environ- mental differences and seed from the small plant if grown under optimum conditions would produce plants of normal size. On the other hand, it is known that many minute differences in organisms are heritable. 4. According to direction variations are classed as orthogenetic and fortuitous. Orthogenetic variations are those differences found in indi- viduals related by descent which form progressive series tending in a definite direction. Many remarkable illustrations are found among paleontological records of the evolution of animals. Occasional examples are found among short-lived or vegetatively propagated species. The remarkable series of variations of the Boston fern described in Chapter XVI is a good example. Fortuitows variations are chance differences occurring in all directions. 5. According to cause variations are either ectogenetic, differences arising from conditions acting upon the organism from without; or autogenetic, differences resulting from strictly internal relations between germ and soma. Variation and Development.—Somatogenesis, in sexually produced multicellular organisms, includes the entire history of cellular multipli- cation and specialization from the first cleavage of the fertilized (or parthenogenetic) egg to the completion of all adult features. From the standpoint of individual development it includes gametogenesis, for the production of sexual glands and of secondary sexual characters are merely phases of differentiation. Cell growth and cell function depend directly upon the activity of the living substance within the cell. The nature and degree of this activity depends upon two sets of determining causes acting simultaneously. First, there are the specific hereditary determiners or genetic factors, which react with the other elements of the protoplasm and, under favorable circumstances, condition normal development. Second, there are all the conditions external to the cell which stimulate or inhibit protoplasmic activity. These ‘developmental stimuli”’ are chem- ical and physical changes wrought by energy from without the organism or caused by its own physiological activities. Chemical stimuli are exerted mainly through the medium of the circulating liquid which surrounds each living cell. Normally this fluid contains the elements essential for maintenance of life as well as various waste products. It may also bear toxic substances that suppress or inhibit the cel! functions and in higher animals it contains the secretions of the ductless, sexual and other glands that profoundly affect development. Physical stimuli are exerted chiefly from without and upon the organism as a whole. They include changes in temperature, light and density of medium, the effects of electric and radiant energy, force of gravity, etc. Obviously, so many VARIATION ait interrelated causes acting simultaneously, éach being independently capable of inducing a change in the end product, may cause an infinite number of differences in substance and in degree of development. Variation and Environment.—External stimuli affect the develop- ment of characters in three ways: (1) they modify the development of inherited characters; (2) they actually condition the production of charac- ters whose hereditary determiners are present in the germ-plasm; (3) they may cause germinal variations which result in the appearance of new heritable characters. The following are illustrations of these effects with reference to particular environmental factors. Fig. 4.—Sedum spectabile. The three shoots (taken from a single plant) were planted in small pots on March 12, 1904, and placed in different greenhouses: J, in blue light; JJ, in mixed white light; JJJ, in red light. Photographed on Sept. 30, 1914. (After Klebs.) 1. Environment Modifies Development of Inherited Characters.— (a) Light and Function.—Klebs reports the results of growing the Showy Sedum (Sedum spectab:le) in white, red and blue light. The diverse effects of the three kinds of light are clearly shown in Fig. 4. Although the visible differences between the three plants were very pronounced the experiment was carried much further. During 1905-06 observations were made on the numbers of stamens in the flowers of plants similarly propagated under white, red and blue light and under various conditions of temperature, moisture, andfood. About 20,000 flowers were examined 22 GENETICS IN RELATION TO AGRICULTURE and six distinct types were found, according to the variation in number of stamens. These had the following average numbers of stamens: (1) 9.68, (2) 8.45, (8) 6.54, (4) 5.05, (5) 9.47, (6) 7.33. Finally, Klebs subjected similar plants from white, red and blue light to chemical analysis in order to secure further evidence of the physiological effects of light of different wave lengths. Table I shows the composition of the leaves in three plants like those shown in Fig. 4. They were in their respective greenhouses from June 6 to September 7. The percent- ages shown are per 100 g. of dry substance. In compar- ing these percentages it should be remembered that TasLe I.—CuHemicaL CoMPosiTION OF THREE Puants oF Sedum Spectabile GROWN IN Waits, Rep AND BuveE Lieut. Substance White Red Blue the plant in white light pro- ie | duced 1324 flower buds and Se aeeu atten eel foretiele) «euler 13.20 1133. 240) 18.60 the plant in red light 405, UGAT s/s 656 aralevs wn ove 11.04 15.40 2.40 : : : Calcium malate..... 22.29 18.02 18.10 while the plant in blue light Free nitrogen.......| 0.16 0.33 0.59 produced none. ‘This ex- StALGhigte ner. ye) manos 3.66 1.20 plains the higher percentage Crude protein...... 5.33 Geils 7.64 = of ash, nitrogen and protein in the last. On the other hand, the amounts of starch and sugar found in the plant from white light are decidedly larger than the one from blue light. In short, according to Klebs, in comparison with normal white light, the production of organic substances, such as starch and sugar, is diminished under the influence of blue light as microchemical and macrochemical tests distinctly show. In consequence of this di- minished assimilation of carbon dioxide the rosettes become purely vegetative. In red light the carbon assimilation is greater than in blue light but less than in white. These experiments prove that the transfor- mation of a plant ‘‘ripe to flower’ into a vegetative one is possible on the one hand by an increase of temperature and of inorganic salts and on the other hand by a decrease of carbon assimilation. (b) Temperature and Pigmentation.—Many experiments in the rearing” of moths and butterflies under controlled temperatures prove that degree of pigmentation is profoundly influenced by the temperature at which the pupz are kept. Some species exhibit seasonal dimorphism in the wild state. By taking pupe of the common European form of the swallowtail butterfly, Papilio machaon, and subjecting them to a tempera- ture of 37° to 38°C., Standfuss obtained the characteristic summer form which occurs in Palestine. Again it has been shown by temperature experiments that many variations found among insects in nature are merely aberrations due to temperature effects. Goldschmidt by arti- ficially controlled temperatures has produced a series of forms of the VARIATION 23 diurnal peacock butterfly, Vanessa io, which show the fading out of the “peacock eye” mark (see Fig. 5). (c) Food and Structure.-—Woltereck was able to prove that the form Fic. 5.—The diurnal peacock-butterfly (Vanessa io), above, and below, forms produced by subjecting the pups to unusual temperatures. (After Goldschmidt.) (hence the structure) of the fresh water crustacean, Hyalodaphnia, varies directly with the food supply. These minute animals produce many generations during a season and the successive generations from the same Saeinee 15-1X 30-VII Fig. 6.—Morphological cycle of head-height and shell-length in Hyalodaphnia. Roman numerals designate months. (After Woltereck, from Goldschmidt.) water exhibit a morphological cycle, the earlier and later generations having shorter heads and the generations produced from midsummer to autumn having longer ones. Fig. 6 is a reproduction of Woltereck’s diagram of the morphological cycle in Hyalodaphnia showing variation 24 GENETICS IN RELATION TO AGRICULTURE in head and shell length as found on successive dates from June 3 to January 3. By raising these animals under constant temperature condi- tions and varying the strength of the nutrient solution, Woltereck proved My > > Fig. 7.—Schematic curves of head-height in Hyalodaphnia as grown in media of three different food values. (After Woltereck from Goldschmidt.) that the relative size of body parts varied with the food. In Fig. 7 the . » . percentages of head height to_shell length are plotted as abscissas and the numbers of individuals as ordinates. Animals from three strengths a b Cc d e Fie. 8.—a, Typical wild pigeon, Scardafella inca; b, the form dialeucos; ec, braziliensis; d, ridgwayi; e, S. inca after three moultings in a moist atmosphere. (After Beebe from Goldschmidt.) of nutrient media were measured, the curves of those from the weaker, the medium and the richer media being shown at m1, Me. and mz respectively. VARIATION 25 (d) Moisture and Plumage Color.—Beebe experimented with the pigeon, Scardafella inca. This species, as found in North and Central America, is very constant in color of plumage, but in the moist tropics the following darker colored forms occur: in Honduras, dialeucos; in Venezuela, ridgway?; in Brazil, braziliensis; and these differ in the amount of pigment in the feathers. By subjecting birds of the northern type to an especially moist atmosphere, Beebe caused them to be so influenced that with each new moulting, whether natural or artificially induced, they always de- veloped darker feathers. Thus a wild bird having pigment in 25.9 per cent. of its area, would have after the second moulting under experimental conditions, 38 per cent. and after the third, 41.6 per cent. Thus during the experiment the typical form assumed the appearance of the three other forms and finally developed plumage markings which have never been seen in nature. Fig. 8 shows the type form, inca, the three geographical variants, and the darkest artificially produced form. Fig. 9.—Plants of Scilla, started alike but the pot on the right was kept in a dark room. (From Ganong.) 2. Environment Conditions Development of Inherited Characters.— (a) Light and Metabolism.—In a general sense light conditions life in all normally green plants. It certainly conditions normal development in such plants. Potatoes sprouted in a dark room develop no chlorophyll in the stems and the rudimentary leaves are abortive. In many bulbous plants, however, the influence of moisture and heat are sufficient to induce leaf growth and even development of the inflorescence, but it is all done at the expense of the food stored up in the bulb as is shown in Fig. 9. 26 GENETICS IN RELATION TO AGRICULTURE (b) Temperature and Flower Color.—Baur reports an experiment with a red variety of the Chinese primrose, Primula sinensis rubra. If plants of this variety are raised by the usual method until about one week before time to bloom and then some of the plants are put in a warm room under partial shade (temperature from 30° to 35°C.) and the re- mainder in a cool house (temperature from 15° to 20°C.), when they bloom those in the warm temperature have pure white flowers while those in the cool temperature have the normal red color of the variety. Moreover, if plants are brought from the warm into the cool temperature the flowers which develop later on will be normal red in color. Thus it cannot be said that this primula inherits either red or white flowers. What it really inherits is ability to react in certain ways under the influence of temperature. (c) Food and Fertility—It is well known that the kind of food supplied to the larve of bees determines whether the females shall be fertile (queens) or infertile (workers), (Fig. 10). The striking differences in Frag. 10.—The three forms of bees: a, drone; 6, queen; c, worker. The twolatter develop as the result of difference in the food supplied to the larve. (After Harrison.) structure and instincts of the two classes of females are all conditioned by the food provided for the larve. Each larva inherited the capacity to react in either way according to the stimulus received. (d) Moisture and Structure—Morgan reports a variety of the pomace fly, Drosophila ampelophila, with abnormal abdomen (Fig. 60); “the normal black bands of the abdomen are broken and irregular or even entirely absent. In flies reared on moist food the abnormality is extreme; but even in the same culture the flies that continue to hatch become less and less abnormal as the culture becomes more dry and the food scarce, until finally the flies that emerge later cannot be told from normal flies. If the culture is kept well fed (and moist) the change does not occur but if the flies are reared on dry food they are normal from the beginning.”’ 3. Environment May Cause New Heritable Characters.—As yet there is a dearth of evidence which can be accepted as scientific proof that external stimuli actually cause germinal variations. At the same time there is an abundance of data which falls into the class of cireum- stantial evidence in favor of such a doctrine. Moreover, there are a few VARIATION 27 cases in which new heritable characters have been artificially produced _by carefully controlled external stimuli. Hence some germinal variations are apparently caused by known environmental conditions and we are justified in recognizing this third category of developmental differences due to environmental effects. Considerable evidence of permanent changes in both morphological and physiological characters has been secured from experiments with the culture of bacteria and yeast, in unusual culture media, in the presence of toxic solutions, or under extreme temperature conditions. The sig- nificant results of four investigators who worked independently, Hansen, Barber, Wolf and Jordan, have been reviewed and discussed in regard to their bearing on genetic theory by Cole and Wright. The four investi- gators mentioned above used refined methods and three of them began by isolating a single organism from whose progeny they obtained dis- A) O Fie. 11.—0O, Portion of leaf of parental Scrophularia showing branching lateral vein; D, branching vein replaced by two laterals in leaf of a seedling grown from seed produced by an injected ovary. Also note difference in size and margin of leaves. (After Mac Dougal.) tinct strains or biotypes which remained constant for hundreds of test- tube ‘‘generations.’’ It must be admitted that in most of these cases no specific influence can be named as the direct cause of the inherited variation. But there is no longer any doubt that permanent, discon- tinuous variations do occur spontaneously in these lowest organisms, and it is highly probable that certain incidental, external forces play an im- portant part in inducing such variations. Direct experimental attack upon the germ cells themselves has been made with plants by a number of investigators, notably by Mac- Dougal, who injected very dilute solutions of potassium iodide, zine sulphate, sugar, etc., directly into the ovaries of various plants imme- diately before fertilization. Consequently somatic changes have been produced which were inherited throughout several generations. By means of check experiments and observations it was found that these germinal variations were not caused by the wounding of the ovary and it is thought that they must have been induced in some way by the presence of the foreign chemical solution in the ovary. Fig. 11 shows a mor- phological change which appeared in a seedling of an unnamed species 28 GENETICS IN RELATION TO AGRICULTURE of Scrophularia as a result of ovarial injection. Having tested this species sufficiently to determine that it was a simple one, MacDougal | treated several ovaries with potassium iodide, one part in 40,000 and se- cured seed. No other species of Scrophularia grew near the cultures. From this seed only three plants were raised. “One formed a shoot fairly equivalent to the normal, finally producing flowers in which the anthocyans were of a noticeably deep hue. The two remaining plant- lets were characterized by a succulent aspect of the leaves and by a lighter and yellow color of the leaves and stems. The flowers on one of the derivatives, as they may be called, were so completely lacking in color as to be a cream-white, this derivative being designated as albida, while the other showed some marginal color and a rusty tinge and was designated as rufida . . . . . Seeds of the original two derivatives were sowed in the greenhouse. But one plant of albida, the most extreme departure, survived, while four of rufida were secured.” MacDougal compared these second generation seedlings with seedlings from the original stock of the species, noting differences in size and margin of leaves, length of petioles and number of marginal glands. He found that the differences shown by the first generation appeared again in the second generation. Striking as these results appear it must be admitted that it would be difficult, on account of the small numbers of individuals differing from the parent type, to prove satisfactorily to the biome- trician that they were not mutations which would have occurred regard- less of the ovarial treatment. What appear to be germinal variations in the tomato have been induced by intensive feeding. T. H. White tested the effect of dried blood, dis- solved phosphate rock, sulphate of potash and iron filings all in excessive amounts, and (with the exception of the iron) in various combinations, on the Red Cherry tomato. The lack of data on control cultures of seedlings from the same parent as the experimental cul tures makes it impossible to compare the actual amount of permanent variation produced. T. H. White states that measurements “show that the plants of the sixth gen- eration grown under the influence of the dried blood are one-third larger in height, length of leaf and size of fruit, than those of the second’’; (see Fig. 12). The author concludes that ‘‘there can be no doubt . . . that, in the case of Red Cherry treated with dried blood, there is permanent variation to the third generation.” If these results are corroborated by more carefully planned and rigidly controlled experiments they will add the weight of scientific proof of a principle in plant breeding long since recognized on empirical grounds, to wit, that the introduction of wild plants into intensive cultivation induces variation. Furthermore, it suggests a possible means for rapid permanent improvement of wild forms with which hybridization may be impracticable. VARIATION 29 In experiments on lower animals, e.g., the protozoa, the same difficulty is met with as has been encountered in bacteria and yeasts, in that it is manifestly impossible to distinguish between somatic and germinal variations. Moreover, in most of these experiments, as with most of those on higher animals, the necessary conditions for rigid scientific analysis have been lacking. Either the same strain as was subjected to artificial conditions was not grown for comparison under natural condi- tions or else the conditions themselves were not sufficiently well con- Fie. 12.—Leaf and cluster of fruit of Red Cherry tomato of the second generation (right) ; same of the sixth generation (left) of continuous treatment with excessive amount of dried blood. (Photo by T. H. White.) trolled to permit of certain analysis. It is interesting to note that the pomace fly, Drosophila ampelophila, which has produced more mutations so far as we know than any other organism, was subjected to the effects of ether on a grand scale and under controlled conditions by Morgan, but that not a single mutation was observed to result from this treat- ment. However, mutations have subsequently appeared again and again in cultures of ‘ wild”’ flies not only of this species but also of other species of Drosophila. Thus it appears that germinal variations fre- quently occur independently of external stimuli. It also seems that a tendency to produce mutations may be inherited. 30 GENETICS IN RELATION TO AGRICULTURE With animals the best known experiments on the artificial production of germinal variations are those of Tower who worked with the Colorado potato beetle, Leptinotarsa decemlineata, and related species. Like other arthropods these beetles are more directly under the influence of tempera- ture changes at least than are warm-blooded animals. Tower first de- termined the period in ontogeny when external stimuli will affect the germ cells. He found that in Leptinotarsa the germ cells do not become susceptible to external stimuli until after the time in ontogeny when the color pattern of the individuals subjected to the stimuli can be influenced. He found that eggs were most susceptible just before and during maturation and this observation is in agreement with those of Fischer, Standfuss, Weismann and others who have conducted similar Fic. 13.—A, Leptinotarsa decemlineata and three mutants; B, tortuosa; C, pallida; D, defectopunctata. (After Tower.) investigations. Tower concluded that certain individuals from the germ cells of a stimulated parent ‘‘show intense heritable variations, whereas those not acted upon do not show these changes.’’ Most of the inherited variations involve changes in the pigmentation of the body parts. In certain cases there was an actual change in the color pattern (see Fig. 13). It is to these results that Tower attaches the greatest significance inasmuch as most similar experiments have not succeeded in causing pattern changes. In spite of the elaborateness of Tower’s methods con- siderable skepticism exists regarding the validity of his conclusions, and this has not been lessened by the non-appearance of confirmatory data. In a recent paper he reports the production of very striking germinal modifications in L. decemlineata as a result of subjecting a morphologically homogeneous race to an extreme change in environment. However, it is still a question whether the material used may not be heterogeneous as regards the germinal factors that condition certain ‘physiological characters. Stockard’s investigations on the effect of alcohol on the progeny of guinea pigs have shown that the germ cells as well as the somatic tissues VARIATION dl of the alcoholized animals are injured. This case is considered further in Chapter XXX. On the whole it must be admitted that the experimental induction of heritable variations is still largely an unworked field. The complex conditions to be considered and consequent obstacles to be overcome are appreciated by no one more fully than by those who have attempted such investigations. For, as Tower has said: “It is evident that the problem of germinal change is one of difficulty, and involves more of indirect than of direct methods of investigation. There is little reason to expect that present biochemical methods can give a solution, but they may give valuable suggestions for further indirect investigation. It seems not improbable, however, that this problem like so many others in biology, must await the solution of the larger question of what life is before it will be possible to express in exact terms the nature of germinal changes. Our present status, with several methods of production and much knowledge of the behavior of induced germinal changes available, is a basis from which great advances in knowledge and in operation may reasonably be expected.” CHAPTER III THE STATISTICAL STUDY OF VARIATION In the present chapter we shall consider the application of purely statistical methods in the analysis of biological phenomena especially the phenomena of variation. The treatment given here does not pretend to be exhaustive or rigorous, but it presents the commonly used method of recognized biometricians, from several of whom valuable suggestions have been received. We shall have occasion to refer to the utilization of statistics in the study of heredity by the “biometrical school,’’ but the application of statistical methods in the analysis of specific Bemeile problems will be deferred until later chapters. Causes of Fluctuations.—Continuous variations, or the slight differ- ences normally found in organisms, are generally referred to as fluctuating variations or fluctuations. It is frequently assumed that “fluctuating variability’ is due entirely to differences in environment. But, as was stated in the preceding chapter, either the modifications in development due to environment, or individual differences which are caused by seg- regation and recombination of genetic factors, may display the normal curve of variation when examined statistically. Hence fluctuations may be due to either of two causes and before conclusions may be drawn from the study of frequency distributions and statistical constants, the causes of the variations studied must be clearly differentiated. The only way to accomplish this is to make one set of conditions or the other as uniform as possible. If the object be to examine modifications, only pedigree material should be used and, on the other hand, if variations due to recombinations are to be considered, the environmental conditions must be as uniform as possible or else due account must be taken of exist- ing irregularities. Certain technical requisites to the biometrical method will be mentioned later. This difference in the nature of fluctuating variations according to their cause is of such fundamental importance that it should be clearly understood at the outset. Law of Statistical Regularity——This fundamental principle, which is also known as the law of probability or law of chance, may be most simply introduced by means of an illustration. Suppose two persons, blindfolded, were each to pick about 500 beans from a bag containing a million beans of any standard variety. The average weights of the beans picked out by the two persons would be almost identical even though the 32 THE STATISTICAL STUDY OF VARIATION 33 individual beans varied considerably in size. Furthermore if one were to obtain the average weight of the whole million, it would not differ, essentially, from the average weights of the smaller groups. The prin- ciple involved here may be stated in various ways. Weld expresses it 3 os : = i ; 3 3 : 3 : é 3 13 & eos, 900 (eT Fig. 14.—Frequency distribution of 500 Broad Beans arranged in classes according to width. as follows: “If a number of different events are equally possible as regards constant conditions (that is, if there is no persistent reason why one should occur rather than another), and all are repeatedly given opportunity to occur, they will in the long run occur with equal average frequency.” While this is a satisfactory general statement of the law of probability, the same 3 34 GENETICS IN RELATION TO AGRICULTURE principle has been expressed by King in terms, which fit well the imaginary case under discussion, as follows: ‘‘A moderately large number of items chosen at random from among a very large group are almost sure, on the average, to have the characteristics of the large group.”’ It must not be inferred that any partial group of individuals no matter how large, will give exactly the same results as would be obtained by the use of the entire mass. But the averages will be close and the probability of in- accuracy due to accidental errér diminishes as the numbers increase because individual errors tend in the long run to counteract each other. Law of Deviations from the Average.—lIf, now, one lot of 500 beans be measured to the nearest millimeter and then arranged in columns from left to right according to width beginning with the narrowest beans, the result will be very similar to Fig. 14. It will be noticed first that the middle classes contain the most beans while the classes on the extreme left and right are very small. The black vertical line M indicates the average width or mean of all the beans and the column with the most beans in it represents the most frequent width of beans and is called the mode. The columns nearest the average value on either side contain the most beans and the further the column is from the average the fewer the beans in it. Thus we see that the majority of the beans show only slight deviations from the average while a few exhibit wide deviations therefrom. Statistical study has proved that it is a general rule with fluctuations that individuals showing extreme deviations in either direction for a given character are comparatively rare, while individuals exhibiting smaller deviations, and hence occupying a position inter- mediate between the two extremes are especially frequent. In other words, continuous variations usually appear in frequencies such that, if we represent these frequencies graphically, we obtain a polygon which resembles more or less the normal variability curve. Such a polygon is produced by connecting the ends of the columns in Fig. 14. The Normal Curve and its Significance.—The normal variability curve is a theoretical curve which pictures the result of expanding the binomial (a+ 6)" when a =b=1 and n is assumed to be indefinitely great. By the binomial theorem + ePaper K SOND Or Wb Saas ano pee WNNRRE OWe S200 Re O1 © = + — WE Se Te al SS + ae on THE STATISTICAL STUDY OF VARIATION 35 From Fig. 15 it is evident that as n becomes larger the straight lines of the polygon more closely approximate the normal curve. The normal curve is perfectly symmetrical because it represents the distribution of an indefinitely large number of items and it assumes all causes to be of equal strength or value. It is assumed that certain biological frequency polygons should simulate this curve for these reasons. It is probable that the environment of any organism is made up of a large number of factors each of which may vary around a mean independ- \ : Fie. 15.—Polygons representing expansion of the binomials (a + b)® and (a + b)!° as compared with the normal curve. ently of the others. Now if a frequency polygon is to be made regarding a character of a population composed of individuals alike in zygotic constitution, such as a field of potatoes of the same variety, the differences found in the development of any character are due wholly to these en- vironmental factors. Hence it is likely that the mean of the distribution is made up of observations on individuals upon which an equal number of favorable and unfavorable forces have acted and the deviates are those upon which a greater or less number of favorable or unfavorable forces have acted. But in sexually reproduced allogamous species the in- dividuals are not alike in zygotic constitution. Moreover, the causes affecting a given character may have an unequal mass effect according to ecological conditions. Either of these factors may cause a high degree of asymmetry in a polygon of variation. Graphs in which the mode is rather far removed from the mean are called skew polygons or curves, 36 GENETICS IN RELATION TO AGRICULTURE The significance of the normal curve as an index of variation is based on the conception that the area within the curve represents an indefinite number of individuals and that the constants of the curve indicate the distribution of these individuals with respect to a given character. If in any curve (Fig. 16) the perpendicular erected at M divides the area of the curve into two equal parts, this line is the median and the point M represents the average or mean of all the values from which the curve is constructed. The perfect symmetry of the normal curve causes the | median to coincide with the mean and the mode; but in actual cases ‘Gie oe 1 aa Fig. 16.—A normal curve divided Fig. 17.—A normal curve of exactly the into quartiles by the perpendiculars same area as the curve in Fig. 16, but with flat- erected at M,Q:,Q3. ter slope and correspondingly greater breadth. The distribution pictured by this curve pre- sents a greater range of variation than in Fig. 16 as is indicated also by the value of Q. ‘these three values will not coincide because the curve will not be sym- metrical. If a perpendicular be erected in either half of the curve at such a distance from M that it divides the area enclosed by the median, the base and half of the curve into two equal parts, the distance of such a perpendicular, Q; or Q; from M is the quartile, g. Then in the normal curve g = MQ: = MQ;. Now the slope of the curve is an index of the amount of variability. The steeper the slope supposing the area (the number of individuals) to remain the same, the nearer to the median will be the position of the quartile and hence the position of the quartile is also an index of variability (Fig. 17). Since curves constructed from actual distributions are never symmetrical, in practice the index taken is nw. However, the measure of variation in common use is the standard deviation, a which in the normal curve represents a distance from the median COU TO 4 0. ae Requirements of Biometrical Study.—The data for statistical analysis are obtained by counting, by measurement, or by arbitrary graduation of continuous differences like degree of pigmentation. In order that such THE STATISTICAL STUDY OF VARIATION 37 data may be compared with other similar data some sort of precise de- scription must be prepared. Graphical representation is good as far as it goes; a frequency polygon conveys to the eye more knowledge than one would have without it. But in order to secure the best description of organisms with reference to specified characters, some mathematical expression for the degree of variation must be deduced from the data. This process involves two essential steps: (1) To obtain a measure of type for the group under observation; (2) to derive an expression for the amount of variation from the type. There are three measures of type, the median, the mode and the mean, and we have seen that in the tlieo- retical normal curve they always coincide. In actual cases they may be widely separated. There are three commonly used measures of variation from type, viz., the range, or the distance from one extreme to the other, the quartile, and the standard deviation. These expressions and others derived from them are known as the constants of the normal curve. In practical work the mean, or arithmetical average, is commonly used as the measure of type and the standard deviation as the absolute measure of variation. A relative index of degree of variation is derived by divid- _ ing the standard deviation by the mean; this is called the coefficient of variation. ‘These three constants are the indispensable mathematical tools of the biometrician. Some knowledge of their calculation and significance is necessary for an intelligent appreciation of considerable important biological and agricultural literature. Before proceeding to discuss these constants it will be necessary to present a few technical terms and methods. Some Biometrical Terms.—An Individual may be either an entire organism or only a single part as the leaves of a tree or seeds of aplant. Individuals are also called variables. A Sample is any group of individuals which are measured or com- pared with a standard. Samples may be divided into sub-samples for definite reasons; for example, corn from different parts of a field. The Population is the general mass or entire group from which sam- ples are taken. A Variate is a single magnitude-determination of a character. A Class includes variates of the same or nearly the same magnitude. The class range gives the limits between which the variates of any class fall. : Requisites to Reliability—a1. Biological Soundness.—Three great sources of untrustworthiness in biological work are: (a) Differences due to age; different ages must not be lumped to- gether without taking account of it. (b) Heterogeneity due to conditions of environment; for example, corn from a field in which the soil is definitely heterogeneous. 38 GENETICS IN RELATION TO AGRICULTURE (c) Mixing of distinct varieties, which must never be permitted if known in advance. 2. Definition of Population The population must be so defined that conclusions reached will not be wrongly applied to other populations. 3. Typical Sample-—The sample must be really typical of the species, variety, breed, strain or race. Otherwise the results are not applicable to large populations. Also the sample must be large enough so that con- clusions may be drawn fairly. 4. Sufficient Accuracy—Measurements must be made with a suf- ficient degree of accuracy. It might be thought that a coarse or slightly variable scale of measurement would satisfy since the measurements are to be grouped, but the relative size of the groups is a most critical matter so that the size of scale and degree of accuracy are very important. Yet perfect accuracy is hardly obtainable. Relative not absolute ac- curacy is the desideratum. As stated by King: For every statistical problem there should be determined in advance a definite standard of accuracy for each item and every endeavor should be made to bring each recorded instance up to this standard. Grouping Variates into Classes.—When the individuals have all been measured the collection of variates must be grouped. The following rules should be observed: 1. Classes should be of equivalent ranges. One must not neglect the extremely large and small variates. Employ a uniform scale through- out all classes. 2. Arrange the classes so there will be no possibility of mistake by the reader. Calculations may be based on the centers of the class intervals or on the upper limits of the intervals for certain purposes. The Frequency Table.—A list of the classes formed by the grouped variates together with the number of individuals in each class is called a frequency table. For example, Love and Leighty give the data on total yield of. plant in grams of Sixty Day oats for the year 1910 at Ithaca, N. Y.- These are presented in the form of a frequency table in Table II. Taste I].—FREQUENCY TABLE SHOWING VARIATIONS IN YIELD OF SIXTY Day Oats. (After Love and Leighty) (Class value = V (Frequency = f) _ (Class value = V) (Frequency = fs) Grams of oats Number of plants Grams of oats Number of plants 0-1 = 0.5 3 5-6 = 5.5 42 1-2, = 1.5 50 6-7 = 6.5 7 2-3 = 2.5 106 7-8 =7.5 2 3-4 = 3.5 109 8-9 = 8.5 1 4-5 = 4:5 80 THE STATISTICAL STUDY OF VARIATION 39 Frequency Graphs.—To graphically represent the data in the above frequency table, indicate a base line on a sheet of coérdinate paper, mark off equidistant points for class intervals and midway between the limits of each class indicate the class center. In this case the class intervals are 0-1, 1—2, 2-3, etc., and the class centers are 0.5, 1.5, 2.5,etc. Counting each space above the base line as one or more individuals (according 109 106 M =3.458 + .045 o =1.323+ .032 C =38.259 + 1.037 0.5 cS 2.5 3.5 4.5 5.5 6.5 7S 8.5 Fig. 18.—Frequency polygon showing variation in total yield per plant in grams of Sixty Day oats at Ithaca, N. Y., 1910. (Data from Love and Leighty.) to the modal number and size of sheet), either construct rectangles of proper altitude to represent the frequency of each class or merely indicate the points of intersection of the frequencis plotted as abscissas and the class centers as ordinates. The latter method is usually employed since it is more rapid and the polygon more truly represents the distribution of classes in a sample showing continuous variation in the character in question. This method is illustrated in Fig. 18. The area within the polygon represents the actual data for which purpose a curve should never be employed. 40) GENETICS IN RELATION TO AGRICULTURE The Mean, Calculation and Significance.—To compute the mean of a series of single variates summate the variates and divide by the number of variates. Thus if « = any variate and n = the number of variates, then 2 (a the mean, M, = i. where > indicates summation. For a series of groups of variates (classes), first multiply each class value (V) by the number of variates in the class or frequency (f) then summate and divide by n. Thus fie BEV nN In the calculation of this and other constants it is important that the work ‘be indicated in a systematic manner. The form as indicated in Table III is usually preferred. | f f.V The data are the same as in the ; frequency table (Table IT). Tasue II].—To Comrute THE MEAN Torat YIELD OF PLANT IN GRAMS = ~ | | 0.5 | 3 1.5 A valuable short method of Hed a es computing the mean consists in the 2.5 106 265.0 : 35 109 381 5 use of an assumed mean which 45 80 360.0 removes the necessity of multiply- 5 42 231.0 ing the class values by their fre- 6.5 7 45.5 quencies and hence greatly reduces ae : ava the actual labor in dealing with xy gah largenumbers. For the same data nm = 400 > = 1383.0 theshort method is shown in Table IV. The rule is as follows: To eas M = 409 = 3-458. compute the mean of a series of classes of variates, write the fre- quency of each class in a column on the right of the class values, then the deviation of each class from an assumed mean, and lastly the product of each deviation by its corresponding frequency. Summate the devia- tion-by-frequency products, divide by n, and add algebraically the correc- tion factor thus obtained to the assumed mean (in this case, 3.5). —17 AO — 0.0425 M = 3.5 + (— 0.0425) = 3.458. Thus for the computation of the mean by the short method we have the formula Correction factor = w = Si(V-O)] n The mean is the best measure of type in organisms because it takes into account all the individuals measured. For this reason the sum of the THE STATISTICAL STUDY OF VARIATION 41 variations from the true mean of all the items in the table equals zero. Unlike the mode it is affected by every item in the group so that its location can never be due to a single class; moreover it gives weight to extreme deviations. The measure of type used and its value should always be indicated . Taste LV.—To Compute THE MEAN ToTat YIELD oF PLANTIN GRAMS. Let G = assumed mean = 3.5 on agraph. For a precise 4 f V-G f (V-@) description of the variation " within a group it is neces- 0.5 3 es ar ee sary to have something 1.5 50 <2 — £00 i more than a measure of the te ss He akg ae type. Knowing the arith- ee 80 1 80 metical average is not 5.5 42 Perea esc sufficient to permit com- 6.5 if 3 21 parison of the variation in 7.5 2 4 8 different opulations. eal P ee popula 5 tie, tee LS There is needed some mea- 4 22200 eee, sure of variability. ! The Standard Deviation, Calculation and Significance.—Examination of the original records of weighings of the total yield of the 400 oat plants would reveal a certain amount of variation in the yield of each plant from the mean yield, 3.458 g. The plants were grouped into classes in com- puting the mean yield and they can be treated similarly in calculating the average amount of variation from the mean yield for the whole sample. It may be noted that the simplest measure of the absolute variation within the sample is the average deviation, which is simply cal- culated by summating the products of the deviation of each class from the true mean multiplied by its frequency and dividing this sum by n. The standard deviation is universally preferred as an absolute measure of variability. The standard deviation differs from the average deviation in one important feature, viz., that in calculating the standard deviation each individual variation from the mean is squared. This gives addi- tional weight to the extreme variations which is especially desirable in biometrical work. In calculating the standard deviation (Table V) the regular procedure is as follows: Write the minus and plus deviation (d) of each class from the mean, square each deviation (d?), multiply each d? by the frequency (f), summate the products, divide by n and extract the square root. This is expressed by the formula 42 GENETICS IN RELATION TO AGRICULTURE TaBLE V.—To CompPutsE THE STANDARD DEVIATION IN MEAN ToTAL YIELD OF PLANT IN Grams (Complete Process Including Calculation of the Mean) V f fV o a fa? 0.5 3 5 —2.958 8.750 26.250 1.5 50 75.0 —1.958 3.854 191.700 2.5 106 265.0 —0.958 0.918 97.308 33503) 109 381.5 0.042 0.002 OBZ 4.5 80 360.0 1.042 1.086 86.880 505 42 PBL AO) 2.042 4.170 175.140 6.5 0 Ann) 3.042 9.254 64.778 Wo 2 15.0 4.042 16.338 32.676 8.5 1 8.5 5.042 25.442 25.442 n= 400] X(f.V) = 1383 X(f.d2) = 700.392 M =3.458 700.392 bey | ee IGE | 4 400 * Taste VI.—To Compute THE STANDARD DEVIATION BY THE SHORT METHOD Let assumed mean = G = 3.55 V—G=d' V | f d’ fa’ fa” fd +12 0.5 3 —3 — 9 Dill 12 1:5 50 —2 — 100 , 200 50 Diy 106 —]1 —106 —215 106 0 Seth) 109 0 0) (0) 109 4.5 80 1 80 80 320 5.5 42 2, 84 168 378 6.5 ai 3 PAI 63 aly 7.5 By 4 8 32 50 8.5 1 5 5 198 25 36 n = 400 [a1 701 _ 1067 400 0.0425 400 1.7525 iM Glee w? = 0.0018 | = 35 — (010495) 1.7595’ 0.0018 a n07 = 3.458 = 4/17507.= 1888 Check: =(f) + 22(f.d') + Z(f.d’2) = 1067 = [f(d’ + 1) The short method for computing the standard deviation is based upon the same principle as the short method for the mean. The rule, therefore, is as follows: Select some number approximating the mean (@); write the minus and plus deviation therefrom (d’); multiply each deviation THE STATISTICAL STUDY OF VARIATION 43 by the corresponding frequency (f.d’); divide the difference between the minus and plus products by n to obtain correction factor (w); then multiply each f.d’ by d’ to get f.d’?; summate the last products and divide by n; from the quotient subtract w? and then extract the square root. The illustration, Table VI, is based upon the same data as the preceding. It will be noted that this value of the standard deviation is slightly larger than the value as computed by the regular method. The short method is the more accurate because of the elimination of many decimal places. In additon to the complete short method there is shown in the last column on the right a very useful method of checking the computa- tion. Each f(d’ + 1)? is calculated algebraically. Thus in the first case f = 3 and d’ = —3; substituting we have 3(—3 + 1)? = 12. In the same way (f) + 2>(f.d’) + d(f.d’*) is computed algebraically. Substituting we have 400 + (—34) + 701 = 1067. The standard deviation, being a measure of absolute variation, is exceedingly useful in comparing the variability of one variety with another with respect to the same character, or of the same variety in. different years with respect to a given character, or of one character with another in the same or different species. For example, Love and Leighty in their memoir on “ Variation and Correlation of Oats” give the means and standard deviations for total yield of plant in grams (as well as for eight other characters) for the same pure strain of Sixty Day oats for three years as follows: 1909 — M = 4.082, o = 2.249 1910 — M = 3.458, o = 1.323 1912 — M = 7.962, o = 3.353. The differences between these values are due mainly to differences in climatic conditions during the three years, the year 1910 having been especially dry and hot. Similar differences appear in the means and standard deviations for height of plant, number of culms and number of grains produced. This particular observation leads to no new con- clusion as it is well known that climatic conditions profoundly influence crop yield, but it illustrates the significance of the standard deviation as a measure of variation. Furthermore it is of interest to note that drouth not only reduces plant growth and yield in this variety but the amount of variation as well. In 1910 the amount of absolute variation was only one-third that of 1912. However, the amount of relative variation was not so much affected by drouth as might at first appear. When comparing standard deviations of different varieties or of the same variety under diverse conditions, it should be remembered that the means of the groups under 44 GENETICS IN RELATION TO AGRICULTURE consideration may be widely different in value. It may even happen that the characters to be compared were measured in different units, as inches and grams. Hence it is desirable to have an expression of vari- ability in relation to the mean. Such an expression is the coefficient of variability which is the ratio of the mean to the standard deviation ex- pressed in per cent. The formula for the coefficient of variability is In the case of total yield of plant in grams for Sixty Day oats in 1910 substituting the values which have been calculated we have paCOOgles2saey C = agg = 38.259. The coefficients for the other two years are: 1909, 55.779 and 1912, 42.113. Thus the amount of relative variation in yield was much greater in 1909 than in 1912 and although the standard deviation for 1910 is only a third as large as that for 1912, yet the amount of relative variation is almost as great. A measure of absolute variation is very useful but a relative measure is essential, especially when comparing different kinds of material such as total yield in grams and number of culms or milk production and butter fat production. The Theory of Error.—It has been said that the frequency curves of many biological measurements follow the curve made by plotting the points given by the expanded binomial (a + 6)” wherea = b = 1. The reasons why this should be true are not difficult to see. They depend upon the laws of probability or chance that have been generalized into the theory of error. The chance of an event happening in an infinite number of trials is expressed by a fraction of which the numerator is the number of ways it may occur and the denominator is the total number of ways it may occur or fail to occur, if each is equally likely. Thus in tossing a coin a great number of times, the chances that it falls heads is one-half. Further, the probability that all of a set of independent events will occur on a single occasion in which all of them are in question is the prod uct of the probabilities of each event. Hence, the probability that two coins tossed together will fall heads is 144 x 146 = 4. _ Now suppose four coins are tossed at random; what is the probability that any particular number m of them will be heads and the rest tails? The number m may be 0, 1, 2, 3, and 4, and the probabilities are as follows: 0 head and 4 tails = 1(14)4 1 head and 8 tails := 4(1¢)4 2 heads and 2 tails = 6(14)4 3 heads and 1 tail = 4(14)4 4 heads and 0 tail 1(14)4. THE STATISTICAL STUDY OF VARIATION 45 The coefficients that appear are what they are because precisely those combinations are possible. There is but one combination in which there are no heads, there are four combinations consisting of 1 head and 3 tails, there are six combinations possible of 2 heads and 2 tails, there are four combinations of 3 heads and 1 tail, and again but 1 with no tails. But this is simply the expansion of the binomial (1 + 1)‘. The prob- ability that when n coins are tossed exactly m of them will be heads and the rest tails, therefore, is given by the m+ Ist term of the binomial expansion (1+ 1)". When n is small a symmetrical frequency polygon is obtained somewhat similar to that given by plotting the yields of individual oat plants. When n is very large more points are obtained Aa) 10 sti Oh Ord I OSI, ee Oe ee FO Fic. 19.—A normal curve or curve of error showing the relationship between the quar- tile, i.e., the probable error of a single variate, and the standard deviation. Q = .6745c. In this curve the mode, median and mean are identical. The quartile equals the probable error of a single variate because by definition one-half of the variates lie within its limits; therefore the chances are even that any variate lies within or without it. The proportions of variates within certain areas of the curve are as follows: within M+ Q,50 %ofthe variates, within M+ o, 68.3 % of the variates, within M + 2Q, 82.3 % of the variates, within M + 2c, 95.5 % of the variates, within M + 3Q, 95.7 % of the variates, within M + 3c, 99.7 % of the variates. and the polygon becomes a regular curve, the normal probability curve or curve of error. It is called the ‘‘curve of error” because if a refined set of direct measurements are made and plotted as abscissas, the corre- sponding ordinates represent the frequencies or probabilities that each will occur. The mean is the most probable value and is assumed to be the true value and the deviations from the mean are errors. Positive errors lie to the right and negative errors lie to the left of the mean. Positive and negative errors are equally likely to occur if they are gov- erned by chance only and as the errors increase in magnitude the frequency with which they occur becomes less and less. Let us assume that we have a perfectly normal frequency curve such as that represented in Fig. 19, and we shall be able to demonstrate the meaning of some of the constants that we have learned to calculate for it. This curve represents observations on a large number of individuals and Cc 46 GENETICS IN RELATION TO AGRICULTURE its area represents the general distribution of these individuals. ‘Themean represents the average of the distribution. The standard deviation (plus and minus) represents the ordinates of those points on the curve where the slope changes from convex to concave; it therefore measures the slope of the curve and is a good measure of its variability. Measuring from M to a on each side of the curve, we find that the space enclosed includes 68.3 per cent. of the total number of individuals; within the limits + 2¢ lie 95.5 per cent. of all individuals and within + 3o lie 99.7 per cent. Thus we see that although theoretically the curve never meets the ground line but extends out to infinity, practically all individuals are found within the limits + 3c. Similarly we find that the quartile measures the number of individuals within the limits of the curve that it marks off as follows: M + Q includes 50.0 per cent. of the individuals + 2Q includes 82.3 per cent. of the individuals 3Q includes 95.7 per cent. of the individuals 4Q includes 99.3 per cent. of the individuals + 5Q includes 99.9 per cent. of the individuals. In a normal curve, therefore, the standard deviation and the quartile have a constant relationship such that Q = 0.6745c. From these relationships an idea of the meaning of the term “prob- able error” which is always calculated for any series of observations may be obtained. ‘The probable error tells us what confidence we may place in our work, if the errors are due to chance only and not to avoidable mistakes of method. ‘The probable error is not the ‘“‘most probable error.”’’ The most probable error is 0 and hence is identical with the mean. Probable error is an arbitrary term used to denote the amount that must be added to or subtracted from the observed value to obtain two limiting figures of which it may be said that there is an even chance that the true value lies within or without these limits. The probable error, EH, of a single variate is the quartile,’ since the chances are even that any variate lies within or without the value M + Q; and since 82.3 per cent. of the variates lie within the value M + 2Q, the chances are 4.6 to 1 that the true value of any series of a calculated con- stant is within these limits. Thus the chances that the true value lies within any multiple of # are + E the chances are even + 2E the chances are 4.6 to 1 1The Germans use o as the measure of error. It is known as the error of mean square and is proportionately larger than the probable error as is shown by the fact that 1) Boas ME sore VE M within M + a lie 68.3 per cent. of the variates within M + 2e lie 95.5 per cent. of the variates within M + 86o lie 99.7 per cent. of the variates. THE STATISTICAL STUDY OF VARIATION 47 + 38E the chances are 21 to 1 4H the chances are 142 to 1 5E the chances are 1310 to 1 6# the chances are 19,200 to 1. + + Since biometricians use the standard deviation as the measure of variability, the relation between it and the quartile is utilized in deter- mining all probable errors, even though there is some real error in such a proceeding due to the distribution scarcely ever being exactly normal. The probable error of the mean is found by multiplying the standard deviation by 0.6745 and dividing by the square root of the number of + 0.67450 variates, thus E, = PG ee Hence the reliability of the determi- nation of the mean increases not in proportion to the number of variates but in proportion to the increase of their square roots. The probable errors of the standard deviation and the coefficient of variability are as follows, but it is not necessary here to go into the proof of the determinations. se, = 40.8745 0 a a/2n + 0.6745C CNet Be = an [+ 2(i00) | +0.6745C V/2n approximately if C is not greater than 10 per cent. because, if the group of variates approximates a normal frequency distribution, the value of C will be less than 10 per cent. and the value of the quantity within the brackets will approximate unity and so can be neglected. The significance of probable error is most apparent when comparing statistical results; for example, the standard deviations for average total yield of plant in two or more varieties. Concerning the significance of probable errors Rietz and Smith make the following statement: In the comparison of two statistical results, the difference between the two results compared to its probable error is of great value. In general, we may take the probable error in a difference to be the square root of the sum of the squares of the probable errors of the two results. If the difference does not exceed two or three times the probable error thus obtained, the difference may reasonably be attributed to random sam- pling. If the difference between the two results is as much as five to ten times the probable error, the probability of such differences in random sampling is so small that we are justified in saying that the difference is significant. In fact a difference of ten times its probable error is certainly significant in so far as there is certainty in human affairs. 48 GENETICS IN RELATION TO AGRICULTURE Multimodal Curves.—Thus far we have considered only homogeneous populations, which, when examined statistically, exhibit a certain degree of approximation to the normal curve of variation. Populations fre- quently occur, however, both in nature and among domesticated animals and plants, which are found to be heterogeneous for certain characters at least when subjected to statistical analysis. Graphically shown the data for such a character produces a polygon with more than one mode. In general such data indicate either the permanent influence of different causes affecting only certain individuals or of the same cause acting differently upon a portion of the population. Conditions of bimodal curves are more or less familiar to all. Sexual dimorphism and certain 3 4 5 6 7 8 9 Fig. 20.—Bimodal polygon plotted from data on the earwig. Mean types (x 34) indicated above corresponding modes. Numbers below the base line indicate length of pincersin mm. (From Bateson and Johannsen.) differences in development which are contingent upon sex, such as height of comb in fowls, obviously would result in a “‘notched” graph if the characters were measured and the data plotted. The classic example of dimorphism producing a bimodal curve is found in the length of the pincers of the common earwig (Forficula auricularia) as reported by Bateson. Fig. 20 illustrates the two mean types, each sketch being placed directly above its corresponding modal class in the graph. Other conditions commonly causing mixed populations such as would result in bi- or multimodal curves are the following: 1. Coexistence of groups of different ages; common in birds at certain times of the year. 2. Overlapping of geographical races of the same species—birds, mammals. 3. Coexistence of different races of the same species, for example, many grasses in the wild state and various cultivated grains contain THE STATISTICAL STUDY OF VARIATION 49 several or many different races. Variation in the number and propor- tion of these races in the population would produce wide differences in statistical data. 4. Germinal diversity among the individuals of a population due to hybrid ancestry. Analysis of the causes contributing to bi- and multimodal curves is possible by means of experimental breeding. By testing individuals typical of the various groups indicated by the statistical examination and examining their progeny statistically, the elements composing the original population can be differentiated. It should be noted that the close proximity of two different races sometimes causes contamination of material and consequent skewness of the variation polygon but not necessarily a bimodal curve. Correlation.—All of the biometrical principles considered in the pre- ceding pages pertain to the analysis of variation in a single character. One of the most striking facts of somatogenesis, however, is the physio- logical interdependence of characters in multicellular organisms. From the earliest stages of embryogeny it is possible to trace associations in the development of various characters. This physiological correlation of characters is one of the most important considerations in the modern study of heredity and it is given due attention in Chapter VI. As regards the statistical study of variation the question to be considered is whether the continuous variations in adult somatic characters are in any cases mutually related or interdependent. It is obvious that, if such a condi- tion be found to exist, it will have an important bearing upon plant and animal breeding inasmuch as selection with reference to a single character would in all likelihood have a definite effect upon certain other characters. The most satisfactory method of investigating this matter is to consider the variation in two characters at a time. The Correlation Table.—In preparing a correlation table the observed data are transferred directly from the original record by the simple method of tallying. In order to prepare a correlation table either the indi- viduals to be examined must be labelled with permanent numbers or else the observation on the two characters must be made for each indi- vidual before passing on to the next. In either case the datum on each character is recorded under the individual number. Next a table is ruled off with a number of horizontal rows corresponding to the total number of class values for one of the characters and a number of columns equal to the total number of class values for the other character. It is under- stood that a frequency table for each of the two characters has been previously prepared so that the range of class values is known. In Fig. 21 the material examined consists of the same 400 plants of Sixty Day oats that we have studied with reference to total yield of plant. 4 50 GENETICS IN RELATION TO AGRICULTURE The character of yield is now to be considered in relation to the number of culms per plant. Hence there will be nine rows and seven columns in the correlation table. The class values are indicated in consecutive order beginning usually at the upper left-hand corner. In the present instance, oat plant No. 1 yielded 0.5 g. and had 2 culms, hence this plant etc. etc. Fia. 21.—To illustrate transference of data from original record to correlation table. V. indicates class values for total yield of plant, V,, number of culms per plant. is tallied in the upper left-hand square of the table; plant No. 2 yielded 2.5 g. and had 3 culms, it is tallied in row 3 column 2, and so on throughout the list of 400 plants. Then the tallies in each square are counted, re- corded and transferred to a new table drawn on a smaller scale for future use, the original table being filed as a permanent record. In this way the tables shown in Figs. 22 and 23 were prepared. Interpretation of the Correlation Table.—A correlation table is a record of the frequency distributions for two different characters so arranged as to show the tendency, if any exists, for one character to THE STATISTICAL STUDY OF VARIATION 51 50 134 167 38 10 1 400 Fig. 22.—Correlation table for 400 plants of Sixty Day oats. Total yield of plant in grams, subject. Number of culms per plant, relative. 1910. Coefficient of correla- tion = 0.712 + 0.017. (From Love and Leighty, 1914.) 45-50 50-55 55-60 60-65 65-70 70-75 75-80 80-85 85-90 90-95 50 134 167 38 10 1 400 Fic. 23.—Correlation table for 400 plants of Sixty Day oats. Average height of plant in centimeters, subject. Number of culms per plant, relative. 1910. Coefficient of correlation = 0.042 + 0.034. (From Love and Leighty, 1914.) Vy 7 M, 7 —dy (—dz)(— dy) =d,d, (dz)(— d,) ee d,dy (2) (3) (1) | (4) Vy Ts M, = dy (—dz) (dy) = —d,dy (dz) (dy) — didy , i 7a Fia, 24.—Interpretation of the correlation table. 52 GENETICS IN RELATION TO AGRICULTURE change as the other character changes. The general features of such a table are shown in Fig. 24. The intersection of the two means M, and M,, divides the table into quadrants, which are numbered 1, 2, 3, and 4. The signs of the deviations from the mean of x and y are opposite in the Ist and 3d, while they are the same in the 2d and 4th quadrants. Now the deviation from M of every individual in the table is V, — M, in terms of x and V, — M, in terms of y. As these deviations are to be considered relatively, their products are taken. The products of unlike signs are negative, Ist and 3d, and of like signs, positive, 2d and 4th. After arranging the x and y individuals in arrays, if the larger number fall in the Ist and 3d quadrants, we learn that there is negative correlation or a tendency for one character to diminish as the other Fig. 25.—Interpretation of the correlation table. Shape of ‘“‘swarm” indicates nature * and amount of correlation. increases. If the majority fall in the 2d and 4th quadrants, we conclude that there is positive correlation or a tendency for one character to in- crease as the other increases. If the individuals are uniformly distributed in the four quadrants we find no evidence of interdependence 7.e., zero correlation. These typical distributions are illustrated by the three diagrams in Fig. 25. Comparing the two correlation tables (Figs. 22 and 23) with these diagrams it is evident that the correlation between yield of plant and number of culms is definitely positive, while the nature of correlation (whether positive or negative) between average height of plant and number of culms cannot be inferred from mere observation of the table but that it is very low indeed is clear from the widely scattered distribution. The Coefficient of Correlation.—The interpretation of a correlation table is based upon the fact that the table shows deviations with respect to two characters for each individual or class of individuals. We must remember that the x and y deviations of each class from the mean are multiplied in order to understand how the distribution in the table can indicate plus, minus, or zero correlation between the characters. The product of the two deviations for any individual or class is its product- THE STATISTICAL STUDY OF VARIATION 53 moment, and the summation of all the product-moments divided by n is the average product-moment. expressed by the formula This measure of absolute correlation is ~(d,d Ay. prod.-mom. = 2(dedy), n No. of culms per plant——— z Gz = 4 2S 2B OT AP ag. 8 ida Spate meee : o-11 3 ee: ee 18 —3(—6) i AC Be | | 50] —2} —100) 200 150 —2{-56+(—19)] A Serio ee 2 2% 18 66) 20) 1] |1/106) —1| —106| 106 98 —1[—36+(—66) +1+ 3] 3 —— A iae4 1} 42] 58] 7| 1) |109 0 0 0 7 oi Gy pe ga ad ad g 45 7| 59/11] 3| | 80 1) so) "s0 10 1(—7+11+6) ae ee ee ee ee Ss 546 26)14| 2| | 42 2) 84] 164 36 2(14+4) Perce nas of to | [are 3 6 4| 3| | 7 Snot OL BB 30 3(4+6) i |__| __|—__|__|_|___}__ eH 1] 1 2 4 Si ae 4 4(1) a a ae Md 826 1 1 5 Slee 10 5(2) > a Sh = — — — — — 50, 134/167 38 10 1 400 = au si Ey | | hy == i fe | "~~ 400 400 400 Seepeiy |e| =—.0425 | 1.7425 | wrwy=.0184 d's —2) —1; 0) 1) 2/8) lw%= .0018 | .0018 8716 1.7407 San | Oe os weedeat | f.d’, | —100|—134| 0/38/20|3 400 “lg, = 1.393 | w2, = .1871 a Oe | 421 les 708 Lt. Ler eae =. 400 7 10825 ” (1.323) (.9303) fa’2,| 200- 134) 038409 oASit pth, 0G rd) 8654 ss n .6745(1— .5013) o1= .9303 = + = .0168 aaa 20 rzy=.708+ .017. Fie. 26.—Calculation of coefficient of correlation (r) for total yield of plant in grams and number of culms per plant for Sixty Day oats grown at Ithaca, N. Y., in 1910. But we cannot compare such a number as derived for instance from size of potatoes and starch content with size of beets and sugar content without reducing them to a relative basis. : of the two standard deviations as the best index of variability by.which to divide the average product-moment so as to reduce it to a relative basis. Pearson suggested the product 54 GENETICS IN RELATION TO AGRICULTURE His formula is the one now generally used. If the coefficient of corre- lation equal r, i Then We know the work of computing the standard deviation is lessened by using the short method. Hence this method should be employed in computing the correlation coefficient. On the basis of assumed means from which the deviations are d’, and d’, we have ro EES — wan) CE) n Ox0y from which we read the following rule: To compute the coefficient of correlation, multiply the x and y deviations from G for each class; summate the products and divide by n; from the quotient subtract the product of the two correction factors; divide this difference by the product of the two standard deviations. The application of this formula is based upon the correlation table and is illustrated in the case of total yield of plant in grams and number of culms per plant for Sixty Day oats (Fig. 26). Interpretation of the Coefficient of Correlation.—King gives the following rules for the interpretation of the coefficient of correlation according to its relation to the probable error: 1. If r is less than the probable error, there is no evidence whatever of correlation. 2. If r is more than six times the size of the probable error, the existence of correlation is a practical certainty. 3. In cases where the probable error is relatively small: (a) If r is less than 0.3 the correlation cannot be considered at all marked. (b) If r is above 0.5 there is decided correlation. Applying these rules to the case of variation in yield as related to number of culms we see that r is over 40 times the probable error and under rule 3, the probable error being relatively small, since r = 0.7+, there is very decided correlation. Referring now to relation of number of culms per plant to average height of plant (Fig. 23) we find that r = 0.042 + 0.034 from which it is clear that there is little if any indi- cation of correlation. Biometricians consider the correlation coefficient the most powerful tool the agricultural investigator can have since it is a most excellent measure and is applicable to an immense range of variables. Remember- ing that this constant is an index of the mutual relation that exists between the variations of any two characters, we realize that, if it is THE STATISTICAL STUDY OF VARIATION 55 high, it indicates they are in some way closely related, and, if it amounts to unity it shows that one is the cause of the other or else both are the result of the same causes. The importance of biological soundness as a requisite to reliability in the correlation coefficient must not be over- looked, e.g., see Harris on physical conformation of cows and milk yield. Pearl reminds us that statistical knowledge of correlation is precise only in the same limited sense that similar knowledge of type and deviation from type is precise, viz., as applied to the particular group or groups in the particular instance in time. However, this ability to describe groups in terms of the groups’ own attributes is extremely useful in the practical conduct of scientific experiments. Love and Leighty point out that correlations may be classified as fluctuating and stable, ‘these divisions being based on the behavior of the relationship of the characters concerned when variation occurs in environmental con- ditions, such as exist in different years, or in different locations. As the names indicate, the correlations of the first class may be made to vary considerably by changes in conditions, while those of the second class remain of about the same value or are stable in character.” The prac- tical value of knowledge of correlation is great, especially when one char- acter is easily seen or readily measured and the other is not. Although it is difficult for the mind to grasp the relation which exists between two groups of data on several hundred or thousand individuals, yet when the relation between those data is expressed in a single number as a corre- lation coefficient the difficulty disappears. Regression.—The correlation between parents and offspring when used as a measure of inheritance—Galton thought his measure of somatic resemblance was a measure of inheritance—is usually known as regression. If in an allogamous species parents and offspring be compared with respect to the same character, it is found that the means of the offspring are nearer the mean of the general population of parents than they are to the mid-value of their own parents. In other words, extreme parents do not produce progeny as extreme as themselves. Galton believed this re- gression toward the mean of the general population to be due to “pull”’ of a mediocre back ancestry. He expressed a mathematical law, good under certain conditions, that is directly opposed to biological facts. It expresses the truth, that, if from a general population of mixed heritage in which there is continual crossing, extremes are selected as parents, there will be regression toward the mean of the general population; and continued selection will be necessary therefore to improve the race. But this regression is not due to the pull of a back ancestry; it is due to the fact that individuals whose somatic appearance places them in diverse classes in the frequency distribution are themselves gametically different and will breed differently. Circumstances may come about by which the 56 GENETICS IN RELATION TO AGRICULTURE breeding efficiency is such that the regression will be negative—that is, away from the mean of the general population—as has been proved by Shull, by Emerson and by East in experiments with maize. Further- more, Johannsen, Jennings and others have shown that when the indi- viduals of a population are alike gametically and their differences are due to external conditions only, these differences are not inherited at all and regression is perfect. This means that if a number of beans are alike gametically, selection of extreme sizes will not shift the mean in either direction. More recently biometricians have applied the mathematical principle involved in Galtonian regression in order to express in absolute terms the relative interdependence of characters expressed by correlation coefficients. Used in this sense regression is commonly represented by a straight line which ‘approximates the largest possible number of the subject means in a correlation table. The “regression straight line” is extensively used by some authors as a method of representing the relation between the absolute values of characters. For excellent illustrations consult Harris on body pigmentation and egg production in the domestic fowl. Employment and Value of the Statistical Method.—It: may be as- sumed that biometrical methods are not worth very much if the great biological generalities of the biometricians are misleading. Such an assumption would also be misleading. Statistical methods are a great aid to biologists, but they are only an aid. Trouble has arisen only when biological conclusions have been drawn by mathematicians who ignored certain biological premises. One can only take out of his mathematical mill just what he puts in, but he can take it out in a more comprehendible form. If he has made an accurate biological analysis mathematics are a help; if he has made no biological analysis mathe- matics are a hindrance. Johannsen sums up the whole situation in the sentence: ‘‘We must treat genetic facts with mathematics not as mathe- matics.”’ If the beginner is careful of his biological premises, if he is certain that the material with which he deals is representative—that he has a random sample—if he makes no mathematical deduction unjusti- fied by common sense analysis, he will find that the use of mathematics will remove many a rough place from his road. Biometry will always be an indispensable instrument for the scientific breeder as well as the geneticist. The agronomist and pomologist also have need to resort to statistical methods in order to reach a satisfactory solution of many problems involving variation such as variety testing, seed germination tests, investigation of the value of bud selection, etc. Intelligent em- ployment of the statistical method insures conservative and reliable con- clusions regarding many questions which would otherwise remain in the debatable class. CHAPTER IV THE PHYSICAL BASIS OF MENDELISM Recent investigations in heredity have focused attention upon the chromosome mechanism as the physical basis for the segregation and re- combination of the units of Mendelian inheritance. The importance of cytological phenomena to students of genetics is admirably summed up by E. B.Wilson in the brief statement that “heredity is a consequence of the genetic continuity of cells by division, and the germ cells form the ve- hicle of transmission from one generation to another.” It is appropriate, therefore, to introduce the subject of Mendelism with a formal and brief treatment of the chromosome mechanism and its mode of operation, on the one hand, in the building up of the body from the single cell with which the individual begins its existence, and, on the other hand, in the production of germ cells when the individual reaches the reproductive period of its life cycle. It is the purpose of this chapter merely to deal with the fundamental facts of cytology which are necessary to an under- standing of the connection between cell behavior and Mendelian phe- nomena. Details unessential to such an understanding, however well established cytologically, will not be dealt with in this treatment to the end that the cardinal points may be presented as simply and as clearly as possible. The Chromosomes.— With few exceptions the number of chromosomes in the cells of any individual is constant and characteristic of the species to which the individual belongs. Thus it is characteristic of Drosophila ampelophila that the cells contain eight chromosomes. In maize the cells contain twenty chromosomes, in wheat sixteen, and in man forty- eight, and so on through the entire plant and animal kingdoms. Not only is the number of chromosomes in a particular species con- stant, but the chromosomes themselves possess a definite individuality. Man and tobacco have cells with the same number of chromosomes. It is needless to point out that these chromosomes, however, are quali- tatively very different. Similarly within the species the chromosomes are not all alike; on the contrary, especially in certain forms, they exhibit very marked differences in size and shape. ‘This is peculiarly well illus- trated in Drosophila as shown in Fig. 27. Here it is possible to recog- nize in the female two large pairs of curved chromosomes very similar in size and shape. ‘There is also a very small pair of chromosomes, and 57 58 GENETICS IN RELATION TO AGRICULTURE finally there is a pair of straight ones about two-thirds as long as the large curved chromosomes. In the male the same relations hold except that instead of the pair of straight chromosomes there is a pair consisting of one straight and one somewhat larger hooked chromosome. The significance of this difference in chromosome content in the sexes will be pointed out in a consideration of the inheritance of sex. The pair of straight chromosomes we call the sex or X-chromosomes, the unequal mate of the X-chromosome in the male of this species is called the Y- chromosome. The other chromosomes are called autosomes when it is desired to distinguish them as a class from the sex chromosomes. Drosophila is not unique in possessing chromosomes of such characteristic AIK JIG Fic. 27.—Diagram showing the characteristic pairing, size relations, and shapes of the chromosomes of Drosophila ampelophila. In the male an X-and a Y-chromosome correspond to the X pair of the female. On the basis of X = 100 the length of each long autosome is 159, of each small autosome 12, of the whole Y 112, of the long arm of the Y 71, and of the short arm of the Y 41. (After Bridges.) } shapes and sizes; but more and more as cytology advances it is becoming possible to distinguish individual chromosomes, and _ to recognize them at every cell division. Moreover, the characteristic paired relations which exist among the chromosomes of Drosophila are of general significance. When mature germ cells are formed in an individual, reduction divisions occur by means of which the chromosome number is reduced in the germ cells to one-half that characteristic of the body cells. Thus the germ cells of Drosophila contain four chromosomes as the result of a reduction which takes place in such a manner that each germ cell contains one member of each pair of chromosomes. As a consequence, the germ cell of Drosophila contains two large curved autosomes, representing the two pairs of these chromo- somes, one small autosome, and one X- or one Y-chromosome. The same thing is true for other species of plants and animals—in the reduc- tion divisions the chromosomes are distributed in such a manner that each germ cell receives one member of each pair of chromosomes. It follows from this that in general a definite number of pairs of chromo- somes is characteristic of the body cells of individuals of a given species, THE PHYSICAL BASIS OF MENDELISM 59 and, taking the chromosomes by pairs, one member of each pair is de- rived from one parent and the other from the other parent. From the standpoint of interpretation the chromosomes are aggre- gates of chromatic material which in itself is definitely and highly or- ganized. Our conceptions of this feature of cell organization are based on appearances of the cytological preparations from certain of the more favorable plants and animals and further interpreted by investigations on heredity. Accordingly the entire chromatin content of the nucleus is regarded as made up of a definite number of individual chromatin elements called chromomeres. The number of chromomeresin a cell of any species must run into the thousands. A certain definite group of these elements make up each chromosome, and at every cell division this chro- mosome is reformed from the same group of chromomeres. The individu- ality of the chromosome, therefore, depends on the individuality of the chromatin elements of which it is made up. Not only is each chro- mosome made up of a definite group of chromomeres, but the chromosome is definitely organized with respect to the position or locus occupied by each chromomere. At certain stages in the history of chromosomes, they are simply lines of chromomeres, very much like single strings of beads with each bead corresponding to a chromomere. Now it appears probable that all the chromomeres in a chromosome are different, as though our string of beads had no duplicates throughout its length. Moreover, each chromomere has a definite place or locus in the par- ticular chromosome in which it belongs and it is always found at that particular locus. The chromomeres of this discussion are identified with the factors of Mendelian heredity, and how closely this conception of the nature of chromatin and its complex organization corresponds to the modern view of Mendelian phenomena will be pointed out as each new phase of Mendelism is taken up. Somatic Cell Division.—The phenomena of cell division (called mi- tosis) are represented in outline in Fig. 28, for a species having four chromosomes in its body cell. Bearing in mind the description which has just been given of the organization of the chromatin material we may follow the steps involved in mitosis as they are outlined in this figure. In the “resting” cell at A the chromatin is scattered throughout the nu- cleus in clumps or knots loosely strung together to form an irregular network. As the cell prepares for division the chromatin elements appear in more definite form until at B the chromomeres have arranged themselves in a single row in a long, continuous spireme-thread. This spireme-thread may be considered to be made up of the four chromosomes united end to end with the chromomeres arranged in a linear series. As mitosis progresses to the next stage represented at C, each chromomere of the spireme-thread divides into two so that a double spireme-thread 60 GENETICS IN RELATION TO AGRICULTURE results from the longitudinal splitting of the original thread. Both parts of the thread are quantitatively and qualitatively equal, for, by the splitting of all the chromomeres both of the threads come to possess all of the individual elements of the original spireme thread. Following the splitting of the chromomeres and the formation of a double spireme, the spireme-thread contracts and segments transversely forming four double chromosomes, the number characteristic of the cells of this individual. This is the stage shown at C where also is shown the origin of the spindle, a part of the mechanism in mitosis. The chromosomes now still further contract until they assume their characteristic shapes and sizes. They next appear in an equatorial position on the spindle as shown at D, where the two pairs of double chromosomes, one larger ° and one smaller,,are diagrammed and the nucleolus, the large black body of the previous steps, is shown cast out and degenerating. The daughter chromosomes of each pair now separate from each other until at E they have moved nearly to the opposite poles of the spindle and are beginning to fray,out and seemingly to lose their identity. At this stage actual division of the cell body has begun. Finally at F, the chromosomes have completely lost all appearance of their identity, the chromatin material is distributed throughout the nucleus as in the origi- nal cell shown at A, and the nucleolus has been reformed in each nucleus. Division of the cell-body has resulted in two daughter cells each of which, so far as chromomeres are concerned, contains exactly the same chromatin elements as the original cell. There are many variations in this process particularly in the order of occurrence of the steps, but these variations in nowise modify the essen- tial fact of mitosis which is that the chromatin material of the cell is converted into a thread which splits throughout its entire length into two halves so that the daughter nuclei receive exactly equivalent portions of chromatin material. This precise division of the chromatin is brought about by a division of each chromomere so that not only do the daughter nuclei receive equivalent portions of chromatin but these portions are also equivalent qualitatively to the entire chromatin content of the mother cell. By this method then each of the cells of the body finally comes to possess not only the whole number of chromosomes contrib- uted by the two parents, but also the entire set of chromatin elements which it received from them. The extreme care with which the cell mechanism partitions the chromatin material in each successive cell division is in itself eloquent testimony of the fundamental importance of this material. The Production of Germ Cells.—In the production of germ cells a different set of phenomena occur which result in a reduction of this num- ber of chromosomes to one-half that characteristic of the somatic cells. THE PHYSICAL BASIS OF MENDELISM 61 Preceding the actual reduction division the chromatin material passes through a complex series of steps which may be included under the term synapsis. (This term is sometimes applied in a specific sense to the pairing of homologous chromosomes and sometimes to the contraction of the chromatin threads in the conjugation stage.) The essential steps in the prereduction process are shown in outline in Fig. 29. At A is diagrammed a “‘resting”’ nucleus at the completion of the multiplication divisions in the germ plasm. As a result of the exact type of mitosis which has been outlined above it contains the full number of chromosomes characteristic of the species. The chromatin of the nucleus next becomes E Fie. 28.—Diagram of mitosis in a species having four chromosomes in its cells. A, The “resting’’ cell. B, Formation of the spireme-thread. C, Longitudinal division of the spireme-thread and transverse segmentation into four chromosomes. D, Separation of the daughter chromosomes formed by longitudional splitting of spireme-thread. H, Beginnings of nuclear reconstruction and division of the celi body. F, Cell division complete and daughter nuclei in the ‘‘resting’’ stage. organized into threads of chromomeres which pair as shown at B. In this diagram the paired threads are taken to represent homologous chromo- somes, and the opposite chromomeres in a pair of threads are considered as the homologous chromomeres of the two chromosomes. ‘The paired threads contract and fuse along their entire length giving the figure diagrammed at C in which the two loops represent two pairs of homolo- gous chromosomes in the conjugation stage, the essential step in synap- sis. Following this stage the two contracted loops of chromatin split lengthwise and unravel in somewhat the manner shown in D. ‘These filaments contract again forming the intertwined pairs of chromosomes shown at #, and the nuclear membrane thereupon begins to disappear. Further contraction and the formation of a spindle results in the reduc- 62 GENETICS IN RELATION TO AGRICULTURE tion figure shown at F, the significant feature of which is the fact that each of the daughter nuclei resulting from this division receives only two chromosomes instead of the four which the original cell at A contained. Since the original cell contained one pair of larger and one pair of smaller chromosomes, the daughter cells which are formed each receive one larger and one smaller chromosome. Cytological investigation is not yet in agreement as to the interpre- tation of synapsis especially as to the manner in which the phenomena therein concerned are connected with preceding mitotic divisions. Con- sidering certain cytological investigations and the results of research in Fia. 29.—The reduction division as represented for a species whose diploid number is four. A, ‘‘ Resting”’ nucleus of a primary germ cell. B, Formaton of paired threads of chromomeres. C, Conjugation of homologous chromosomes (synapsis). D, Loosening of thesynaptic knot. H#, Condensation of the chromosomes and disappearance of the nuclear membrane. F, Homologous chromosomes about to pass to opposite poles, thus giving each secondary germ cell a member of each pair and one-half the somatic number. heredity together, it appears that the threads which pair in stage B rep- resent pairs of chromosomes with homologous chromomeres occupying corresponding positions along their entire length. Likewise the contrac- tion stage at C is taken to represent a conjugation of the members of pairs of chromosomes which later again separate. Other cytological evidence indicates that in some forms the conjugation of pairs of homologous chro- mosomes is brought about in another way. However, the essential fact is the same in either case. In the reduction figure the members of each pair of chromosomes are distributed to the opposite poles of the spindle so that the daughter nuclei received only one member of each pair. The significance of synapsis lies in the conjugation of homologous THE PHYSICAL BASIS OF MENDELISM 63 chromosomes. In the mitoses which have preceded this particular divi- sion, the chromosomes were each time conceived to be reformed from the identical group of chromomeres which they contained originally. In synapsis, however, as shown at B there is a certain amount of intertwin- ing of the paired threads and in the unraveling of the chromosomes after the contraction stage there is likewise a twisting of the filaments about each other. The indications are, therefore, that in synapsis there is a possibility of interchange of chromatin material between the members of a pair of homologous chromosomes. In all cases, however, in order to uphold our conception of the definite organization of the chromosomes with respect to the chromomeres which they contain, this interchange of material must involve exactly equivalent portions of the two chromo- } Fig. 30.—Diagram of chromatin interchange between homologous members of a pair of chromosomes. (After Muller.) somes. The chromosomes of the reduction division shown at F may not, therefore, be identical with the four originally present in A, but may represent various combinations of portions of both members of a par- ticular pair of chromosomes. ‘The results of such interchange between members of homologous pairs of chromosomes is shown in Fig. 30. At the left is shown a pair of chromosomes one in outline the other in full black. In the middle the steps in chromatin interchange are diagrammed and finally at the right this interchange results in a pair of chromosomes each of which is made up of parts of both members of the original pair of chromosomes. Various combinations may result depending on the points at which interchange takes place, but in every case the exchange involves corresponding portions of the two chromosomes. Independent Distribution of Chromosomes.—In Fig. 31 are illus- trated diagrammatically the chromosomes of Drosophila, with particular reference to their size and form relations and to their characteristic pairing in the cell. One member of each of these pairs of chromosomes was contributed by the female parent and one member by the male parent. In the reduction divisions these chromosomes are separated so that each germ cell contains one member of each pair of chromosomes. ‘The simplest condition which could obtain is that of independent distribu- 64 GENETICS IN RELATION TO AGRICULTURE tion in each pair of chromosomes such that the particular member of one pair which went to a given pole of the reduction spindle would have no influence on the distribution of the members of any other pair. Such independent distribution of chromosomes appears to be actually the type fil nM VE Fig. 31.—Diagram showing consequences of: independent segregation of chromosomes in Drosophila ampelophila. ae si 2 followed in reduction. As a consequence the germ cells contain various combinations of chromosomes with respect to their original parental deri- vation. In Fig. 31 the types of combinations of maternal and paternal chromosomes and their mode of derivation in Drosophila are shown diagrammatically. Two germ cells, one from the female with the chro- mosomes in outline, and the other from the male with the chromosomes in full black, unite to form the female zygote shown in the middle of the figure. The combinations of maternal and paternal chromosomes which THE PHYSICAL BASIS OF MENDELISM 65 result in the production of germ cells in such an individual are shown diagrammatically in the lower portion of the figure. There are eight different ways in which the chromosomes may be grouped in the reduc- tion figures and on the basis of chance any one of these types is as likely to occur as any other. As a result there are sixteen possible combina- tions of chromosomes in the germ cells with respect to the original derivation of the chromosomes, whether from the female or from the male parent. This of course represents only the total number of pos- sible combinations of entire chromosomes. By exchange of chromatin material between homologous chromosomes resulting in the formation of combination-chromosomes the number of actual combinations is greatly increased. The number of chromosome combinations resulting from independent distribution is that number possible when each pair of chromosomes is considered separately, and every combination has an equal chance of occurrence. With a form having but two pairs of chromosomes there would be only four possible combinations, three pairs would give eight, four pairs sixteen, and in general the number of possible combinations is given by the expression 4" in which n is the number of pairs of chro- mosomes in the individual in question. In tobacco which has 24 pairs of chromosomes the number of possible combinations in the germ cells reaches the enormous total of 16,772,216. This means that in the for- mation of zygotes in a self-fertilized tobacco plant the actual parental combinations, 7.e., combinations identical with those of the germ cells which united to form the individual in question, occur only twice in over sixteen million times, and this proportion is still further lessened when the interchange of chromatin material between homologous chromosomes is taken into account. The condition of independent distribution although simple in itself results in a rapid increase in complexity with the increase in the number of pairs of chromosomes involved. Chromosomes and Sex in Drosophila.—The relation between inherit- ance and the chromosome mechanism is perhaps most simply displayed in the inheritance of sex in those animal forms in which the sexes occur in approximately equal proportions. Thus in Drosophila as indicated in Fig. 32 there are three pairs of autosomes which are alike in both the male and the female. The remaining pair of chromosomes, however, differ, for the female possesses two X-chromosomes whereas in the male a single X-chromosome is paired with a Y-chromosome and these differ- ences are characteristic of all normal males and females of this species. The bearing of these differences on the inheritance of sex is shown diagram- matically in Fig. 32. Beginning with the parents, the diploid number is shown in the circles representing the female and the male. In the female the three pairs of autosomes are outlined and the X-chro- 5 66 GENETICS IN RELATION TO AGRICULTURE mosomes only are drawn in black to indicate that they are the ones pri- marily concerned in the determination of sex. Similarly in the male the three pairs of autosomes which are exactly like those in the female are outlined but the X-chromosome and the Y-chromosome are drawn in | | | REDUCTION | ( DIVISIONS \ . ( Fig. 32.—Diagram to show chromosome relations in the inheritance of sex in Drosophila ampelophila. black. The reduction division in the female results in a separation of the members of each pair of chromosomes, so that every secondary germ cell (or egg) contains two large curved autosomes, a.small autosome, and an X-chromosome. Consequently as far as chromosome content goes the eggs are all exactly alike. In the male, however, the separation of THE PHYSICAL BASIS OF MENDELISM 67 the members of the chromosome pairs results in sperms half of which contain an X-chromosome and half a Y-chromosome in addition to the three autosomes. The reduction division in the male insures an equality in numbers for the two kinds of sperm cells and the chances that either kind of sperm will fertilize an egg-cell are equal. By this arrangement the numerical equality of the sexes is maintained. When, later, the egg cells of the female are fertilized by the sperm cells of the male, as shown in the lower portion of the figure, half of them being fertilized by sperm cells which contain an X-chromosome will give females, and half uniting with sperm cells which contain Y-chromosomes will produce males. The inheritance of sex in Drosophila provides a beautiful illustration of the parallel behavior of the chromosome mechanism and a somatic differ- ence, in this case sex. To recapitulate, the essential phenomena of cell behavior which fur- nish the mechanism for the distribution of hereditary factors are these. 1. Every species is characterized by a definite number of chromosomes, each of which is made up of a definitely organized group of chromomeres. The chromosomes occur in pairs, in each of which one member is derived from each parent. In ordinary somatic mitosis the distribution of chro- matin is such that each daughter cell receives a full complement of chro- mosomes which are equivalent qualitatively to those of the mother cell. 2. In germ cell formation the homologous chromosomes conjugate during synapsis, then separate, and pass into a division figure in which entire homologous chromosomes are opposed to each other. The re- sulting reduction division gives daughter cells with half the number of chromosomes characteristic of the species, the half number being made up of one member of each pair of chromosomes. During synapsis there is an opportunity for the members of a pair of chromosomes to ex- change chromatin material. When such interchange takes place equiva- lent portions of chromosomes both qualitatively and quantitatively are involved. In the reduction division segregation within one pair of chro- mosomes is entirely independent of that of any other pair so that the combinations of parental chromosomes in the germ cells represent all those to be expected on the basis of chance distribution. The student should constantly endeavor to harmonize this conception of the distributing mechanism of the chromatin material with the Men- delian interpretations of hereditary phenomena which will be presented in what follows, to the end that he may obtain a clear and definite idea of the interrelations between the known facts of heredity and cell behavior. CHAPTER V INDEPENDENT MENDELIAN INHERITANCE Essentially Mendelism is an attempt to explain the result of heredity on a rigid, statistical basis. Morgan has stated that the cardinal feature of Mendelism is the fact that when the hybrid forms germ cells the factors segregate from each other without having been contaminated one by the other. The presence or absence of any contamination of factors is still a debatable subject as will be apparent from later discussions, but for all practical purposes the absence of such contamination may be re- garded as an established fact. ‘The other implications of this statement that the two germ-cells which unite to produce the individual each con- tribute an homologous set of hereditary units or factors which determine the characters of the individual and that these units again separate from each other in germ-cell formation are the fundamental conceptions of Mendelism. When the units are considered pair by pair one member of each of which has been derived from each parent, it is clear that the im- portant feature of Mendel’s discovery lies in the segregation of the members of each pair in germ-cell formation. The statistical laws of segregation of characters were first announced by Johann Gregor Mendel, Augustinian monk and later Prilat of the ‘Kénigskloster at Briinn, Austria. In 1865 after 8 years of thorough and painstaking research which is even today a model of genetic inves- tigation, he read the results of his investigations before a meeting of a local scientific society, the Natural History Society of Briinn, and the following year the paper was published in the transactions of this society. Unfortunately, however, the announcement of the work was made at a time when the scientific world was not in a position to appreciate its full significance and was busy with other things. The results, therefore, were neglected until in 1900, the independent investigations by the three botanists, Correns, von Tschermak, and de Vries, led to similar conclusions and to the rediscovery of Mendel’s paper. By that time experimental research had so far advanced that the importance of Mendel’s work was immediately recognized and it was not long before a vast series of investigations had been reported in confirmation of it. The Monohybrid.—The operation of Mendelism is best followed by considering an actual experiment. Mendel crossed tall and dwarf peas and obtained hybrid plants, all of which were tall like the tall parent. 68 INDEPENDENT MENDELIAN INHERITANCE 69 When the progeny of these tall hybrid plants were grown three-fourths of the plants were tall, like the original tall variety, and one-fourth were dwarf, like the original dwarf variety. Although like the tall plant in appearance, therefore, the tall hybrid plants which were produced by crossing a tall and a dwarf plant displayed their hybrid nature in the kind of progeny they produced. To distinguish them from the tall parents which produced only tall plants, they are accordingly called tall hybrids. Continuing this experiment, Mendel found that the dwarf segregants of the second generation bred true, they produced only dwarf plants; but of the tall plants one-third only bred true, and the other two-thirds proved to be tall hybrids, for three-fourths of their progeny were tall plants and one-fourth dwarfs. The progeny of the original tall hybrid plants, therefore, when subjected to this analysis was found to consist of 1 tall : 2 tall hybrid :1 dwarf. The experimental results of. the hybridization of tall and dwarf peas may accordingly be diagrammed as in Fig. 33. Tall =< Dwarf Tall hybrids —_— —. 1 Tall 2 Tall hybrid 1 Dwarf Tall. 1 Tall 2 Tall hybrid 1 Dwarf Dwarf Tall Tall 1 Tall :2 Tall hybrid :1 Dwarf Dwarf Dwarf Fig. 33.—Results of hybridization of tall and dwarf peas. Mendel studied hybrids involving several different pairs of contrasted characters and found that in every case one member of each pair of characters was expressed unchanged in the hybrids, whereas the other member of the pair became latent and its presence could be detected only by growing the progeny of the hybrid. Those characters which were expressed unchanged in the hybrid Mendel termed dominant, the latent characters he called recessive. In the above experiment, for example, tallness was dominant and dwarfness, recessive. Mendel saw that the dominant character, therefore, in these experiments possessed a double significance, that of parental character in which case a uniform progeny of dominants is produced and that of a hybrid character in which case one-fourth of the offspring display the contrasted recessive character. In the above experiment the parental dominants are the tall parents and the hybrid dominants are the tall hybrids. The condition of dominance for a character, therefore, is determined by the fact that in the hybrid that character is expressed to the exclusion of its contrasted character. Dominance is by no means a universal phenomenon, but in Mendel’s 70 GENETICS IN RELATION TO AGRICULTURE experiments it so happened that one member of each of the seven pairs of characters displayed complete dominance. The explanation for the appearance of the recessive character in the second generation and in subsequent generations rests on the fact /Pollen oh Grains TALL HYBRID Fic. 34.— Diagram showing factor history in a cross between tall and dwarf peas. that the contrasted characters are represented in the germ cells by units or factors. The factor for tallness may be represented by 7 and the factor for the contrasted character dwarfness by ft. The relations which exist when plants bearing these dif- ferent factors are crossed are shown in Fig. 34. In the tall race of plants the gametes all bear the factor T, so that since any individual of this race arises from the union of two germ cells its genetic constitution with respect to this particular factor is T'7. Similarly the genetic con- stitution of any plant of the dwarf race is represented by tt and it pro- duces germ cells each of which bears the factor ¢ When tall and dwarf plants are crossed, the hybrid receives a factor T from one germ cell and a factor ¢ from the other, so that the tall hybrids which are produced are of the genetic constitution Tt. In the production of germ cells and in the union of these germ cells to produce the individuals of the second generation is seen the opera- tion of Mendelian principles. The contrasted units T and ¢ separate in the germ cells of the offspring so that a particular germ cell receives only one of these factors, either 7’ or f. Half the germ cells consequently bear the factor 7 and half bear the factor t, and this is true of both pollen grains and ovules. When a tall hybrid plant is self-fertilized, therefore: a T ovule may be fertilized by a T pollen grain producing a TT plant, tall, a T ovule may be fertilized by a ¢ pollen grain producing a T¢ plant, tall hybrid, a t ovule may be fertilized by a T pollen grain producing a Tt plant, tall hybrid, a t ovule may be fertilized by a ¢ pollen grain producing a tt plant, dwarf. INDEPENDENT MENDELIAN INHERITANCE ak Since there is an equal chance for the occurrence of any one of these types of combinations the progeny of a tall hybrid plant are in the ratio 3 tall: 1 dwarf. One-third of the tall plants are of the genetic constitu- tion TT and they consequently will produce only tall plants, whereas the other two-thirds are of the genetic constitution 7't and will display segrega- tion in the ratio 3 tall: 1 dwarf. Thedwarfsare all of the genetic constitu- tion tf, consequently they can produce only dwarf plants. The explana- tion, therefore, satisfies all the requirements laid down by the experimental results. Mendelian Terminology.—As a result of the rapid development of Mendelism during the past few years, a special terminology has grown up which is used by practically all investigators in heredity. For those terms which are in most common use, the following brief statements are intended as interpretations of meanings and significance rather than as mere definitions. The germinal representatives of Mendelian characters are variously termed genes, factors, or determiners, three terms which are used synony- mously in Mendelian literature. A Mendelian factor may be defined as an independently inheritable element of the genotype by the presence of which the development of some particular character in the organism is made possible. The word: gene was introduced by Johannsen to designate an internal condition or element of the hereditary material upon which some morphological or physiological condition of the organism is de- pendent. These definitions do not hold rigidly as is always the case with attempts to define something about which very little is known. Of the terms, the term gene as introduced by Johannsen expressly denies any assumptions as to the ultimate nature of the unit in question. The word determiner on the other hand since it implies a rigid relation between an hereditary unit and its end product, the character, is falling into disre- pute, for very probably many hereditary units are concerned in the pro- _duction of all characters. The term factor, as applied to the units of Mendelian heredity is perhaps more frequently used than any other and is just as free from undesirable implications as to the nature of these units on the one hand or their relation to the characters of the individual on the other hand. It will consequently be used more frequently in this book. Unit characters are those characters of the individual which behave as units in heredity. Thus tallness and dwarfness in peas, since they behave as units in heredity are called unit characters. To behave strictly as units in heredity, character contrasts must depend on single factor contrasts, as for example the character contrast of tall vs. dwarf in peas depends upon a contrast of the factors T and t. The term is a survival of the early days of Mendelism when attention was focussed on the 72 GENETICS IN RELATION TO AGRICULTURE character rather than on the factor as is today the case; and we now have numerous examples of characters which behave as units in certain con- trasts, but in others behave as compound characters. It is, therefore, questionable whether in a rigid sense there are any such things as unit characters, but the term has been much used in Mendelian literature, and the conception to which it gives rise, namely that particular indi- viduals or races possess a number of unit characters which may be dis- sociated from them and recombined in various fashions with the unit characters of related individuals or races, is a useful one and is strictly in accordance with experimental results. Allelomorphs are contrasted factors or characters. More rigidly as applied to characters, an allelomorph is one of a pair of characters which display alternative inheritance, 7.e., inheritance in which one or both of the contrasted characters, although obscured, retain their identity and emerge unchanged from the hybrid. With respect to factors allelo- morphism is a relation between two factors such that they are sepa- rated into sister gametes in germ-cell formation; they never both enter the same gamete. The allelomorphic characters in our sample are characters tallness and dwarfness, and correspondingly the factors T’ and t are allelomorphs. The genotype is the constitution of an organism with respect to the factors of which it is madeup. Rigidly the genotype is the sum total of genes or factors of an individual, but it is customary to speak of the sum total of analyzed factors which are under immediate consideration as the genotype. The genotype of the tall race of peas in the above experiment was TT, of the dwarf race tt. The factor arrangement of an individual is also called its genetic constitution when a particular set of factors are concerned and this term is also employed to designate a particular set of factors carried by a gamete. Genotypes of the constitution TT or tt, or in general those which receive the same factors from both gametes are homozygous, whereas those which receive different factors from the two germ cells or gametes are heterozygous, as for example plants of the genetic constitution Tt. Similarly an individual contains a duplex dose of a given factor when it receives that factor from both parents, or a simplex dose if the factor comes in in only one of the germ cells. The substantives corresponding to the adjectives homozygous and hetero- zygous are homozygote and heterozygote, respectively. The phenotype is the aggregate of the externally obvious characters of an individual or a group of individuals. Thus in the second generation of the above.experiment there were two phenotypes, tall and dwarf, and all the second generation plants belonged to one or the other of these classes. Moreover all members of a phenotype do not necessarily possess the same genetic constitution. In the above example the tall phenotype INDEPENDENT MENDELIAN INHERITANCE 73 included tall plants of the genetic constitution 77 and tall hybrids of the genetic constitution 7t. The distinction between the genotypes of a given phenotype is only possible by further breeding tests. In general a hybrid is best detected by crossing it to the recessive form in which case it will produce half dominants and half recessives, whereas the pure dominant will produce only dominants. Such a cross is known as a back cross or sesqui-hybrid. With respect to history an extracted dominant or recessive is one which has been derived from a hybrid form. The historical fact with re- gard to an extracted form that the parent or other known ancestor did not breed true for the character in question is the only distinguishing feature about it, the factors which it contains are the same as those in the parent races. The parents of a hybrid are generally called the P; generation. The progeny obtained by crossing two distinct races is the first filial genera- tion, conveniently designated the F;. The progeny of the F, are the F2 generation and so on. The above terms are constantly employed in even the most simple cases and their application will soon become clear to the student. Other terms are used in connection with more complex cases, but these will be introduced only when their significance may be made clear from the manner in which they are employed. The Chromosome Interpretation.—The chromosome interpretation _ of a case of monohybridism is very simple. It depends on the assumption for the case of tall vs. dwarf peas that the factor T is a chromomere occupying a definite position in each member of a certain pair of chromosomes of the tall race. The factor ¢ is correspondingly located in exactly the same pair of chromosomes in the dwarf race. Aside from this difference in one pair of chromomeres which occupy identical positions in corresponding pairs of chromosomes the chromosomes of the two races bear exactly the same set of factors. Accordingly, of the seven pairs of chromosomes in the cells of the garden pea, only that pair need be considered which bears the factor 7, or in the dwarf races its allelomorph, the factor t. In the hybrid produced by crossing a tall and a dwarf pea one member of the pair of chromosomes bears the factor 7’, and the other the factor ¢. In the reduction divisions the members of this pair of chromosomes are separated and distributed to different germ cells, consequently half the number of germ cells will receive that mem- ber which bears the factor 7’, and half that member which bears the factor t. Recombination of these gametes gives the offspring in the ratio 3 tall: 1 dwarf, which has been pointed out previously. If in Fig. 34 the rectangles containing the factors 7 and ¢ are taken to represent the mem- bers of this pair of chromosomes instead of entire gametes, this figure 74 GENETICS IN. RELATION TO AGRICULTURE may be used to illustrate the history of this pair of chromosomes in hy- bridization. Since chromosome relations have been determined more definitely in Drosophila, we shall follow out in detail a selected case in this species. We have pointed out previously that aside from the pair of sex-chromosomes, the pairs of chromosomes in both the male and fe- male of Drosophila are alike and bear the same factors. But in the male the Y-chromosome appears to have no effect upon the development of the body characters so that the male depends upon a single X-chromo- some for the development of those characters determined by the factors borne in this chromosome. The Y-chromosome may, therefore, be re- garded as a neutral mate for the X-chromosome in the male. Since the distribution of this pair of chromosomes is unique as we have pointed out in the discussion of the inheritance of sex in Drosophila, the history of factors carried by the X-chromosomes furnishes a beautiful illustration of the parallelism existing between chromosome behavior and factor distribution. The inheritance of white-eye color in Drosophila is a case in point. Wild races of Drosophila have red eyes, but Morgan discovered a few white-eyed male mutants in an inbred strain of ‘‘wild”’ flies, 7.e., flies which were directly descended from wild flies. From this muta- tion it was found possible to establish a white-eyed race of flies which breed true to this new eye character. When a white-eyed male is mated to a red-eyed female the offspring all have red eyes, because red eye in Drosophila is dominant to white. In F, red- and white- eyed flies are produced in the proportion of 3 red:1 white. All the fe- males in this generation are red-eyed, but of the males half have red and half white eyes. When the reciprocal cross is made, 7.e., when a white- eyed female is mated to a red-eyed male the results are different. In the F, of such a mating the female flies have red eyes and the males all have white eyes. When the F, flies are bred together an F2 is obtained half the females of which have red eyes and half white eyes, and likewise among the males half have red eyes and half white eyes. The explanation of this type of inheritance is shown diagrammatically in Figs. 35 and 36. The factor for white eyes is represented by w and it is borne in the X-chromosome. ‘The factor W for red eyes, allelomorphic to w, is carried by the X-chromosome of the red-eyed race of flies at ex- actly the same locus as that of w in the white-eyed race. Since these two factors occupy the same locus in the X-chromosome obviously they can never be contained in the same chromosome. In Fig. 35, the two X-chromosomes of the red-eyed female both contain the factor W for red eyes. In a convenient shorthand system the genetic constitution for such a fly may be designated (WX)(WX), the parenthesis indicating that the factor W is carried by the X-chromosome. Each egg from such a female will contain an X-chromosome with a factor for red eyes—in INDEPENDENT MENDELIAN INHERITANCE 75 our shorthand notation they will all be (WX). On the other hand, the white-eyed male will produce sperm cells half of which have an X- chromosome and half a Y-chromosome. The X-chromosome of the sperm cells carries a factor w for white eyes, but the Y-chromosome does not bear Fig. 35.—Inheritance of white eye color in Drosophila. Red-eyed female mated to white-eyed male. Solid lines indicate history of chromosomes of female; dotted and gray lines, of the male. (Adapted from Morgan.) this factor. In the shorthand notation these two kinds of sperm are rep- resented by (wX) and Y respectively. The Y-chromosome is drawn in black to indicate its unknown constitution with respect to the factors it contains. When an egg-cell (WX) of the red-eyed female is fertilized by a (wX) sperm cell, a female is produced of the genetic constitution 76 GENETICS IN RELATION TO AGRICULTURE (WX)(wX), and it will be red-eyed because red is dominant over white. When such an egg is fertilized by a Y-bearing sperm a male is produced of the genetic constitution (WX) Y and it is red-eyed because the X-chromosome of the egg-cell carries the factor W. In the F; female the reduction divi- Fig. 36.—Inheritance of white eye color in Drosophila. White-eyed female mated with red-eyed male. Dotted lines indicate history of chromosomes of female; solid and gray lines, of the male. (Adapted from Morgan.) sions separate the X-chromosome bearing the factor for red eyes from the X-chromosome bearing the factor for white eyes. Consequently the F, female produces two types of eggs, (WX) and (wX). In the F; male the sperm cells similarly produced will be of two kinds (WX) and Y. As shown in the diagram there are four possible combinations of such INDEPENDENT MENDELIAN INHERITANCE 77 egg and sperm cells in Ff, and these give red-eyed females half of which are homozygous (WX)(WX) and half heterozygous (WX)(wX) and equal numbers of red-eyed and white-eyed males (WX)Y and (wX)Y respectively. In the reciprocal cross, Fig. 36, the white-eyed female contains two X-chromosomes each bearing a factor for white eyes. Her genetic constitution, therefore, is (wX)(wX). All the eggs from such a female will be of the genetic constitution (wX)—they contain an X-chromosome bearing a white-eye factor. When such eggs are fertilized by an X- bearing sperm cell from the male, the female produced will be of the genetic constitution (WX)(wX). It will be red-eyed because of the red- eyed factor carried by the X-chromosome of the sperm. On the other hand, when such an egg is fertilized by a Y-bearing sperm cell, the male thus produced will be of the genetic constitution (wX)Y. It will be white-eyed, because of the white-eye factor in the X-chromosome of the egg-cell. Breeding two such Ff, individuals together will result in the F, distribution shown in the diagram. Females will be produced half of which are of the genetic constitution (WX)(wX) and half (wX)(wX), hence red-eyed and white-eyed respectively; and males half of the genetic constitution (WX)Y and half (wX)Y, hence red-eyed and white-eyed respectively. The peculiar relations exhibited in the inheritance of white-eye color in Drosophila, therefore, admit of a logical chromosome interpretation, if we assume that the factors involved are borne by the X-chromosomes. The type of inheritance which is apparently dependent on factors borne in the sex-chromosomes is called sex-linked inheritance. It will be treated more fully in Chapter XI. Mathematical Adequacy of Mendelism.—Mendelian principles do not apply to isolated phenomena of inheritance alone, but they are of general significance. It is consequently of interest to know how well experimental results agree with theoretical expectations when Mendel- ian analyses are rigidly applied. Particularly is this true of the mathe- matical relations involved, which have often been used to confute the arguments of Mendelian interpretations. We shall accordingly con- sider the results of Mendel’s original investigation from this standpoint, and a few other cases which have been investigated in particularly large progenies and under circumstances which practically eliminate personal bias. Mendel’s investigations with peas included a consideration of seven pairs of contrasted characters as follows: 1. The Difference in Form of Ripe Seeds——These are either round or roundish, the depressions, if any, occur on the surface, and are at most only shallow as in the indent type; or they are irregularly angular and deeply wrinkled. 78 GENETICS IN RELATION TO AGRICULTURE 2. The Difference in Color of the Cotyledons—The cotyledons of the ripe seeds are either pale yellow, bright yellow, or orange-colored, or they possess a more or less intense green tint. 3. The Difference in Color of the Seed Coat.—This is either white, with which character white flowers are constantly correlated; or it is gray, gray-brown, leather brown, with or without violet spotting, in which case the color of the standards is violet, that of the wings purple, and the stem at the base of the leaves is of a reddish tint. 4. The Difference in Form of the Ripe Pods.—These are either simply inflated, not contracted in places; or they are deeply constricted between the seeds and more or less wrinkled. 5. The Difference in Color of the Unripe Pods.—They are either light to dark green, or vividly yellow, in which coloring the stalks, leaf veins, and calyx participate. 6. The Difference in Position of the Flowers.—They are either axillary, that is distributed along the main stem; or they are terminal, that is bunched at the top of the stem and arranged almost in a false umbel; in this case the upper part of the stem is more or less widened in cross- section. 7. The Difference in Length of the Stem.—The length of the stem is very various in some forms; it is, however, a constant character for each, in so far that healthy plants, grown in the same soil, are only sub- ject to unimportant variations. In the experiments a long axis of 6 to 7 feet was always crossed with a short one of 34 to 114 feet. The results of segregation in F, in these seven series of experiments have been summarized from Mendel’s paper in Table VII: Taste VII.—SumMaAry oF MENDEL’S EXPERIMENTS WITH PEAS No. Character contrast No. in F2 | Dominants Recessives Ratio per 4 1 Hormvot seeds eas wee oe 7,324 5,474 1,850 2.99 :1.01 2 | Color of cotyledons..... 8,023 6,022 2,001 3.00 :1.00 3 | Color of seed coats..... 929 705 224 3.04 :0.96 A! Horm sof fOde. feiss. cs). 1,181 882 299 |2.99:1.01 See lNColonOtmodars ..e +. 580 428 152" | 2,95 31-05 6 | Position of flowers...... 858 651 207 3.03 :0.97 7‘) Length of etem... 2... 1,064 787 277° | 2:92:31.08 NOUS Sy oe GOS Oe oe ee 19,959 14,949 | 5,010 (2.996 :1.004 Mendel observed no transitional forms in these experiments so that the ratios he obtained are based entirely on unprejudiced observations. The ratios in no case differ significantly from the ideal 3:1 ratio. Several investigations have furnished confirmatory evidence as to the correctness INDEPENDENT MENDELIAN INHERITANCE 79 of these observations, particularly with respect to that pair of characters concerned with cotyledon color. Johannsen has summarized these results and examined them with reference to their agreement with the conditions imposed by the laws of chance. Table VIII which has been adapted from Johannsen shows that in a sum total of 179,399 counts by seven different investigators the ratio was 3.0035:0.9965. The probable error for this number of observations is + 0.0028 so that the deviation from the ideal ratio is slightly greater than the probable error, but only so great that such a deviation would be expected approximately twice in five times. Another case which has been investigated with very large numbers is that of the contrasted characters starchy and sweet endosperm in yt PRE SM dey Nea . ~~sene MORO ASS EY PEMRERIAZS, Oe: TEES eT rae rant Bite trecas Fig. 37.—Results of crossing starchy and sweet corn: a, Sweet parent; c, starchy parent; b, the Fi showing complete dominance ,of starchiness; d, the F2: showing monohybrid segregation; e, f, g, and h, F3 populations, the last three obtained by planting F2 starchy grains, the sweet ear, e, by planting an F2 sweet grain. (After East and Hayes.) maize. Those varieties of maize which have starchy endosperms have smooth opaque grains whereas the varieties with sweet endosperms have translucent, wrinkled grains. The difference is due to the fact that in ripening there is a progressive formation of starch in starchy races, but in sweet races the starch grains formed are small and angular and there is an actual breaking down of endosperm materials into various kinds of sugars. Correns has shown that starchiness is completely dominant and segregation is sharp and unquestionable aside from very exceptional cases of intergrading. Fig. 37 illustrates very well how sharply segre- gation occurs in hybrid ears. The results of East and Hayes’ extensive investigations of segregation for this pair of characters are summarized in Table IX. In this table families have been entered separately so that 80 GENETICS IN RELATION TO AGRICULTURE the close correspondence to expectation can be seen all along the line. Moreover, these families represent crosses between many different starchy and sweet races, so that the observations are not based on a single hybrid. The deviation of the total ratio from the 3:1 ratio is very slight and falls far within the probable error set by mathematical conditions. TasiEe VIII. —SumMmary oF INVESTIGATIONS ON INHERITANCE OF COTYLEDON COLORIN Pisum (After Johannsen) Investigator Yellow Green Total Ratio per 4 Probable errors Mendel, 1865... 6,022 2,001 8,023 |3.0024:0.9976| +0.0130 Correns, 1900.. 1,394 453 1,847 |3.0189 :0.9811| +0.0272 Tschermak,1900 3,580 1,190 4,770 |3.0021 :0.9979) +0.0169 Hurst, 1904.... 1,310 445 1,755 |2.9858 :1.0142) +0.0279 Bateson, 1905.. 11,902 3,903 15,806 {3.0123 :0.9877;| +0.0093 Lock, 1905... 1,438 514 1,952 |2.9467 :1.0533) +0.0264 Darbishire, 1909; 109,060 36,186 145,246 |3.0035 :0.9965) +0.0030 mhOtalssaccs.4 134,707 44,692 179,399 {3.0035 :0.9965) +0.0028 TasBLe IX.—SpGREGATION OF STaRcHY vs. SWEET ENDOSPERM IN Maize Family No. Starchy Sweet Total Ratio (15 X 54) 1,746 623 2,369 2.948 1.052 (15 X 54) — 2 2,293 728 3,021 3.036 :0.964 (24 X 54) 2,288 801 3,089 2.963 :1.037 (24 X 54) —1 a: 269 1,040 2.965 :1.035 ( 5 X 18) 1,509 492 2,001 3.017 :0.983 (11 X 18) 873 319 1,192 2.930 :1.070 (17 X 54) — 1 328 102 430 3.051 :0.949 (18 X 58) —1 332 102 434 3.060 :0.940 (7 X 54) 872 268 1,140 3.060 :0.940 (8 X 54) 1,505 530 2,035 2.958 :1.042 (8X54) —-1 3,524 1,163 4,687 3.008 :0.992 (8 X 54) —5 2,190 725 2,915 3.005 :0.995 (19 X 7) 783 230 1,013 3.092 :0.908 (19 X 7) -—5 304 109 413 2.944 :1.056 (19 xX 8) 1,813 602 ; 2,415 3.003 :0.997 (60 — 5 X 54) 1,150 379 1,529 3.009 :0.991 (60 — 38 X 54) 799 267 1,066 2.998 :1.002 (60 — 8 X 54) 451 138 589 3.063 :0.937 AUG Call Screen qe. teen. 23,531 7,847 31,378 2.9997 : 1.0003 Probable error +0.0062 Pe INDEPENDENT MENDELIAN INHERITANCE 81 These two cases illustrate very well how closely the results of Men- delian investigations fulfill mathematical requirements, and their signifi- cance cannot be doubted when it is considered how little difficulty is experienced in classifying this particular kind of material. Nevertheless the mathematical requirements are very often not fulfilled on account of the action of external conditions of various kinds. Here as elsewhere the disturbing influence of biological factors must ever be kept in mind in judging the significance of the application of any strict mathematical tests. Dihybridism.—When two pairs of factor differences are involved in a hybrid the same laws apply in segregation and recombination as apply in the monohybrid. The two pairs of factors segregate independently Y re ‘ aay Bissagavas Fie. 38.—Maize ear showing F'2 segregation of grains in the ratio of 3 purple, 1 white. of each other and give character combinations in Ff’, to be expected on the basis of chance factor distribution. In maize there are varieties which have a deep purple aleurone color which gives the entire grain a black appearance. When such varieties are crossed with certain white varie- ties which possess no aleurone color the F; is purple and in F, the grains are in the ratio of 3 purple: 1 white. An ear displaying such F» segrega- tion is shown in Fig. 38. The factors involved in this case are W for pigment production in the aleurone layer and w for no pigment pro- duction in this tissue. The hybrid Ww since it is a monohybrid will, therefore, give in F’, genotypes in the ratio 1 WW :2 Ww:1 ww, which are distributed in two phenotypes in the ratio 3 purple: 1 white. We have shown similarly how starchy corn when crossed with sweet gives a starchy F, and in F,2 3 starchy:1 sweet. Here the factors involved are S for starchiness and s for sweet. A purple sweet corn, therefore, will have ‘the genetic constitution WWss with respect to the above factors, and a white starchy corn, the genetic constitution wwSS. When a purple sweet corn is crossed with a white starchy corn the F, will be purple starchy—it will display the dominant characters of both parents to the exclusion of the recessive characters, white and sweet. From the purple sweet corn the F; receives gametes of the genetic consti- tution Ws and from the white starchy wS. Consequently its genetic 6 82 GENETICS IN RELATION TO AGRICULTURE constitution is WwSs, and it contains two pairs of factor contrasts. Such a hybrid produces gametes representing all possible combinations containing one member of each pair of factors. The gametes, therefore, will be produced in the combinations and proportions 1 WS:1 Ws:1 wS:1 ws. This series of gametes will be represented in both the pollen grains and ovules, so that if each kind of ovule has an equal chance of being fertil- ized by any one of the four kinds of pollen grains the following combina- tions will result. TABLE X.—CoMBINATIONS OF FacTORS AND CHARACTERS RESULTING FROM SELF-FERTILIZATION OF A PURPLE STARCHY CORN OF THE Composition Ww/Ss. WS ovule and WS pollen grain gives WWSS, WS ovule and Ws pollen grain gives WWSs, WS ovule and wS pollen grain gives WwSS, WS ovule and ws _ pollen grain gives WwSs, purple starchy purple starchy purple starchy purple starchy Ws ovule and WS pollen grain gives WW/Ss, purple starchy Ws ovule and Ws pollen grain gives WWss, purple sweet Ws ovule and wS pollen grain gives WwSs, purple starchy Ws ovule and ws_ pollen grain gives Wwss, purple sweet wS ovule and WS pollen grain gives WwSS, purple starchy wS ovule and Ws pollen grain gives WwSs, purple starchy ovule and wS pollen grain gives wwSS, wS ovule and ws pollen grain gives wwSs, WSs WS WS WS When the F». grains are classified according to their phenotype, ovule and WS pollen grain gives WwSs, ovule and Ws pollen grain gives Wwss, ovule and wS pollen grain gives wwSs, ovule and ws pollen grain gives wwss, they are distributed as follows: white starchy white starchy purple starchy purple sweet white starchy white sweet 9 grains with purple aleurone and starchy endosperm 3 grains with purple aleurone and sweet endosperm 3 grains with white aleurone and starchy endosperm 1 grain with white aleurone and sweet endosperm Just as the 3:1 ratio is typical for the monohybrid when one of the contrasted characters is dominant, so the 9:3:3:1 ratio is charac- teristic of dihybrids when one member of each pair of characters is domi- nant. This ratio is clearly derivable from the simple 3:1 ratio, for considering first aleurone color, the segregation is in the ratio 3 purple: 1 INDEPENDENT MENDELIAN INHERITANCE 83 white. When the endosperm segregation into starchy and sweet is taken into account in the same hybrid the segregation will be in the ratio of 3 starchy:1 sweet in each of these classes, for these characters segregate independently of the aleurone color. This gives, therefore, 3 purple (3 starchy:1 sweet):1 white (3 starchy: 1 sweet) which becomes on expansion: 9 purple starchy :3 purple sweet :3 white starchy: 1 white sweet. The correlation of the above facts with chromosome behavior is again very simple. The factors W and w lie in identical positions in one pair of chromosomes and the factors S and s lie in identical positions in a different pair of chromosomes. If the difference between the two varie- ties of maize is only in these factors, then all the other pairs of chro- mosomes in the varieties bear the same set of factors. Accordingly of the ten pairs of chromosomes of maize only those two need be considered Fic. 39.—Chromosome behavior in reduction in F; from a cross between purple sweet and white ‘starchy corn. Factor symbols: w=white, W=purple, s = sweet, S = starchy. which bear the above factors. The relations then are shown diagrammat- ically in Fig. 39. The parents in both cases produce gametes which are all alike. The crossing of these parents produces a zygote in which two pairs of the chromosomes differ in their factor content. One member of one pair bears W and the other w and in the other pair one member bears S and the other s. Two types of F; reduction division are possible and these give four kinds of gametes as shown in the diagram, Since this has occurred in the formation of both ovules and pollen grains, in the self-fertilization of such a plant there are sixteen possible combina- tions of gametes, which distribute themselves in four phenotypes in the ratio 9 purple starchy:3 purple sweet:3 white starchy:1 white sweet. This feature of the case has already been discussed fully and need not be repeated here. The actual agreement of this analysis with experimental results has been shown by several investigators but particularly by East and Hayes. In one case they crossed a white flint corn, Rhode Island White Cap, with a purple sweet corn, Black Mexican. The F; grains were purple starchy and in F, there was sharp segregation for purple and white aleurone and starchy and sweet endosperm. In some cases splashed 84 GENETICS IN RELATION TO AGRICULTURE Taste XI.—Cuass Frequencies AMONG GRAINS From F, AnD F; Ears From THE Cross PurrLe Sweet X Wuire Srarcuy (From East. and Hayes) GIST SAE EL ATOR EB ECE NE NRE TS Dev el ee ee Generation | Ear No. Ee cee ae Witte Total (24 x 54)-1 207 67 67 27 368 207 69 69 23 (24 x 54)-2 170 54 49 19 292 164 55 56 18 (24 X 54)-6 197 65 59 24 345 194 65 65 22 F, (24 X 54)-8 159 41 AL 23 264 148 50 50 16 (24 < 54)-10 166 40 46 19 271 152 51 61 in, (24 X 54)-11 166 55 47 22 290 163 54 54 18 (24 X 54)-13 205 81 59 25 370 208 69 69 23 (245054) =1-2) a6 55 46 13 275 155 52 52 17 (24 X 54)-1-6 171 56 52 19 298 168 56 56 19 BP, (24 X 54)-1-8 | . 180 71 55 19 325 183 61 61 20 (24 X 54)-1-9 79 29 27 7 142 | 80 27 av 9 | F, Motels o<4)-2. 1,270 | 403 368 159 | 2,200 1,287 | 413 413 137 FB; Motels s. 591 211 180 58 | 1,040 685 | 195 196 65 Combined Totalyye salute base oe. 1,861 614 548 217 | 3,240 1,822 | 608 608 202 ene — = INDEPENDENT MENDELIAN INHERITANCE 85 purple grains were obtained, but further breeding tests showed that these were simply heterozygous for purple coloration. A real exception must, however, be made for certain families which showed aleurone color segregation in the ratio 9 purple:7 white. Such results depend on the presence of two color factor differences and they will be explained later. The results in fF. and F3 for these plants of the genetic consti- tution WwSs are tabulated in Table XI. The expected results in each case are given in italics. Throughout the results in this table are substantially in agreement with theoretical requirements. The hypothesis has, however, been sub- jected to the further test of growing /’; populations. Table XII shows the kind of F3 populations which are to be expected when /’, grains from this cross are planted. All these types of populations were secured. The case, therefore, provides an excellent illustration of the way in which a Mendelian experiment is carried out and of the excellent agreement with theory which is given in such experiments. TaBLE XII.—F; Ratios To BE EXPECTED FROM THE DIFFERENT GENOTYPES IN THE Cross WWss X wwSS Ratio in F’3 Phenotype Genotype i i “aes a. Purple Purple White White starchy sweet starchy sweet Purple starchy.....| WWSS All 5 WWSs 35 il Wwss | Fees billie sic cre een 1 Wwss 9 3 3 1 Purple sweet...... WEWISSEME lotto cies xe.nicee All WiiWii “iii Se feb t: Stee Wyn are senses 1 White starchy..... WAVE ats vile ecpdaccslokemme mc aees All UWS eam ool eae cater tosts | craisuaatas shrank 3 1 White sweet....... WAVSS cs Ch cere nMes onus cilities chsrctiarono|(cuer SeMe tens eka} she All In the animal kingdom important work has been done in establishing Mendelian principles by the use of small animals, particularly mice, rats, guinea-pigs and rabbits. Such animals are particularly favorable for investigations in heredity because a large number of generations may be reared in a relatively short space of time. Castle has reported an excellent case of dihybridism in the guinea-pig. Rough coat is dominant to smooth and colored coat to the albino condition. When a smooth black is crossed with rough white the hybrids are rough black. In F2 86 GENETICS IN RELATION TO AGRICULTURE the segregation is in the ratio 9 rough black :3 rough white :3 smooth black :1 smooth white. These relations are shown diagrammatically in Fig. 40. The factor relations are very simple. The genetic constitu- tion of the smooth black race is rrCC and of the rough white race RRcc, where FR is a factor for rough coat and its allelomorph r a factor for smooth coat, and C and ¢ are factors for colored and albino coat respec- ~~ HThalneim. 1 Fig. 40.—Results of crossing smooth white and rough black guinea-pigs. 1 is rough black. F2 is in the ratio 9 rough black :3 rough white :3 smooth black : 1 smooth white. (After Baur.) tively. The F, RrCc is rough black because of the dominance of these two characters over their allelomorphs. When F;, individuals are bred together the F. segregates in accordance with normal dihybrid expecta- tions as shown in the checkerboard in Fig. 41. This experiment shows how easily new races may be established, for in F, two entirely new combinations of characters were obtained, namely, rough black and smooth white. Of the rough black individuals only INDEPENDENT MENDELIAN INHERITANCE 87 one in nine were homozygous for both dominant factors as may be determined from the checkerboard analysis. Consequently for this com- bination of characters it would be necessary to make extensive tests of the individuals in order to determine their genetic constitutions. Mat- ing those which had been determined to be of the genetic constitution RRCC together would insure the production of a race of rough black guinea-pigs which would breed true for these characters. On the other hand, all those which are smooth white are of the genetic constitution rrec; they are therefore homozygous and will produce a uniform progeny when bred together. Dihybridism in Drosophila.—We shall not attempt to follow out the chromosome relations for the guinea-pig hybrid because they are exactly RC Re mC: TC “RC RRCC RRCe RrCC | RrCe rough black rough black rough black rough black Re RRCc RRec RrCe Rrec rough black rough white rough black rough white TC RrCC RrCc rrCC rrCe rough black rough black smooth black smooth black re RrCc Rrec rrCc rrcc rough black rough white smooth black smooth white Fig. 41.—Checkerboard showing F2 segregation in the cross, smooth black X rough white in guinea-pigs. the same as those in maize. In Drosophila, however, the peculiar rela- tions displayed by the sex-chromosomes gives more striking instances of parallelism of chromosome behavior and factor distribution. The inheritance of white-eye color in Drosophila has already been described in detail. Another character, vestigial wings, shows a different type of inheritance. When vestigial-winged flies are crossed with normal long- winged flies the F flies of both sexes are long-winged in the reciprocal crosses, and in F’; segregation is in the ratio of 3 long :1 vestigial in both sexes. The factor for vestigial wings, therefore, must be located in one of the pairs of autosomes. We shall call this factor v and its normal allelo- morph in the long-winged race V. The formula for a vestigial-winged white-eyed female then becomes wv (wX)(wX) and for a long-winged red- eyed male VV(WX)Y. The chromosome relations involved in crossing a vestigial white female and a long red male are shown diagrammatically in Fig. 42. Two pairs of chromosomes are involved, the sex-chromosomes and a pair of 88 GENETICS IN RELATION TO AGRICULTURE autosomes. A vestigial white female produces eggs all of which contain an X-chromosome bearing the factor w and an autosome bearing the fac- tor v. Half the sperms, on the other hand, contain an X-chromosome bearing the factor W and an autosome bearing the factor V, and half con- tain a Y-chromosome and an autosome bearing the factor V. Conse- quently when the sperm cells which contain X-chromosomes fertilize the eggs, Ff, females of the genetic constitution Vv(WX)(wX) will be ee 0 | ie ( 0 ‘y G e Fy Long White 9 | Long White 9 HUCVILOIHVEOE UNE Vestigial ee White 2 Long White d White Long White ¢ | Long White ¢ Long White 2 Fig. 42.—F; gametes and F»2 zygotes from the cross, vestigial white female X long red male. Factor symbols: v = vestigial, V = long, w= white, W = red. produced. Phenotypically they will be long red. When the sperm cells which contain Y-chromosomes fertilize the same kind of eggs the F, males which result will be of the genetic constitution Vo(wX)Y. Phenotypically they will be long white. The reduction divisions in the F, female will result in the production of four kinds of eggs as shown diagrammatically in the figure. They will be of the genetic constitutions: V(WX) v(WX) V(wX) v(wxX). Similarly four kinds of sperm cells will be produced by the F; male and they will be of the genetic constitutions: V(wX) v(wX) VY vY. INDEPENDENT MENDELIAN INHERITANCE 89 The F, population produced by mating two F; individuals will be made up of Ff; gametes as shown in the checkerboard analysis in Fig. 43. When like phenotypes are collected the ratio in each sex is 3 long red:3 long white:1 vestigial red:1 vestigial white. This is very different from the typical 9:3:3:1 ratio of a dihybrid, but it is strictly in agreement with experimental observations and chromosome behavior. The reciprocal cross gives different results and for that reason we V(wX) v(wX ) War vY V(WX)| VV(WX)(wX) Vo(WX) (wX) VV(WX)Y Vo(WX)Y long red 9 long red 9 long red long red V(wX) | VV(wX)(wX) | Vo(wX)(wX) VV(wX)Y Vo(wX)Y long white 9 long white 9 long white @ long white ~ v(WX) Vo(WX) (wX) vo(WX) (wX) Vo(WX)Y vv(WX)Y long red 9 vestigial red 9 long red @ vestigial red 7 v(wx) Vo(wxX)(wX) vv(wX )(wX) Vo(wx) Y vo(wX)Y long white @ vestigial white Q long white @ | vestigial white 7 Fie. 43.—Checkerboard analysis of the F2 obtained by crossing vestigial white 9 with long red o in Drosophila. V(WX) o(WX) VY vY V(WX)| VV(WX)(WX) | Vo(WX)(WX) VV(WX)Y Vo(WX)Y long red 9 long red 2 long red & long red V(wX) | VV(WX)(wX) Vo(WX) (wX) VV (wX)Y Vo(wX)Y long red 9 long red @ long white long white 7 v(WX) | Vo(WX)(WX) vv(WX)(WX) Vuo(WX)Y vy(WX)Y long red 2 vestigial red @ long red #7 vestigial red @ v(wx) Vo(WX)(wX) vv(WX)(wX) Vo(wxX)Y vv(WX)Y long red 9 vestigial red 9 long white “ _- vestigial white ' Fie. 44.—Checkerboard analysis of the F2 obtained by crossing long red 2 with vestigial white o. Reciprocal of cross analyzed in Fig. 43. shall go through it briefly to show that this difference is a necessary con- sequence of the chromosome behavior. When a long red female, genetic formula VV(WX)(WX), is mated to a vestigial white male, vv(wX)Y, the F, individuals are long red females, Vvo(WX)(wX), and long red males, Vo(WX)Y. The F; females then give the same four types of eggs as those of the reciprocal cross, viz., V(WX) V(wX) vo(WX) v(wX). 90 GENETICS IN RELATION TO AGRICULTURE The males, however, produce the following series of sperms: VWX) . 0 WX) VY vy. Mating F, flies of this cross, therefore, results in the Ff, population shown in the checkerboard in Fig. 44. _ When these are collected into like pheno- types the ratio obtained is 9 long red:3 long white:3 vestigial red: 1 vestigial white, but this agreement with the typical dihybrid ratio is only apparent. When the females alone are considered the ratio is 6 long red: 2 vestigial red, and the males are in the ratio 3 long red:3 long white: 1 vestigial red:1 vestigial white. The disturbance in the ratio is occasioned by the unique behavior of the white-eye character which behaves exactly as it did in the simple case which was analyzed previously. The reciprocal ratio, therefore, is additional evidence as to the adequacy of the chromosome theory. The Trihybrid.—The same line of reasoning of course applies to | cases in which three pairs of factors are involved. Such for example is a case which Baur has described in the common garden snapdragon, Antirrhinum majus. In this particular case the factors involved have the following relations. Z—a, factor which conditions the development of the zygomorphic type of blossom which is characteristic of the species. The factor z, its allelomorph, conditions the production of peloric blossoms, 7.e., blossoms which display radial symmetry. The normal form is nearly completely dominant. R—a factor for red color of the blossoms. The allelomorph r gives flowers which are flesh colored. I—a factor for ivory coloration of the blossoms. The allelomorph 7 in this case conditions the production of yellow flowers. FR with J gives flowers red on an ivory background, a magenta type of coloration, whereas R with 7 gives flowers which are red on a yellowish background. It is possible to distinguish these two classes in a mixed population. TaBLeE XII1.—DistTrisuTION oF CLAsseS AMONG THE PROGENY OF AN F, HysBrip SNAPDRAGON OF THE Composition ZzRrli Ratio Phenotypes Observed Expected 27 Zygomorphic, red on ivory.................. 64 52 9 Zygomorphic, red on yellow................ ek! 17 9 Zygomorphic, flesh-colored, on ivory......... 10 17 3 Zygomorphic, flesh-colored, on yellow........ 23 17 9 Peloric, ‘red /Omivory. 045 +5. .< = seem eee: 6 6 3 Peélorie; red; om yellowety .12\5.0 ce. nee 1 6 3 Peloric, flesh-colored, om ivory. ..:..-.2 .222.. 4 6 1 Peloric, flesh-colored, on yellow............. 2 2 INDEPENDENT MENDELIAN INHERITANCE 91 The F, hydrid ZzRrli was the normal flower form and produced blossoms of a magenta coloration, 7.e., red on ivory. From self-fertilized seed of such plants there were produced 124 plants distributed with respect to their phenotypes as shown in Table XIII. Forso small a popu- lation, in number of individuals only about twice that necessary to obtain a triple recessive, the agreement is good. To account for these hybrid results on a chromosome basis it is necessary to assume merely that the three pairs of allelomorphs are borne in different pairs of chromosomes. When germ cells are formed in the F, 0 86 1 <0 ae O08 Oe ot <8 WW (ON Fig. 45.—Diagrammatic representation of reduction divisions in an Ff snapdragon of the genetic constitution ZzRrIi. hybrids the members of the three pairs of chromosomes separate independ- ently so that eight different kinds of gametes are formed. In Fig. 45 are shown diagrammatically the reduction divisions in an F; snapdragon of the genetic constitution ZzRrIv. Since any one of these types of re- duction has a good as chance of occurrence as any other one, the eight kinds of gametes are produced in substantially equal numbers. When such a plant is self-fertilized there are 64 possible combinations as shown in the checkerboard in Fig. 46. In the description of phenotypes in this checkerboard certain differences between homozygous and heterozygous forms have been disregarded. This is true particularly for the Rr individuals as contrasted with the homozygous RR individuals. The GENETICS IN RELATION TO AGRICULTURE | ZRI ZRi ZrI Zri 2RI 2Ri arl 2ri ZRI ZRi ZrI Zri zRI zRi erl 2rt ZRI ZRI ZRI ZRI ZRI ZRI ZRI ZRI ZY g0- zy g0- ZY g0- ZY ZO- zyg0- zy g0- zy g0- zy g0- ZRI E j . A A H F morphic morphic morphic morphic morphic morphic morphic morphic red red red red red red red red ivory ivory ivory ivory ivory ivory ivory ivory ZRI ZRi Zrl Zri zRI 2Ri 2rl 2ri ZRi ZRi ZRi ZRi ZRi ZRi ZRi ZRi : zy Z0- zy Z0- zy Z0- zy go- zy gZo- zy ZO- zy gZo- zy £0- ZRi 5 2 a ; : é fs r morphic morphic morphic morphic morphie morphic morphic morphic red red red red red red red red ivory ' yellow ivory yellow ivory yellow ivory yellow ZRI ZRi Zrl Zri zRI zRi erl ert ZrIl Zrl Zrl Zrt Zrl Zr Zrt Zrl zygo- zy gZo- zZygo- zy Zo- zygo- zygo- Zzygo- zy Zo- ZrI | morphic morphic morphic morphic morphic morphic morphic morphic red red flesh- flesh- red red flesh- flesh- ivory ivory colored colored ivory ivory colored colored ivory ivory ivory ivory ZRI ZRi ZrI Zri zRI zRi zrI 2rt Zri Zri Zri Zri Zri Zri Zri Zri zy g0- zy g0- zy go- zy g0- zy g0o- zy Z0- zy gZ0- zy g0- Zri morphie morphic morphic morphic morphic morphic morphic morphic red red flesh- flesh- red red flesh- flesh- ivory yellow colored colored ivory yellow colored colored ivory yellow ivory yellow ZRI ZRi Zrt Zri zRI zRi erl ert zRI zRI zRI zRI zRI zRI zRI zRI =RI ZY Z0- ZY gZ0- ZY gZ0O- zy go- peloric peloric peloric peloric morphic morphic morphic morphic red red red red red red red red ivory ivory ivory ivory ivory ivory ivory ivory ZRI ZRi ZrI Zri zRI 2Ri zrl zri zRi zRi zRi zRi zRi zRi zRi 2zRi Ri ZYgZO- . zy Z0- zy gZo- zy gZo- peloric peloric peloric peloric morphic morphic morphic morphic red red red red red red red red ivory yellow ivory yellow ivory yellow ivory yellow ZRI ZRi ZrI Zri zkI zRi erl zre art zrl earl erl erl arl zrl zrl zy go- zy gZo- ZY Z0- zy go- peloric peloric peloric peloric zrI_ | morphic morphic morphic morphic red red flesh- flesh- red red flesh- * flesh- ivory ivory colored colored ivory ivory colored colored ivory ivory ivory ivory ZRI ZRi ZrI Zri zRI z2Ri erl 2rt zr 2rt 2ru zrt zru Zr zrt 2rt zy gZo- zy gZo- zy go- zy Z0- peloric peloric peloric peloric ari morphic morphic morphic morphic red red flesh- flesh- red red flesh- flesh- ivory yellow colored colored ivory yellow colored colored ivory yellow ivory yellow Fia. 46.—CHECKERBOARD ANALYSIS OF THE Ff, OBTAINED FROM A Cross INVOLVING THE THREE Parrs oF Factors, Z-z, R-r, I-7 sn gmantimmnoee a — : shoe : a = _- a a A INDEPENDENT MENDELIAN INHERITANCE 93 former are intermediate in coloration between the full-colored RR individuals and the flesh-colored rr individuals, and form a distinct class in themselves. But disregarding these differences the phenotypes in F's are in the following ratio: 27 Plants with zygomorphie, red on ivory flowers. 9 Plants with zygomorphic, red on yellow flowers. 9 Plants with zygomorphic, flesh-colored on ivory flowers. 9 Plants with peloric, red on ivory flowers. 3 Plants with zygomorphice, flesh-colored on yellow flowers. 3 Plants with peloric, red on yellow flowers. 3 Plants with peloric, flesh-colored on ivory flowers. 1 Plant with peloric, flesh-colored on yellow flowers. This 27:9:9:9:3:3:3:1 ratio is typical for trihybrids, if dominance occurs in the three pairs of factors involved. Like the dihybrid ratio it is derivable from the monohybrid 3:1 ratio by subdividing the mem- bers of each term in the 3:1 ratio and then by again subdividing each term of the dihybrid ratio thus obtained in the ratio 3:1. To illustrate with our example, if the contrasted characters zygomorphic and peloric are considered segregation is in the ratio 3 zygomorphic: 1 peloric. When the contrasted characters red against flesh-colored, which also segregate in the simple ratio, are introduced into the analysis the ratio becomes 3 zygomorphic (38 red:1 flesh-colored):1 peloric (8 red: 1 flesh-colored) = 9 zygomorphic red:3 zygomorphic flesh-colored : 3 peloric red:1 peloric flesh-colored. When finally the contrasted characters ivory against yellow are introduced this becomes 9 zygomorphic red (3 ivory: 1 yellow) :3 zygomorphic flesh-colored (3 ivory : 1 yellow): 3 peloric red (3 ivory : 1 yellow):1 peloric flesh-colored (8 ivory : 1 yellow). This gives the final distribution tabulated above. It is important to note that in this phenotypic ratio one member only of each phenotype is homozygous for all its factors and will breed true thereafter. From a Mendelian standpoint an individual is either homo- zygous or heterozygous for a given factor, if it is homozygous it is pure bred with respect to that factor and will breed true thereafter, irrespec- tive of its derivation. It is therefore possible in Ff, to obtain a pure race with respect to any combination of parental factors provided only that a large enough F.2 generation is grown and tested. The increasing diffi- culty of fulfilling these conditions as the number of factors involved increases is obvious, so that from the standpoint of practicability it is usually necessary to work with crosses involving a relatively small num- ber of factor differences. Another fact which is apparent from this trihybrid case is the greater ease with which homozygous individuals may be obtained from the classes which are represented in the smallest numbers. In the above 94 GENETICS IN RELATION TO AGRICULTURE example peloric snapdragons with flowers flesh-colored on yellow are least frequent, but they all breed true on self-fertilization. In the case of the most frequent class, however, the zygomorphic red on ivory only one plant in 27 is homozygous for the three factors involved and con- sequently would breed true. These are features of Mendelism which have a direct practical application. Multi-factor Hybrids.—Very few cases have been worked out which demonstrate conclusively, that more than three pairs of independently Mendelizing characters were involved. Not only are the experimental difficulties in such cases too great, but the scientific interest attached to them is not considerable. From ascientific standpoint accuracy of analy- sis is of chief importance, and accuracy is best attained by working with small numbers of factors at a time. Little and Phillips, however, have conducted an experiment involving four pairs of independently Mendelizing factors in mice. The factors and the character expressions which they produce are listed below: A—factor for agouti coloration. In this type of coloration the pigment is disposed in bands in the hairs giving the peculiar gray or agouti coloration of the wild mouse. The allelomorph a conditions a uniform distribution of pigment in the hairs. B—a factor for black coat color. In this experiment the allelomorph 6} conditions the production of brown coat color. D—a factor for intensity of coat coloration. Animals with the factor D were full colored, whereas those with d were “‘dilute”’ colored. P—a factor for eye coloration. P conditions a dark eye coloration; the allelo- morph p pink-eye color. Taste XIV.—Four-FroLtp Factor SEGREGATION OF Mice (From Little and Phillips) Phenotype Formula | Observed | Expected Observes | HEeoreerl Blacksaeoutiperee Hee ees ABDP 436 373 94.5 81 Pa Cle eesti seen e aBDP 127 124 27.5 27 BROAN AOU 5 sIouecadosueb os oll ANeyye 103 124 22.9 PAT Dihite black agouti-.......26..; | ABdP 130 124 28.2 20 Pink-eyed black agouti........ | ABDp 103 124 22.3 27 BIrOWRA: 2. cgeG tek ete LE abDP 40 41 Sea 9 Dilute brown agouti...........| AbdP 31 41 - 627 9 Wilittesblackns 25 pe). nee bGDae ou 41 8.0 9 Pink-eyed black......... s,s. aBDp 35 41 7.6 9 Pink-eyed brown agouti....... AbDp 38 41 Sez 9 Pink-eyed dilute black agouti...) ABdp 38 41 8.2 9 Dihite brown 22)? jest Be abdP. 11 14 2.4 3 Pink-eyed brown............. abDP 12 14 2.6 3 Pink-eyed dilute brown agouti. .| Abdp 15 14 3.3 3 Pink-eyed dilute black......... aBdp 17 14 3.7 3 Pink-eyed dilute brown........ abdp 7 5 ile) 1 INDEPENDENT MENDELIAN INHERITANCE 95 For the experiment a wild male of the genetic formula AA BBDDPP was mated to a pink-eyed dilute brown female of the genetic constitution aabbddpp. The F\s, AaBbDdPp, displayed all four dominant characters and were like the wild males. The Ff’, segregation is shown in Table XIV. For 1180 individuals only about four times the number of genetic com- binations for a four-factor hybrid, the agreement is satisfactory. As for the chromosome interpretation, it may be made in the same way as in other cases by assuming that four different pairs of chromo- somes bear the factors. Sixteen different kinds of gametes would be formed by such a hybrid, and these together would give the 256 gametic combinations of the F', generation. For higher numbers of pairs of factors the same manner of independ- ent distribution may hold as for those cases which have been outlined in detail. For independent distribution, the chromosome condition is simply that the different pairs of factors be borne in different pairs of chromosomes. Since, however, the total number of factors in any species must greatly exceed the number of pairs of chromosomes, it cannot be ex- pected that every multi-factor hybrid will display independent segregation for all its factors. 'The number of pairs of chromosomes in Drosophila is four, consequently no crosses in this species involving more than four pairs of factors can possibly display independent segregation, if the chromosome theory be valid. Moreover, on the basis of the laws of probability, the chances that any particular case of even fourfold factor hybridization in this species would display independent segregation are extremely slight. Abundant evidence in this species has established the validity of these theoretical deductions. The same principles may logically be extended to other species so that for even as small a number of pairs of factors as five in wheat, which has eight pairs of chromosomes, in peas which have seven, and in corn which has ten, independent segregation would be an exception rather than the rule. Cases where independent segregation does not occur are treated in the next chapter, which deals with linkage. Methods of Dealing with Genetic Data~—Many different methods have been devised for representing the results of Mendelian studies, and as yet the work of any large group of investigators is marked by a consider- able lack of uniformity in this respect. Often the same investigator employs at one time one method of representation, and at another time, another. This is as it should be, for it can hardly be expected in a field of investigations marked by as rapid strides as had been characteristic of genetics in recent years that the ideal method of presentation should have been discovered while only a comparatively small portion of the evidence is at hand. Moreover, the method of presentation is merely a short- hand account of the operation of certain principles; it should not, there- 96 GENETICS IN RELATION TO AGRICULTURE fore, greatly matter what methods are adopted so long as they represent clearly and adequately the operation of these principles. It is necessary for the student to familiarize himself with at least some of the more widely employed methods, remembering that with the basic principles clearly in mind, it should be possible to interpret very easily the methods employed in the presentation of any body of Mendelian data. For ordinary work it is well to have a definite system of interpreting problems. In the following treatment only cases involving the category of independently segregating pairs of factors are considered, but it should be possible to extend the system without great difficulty to other categories as will be pointed out in those sections dealing with such categories. When an individual is heterozygous for one pair of factors two types of gametes are possible. If the factors involved are repre- sented by A and a, the genotypic constitution of such an individual is Aa, and the two types of gametes are A and a. If an in- dividual is heterozygous for two paits of factors, its genotypic formula will be AaBb, and there are then possible four different kinds of gametes, namely AB, Ab,aB and ab. As the number of heterozygous factors increases, e therefore, the number of possible combinations I< of factors increases geometrically so that it is Fra. 47.—Method of writing necessary to adopt a method of writing en ee saeiaee ions of down these possible combinations. The method of dichotomy may be used in such cases, and the diagram, Fig. 47, explains its operation without further comment. With as few as five pairs of factors, therefore, there are possible no ess than thirty-two different kinds of gametes as follows: \ Ad ss] Ad MA / A ABCDE AbCDE aBCDE abCDE ABCDe AbC De aBCDe abC De ABCdE AbCdE aBCdE abCdE ABCde AbCde aBCde abCde ABcDE AbcDE aBcDE abcDE ABcDe AbcDe aBcDe abcDe ABcdE AbcdE aBcdE abcd ABcde Abcde aBcde abcde By following this method consistently it is possible to write out readily the possible combinations of any series of factors. Fortunately or un- INDEPENDENT: MENDELIAN INHERITANCE 97 fortunately the limits of experimental facilities usually preclude the pos- sibility of working with large numbers of factors in any single experi- ment, so that it rarely becomes necessary to handle any large number of combinations. Since F’2 results are commonly obtained by selfing the /; individuals in the case of plants, or interbreeding them in the case of animals, the F, ratios ordinarily represent the product of two like gametic series each consisting of all possible combinations of the different factors involved. There are several methods of obtaining these ratios, each of which has its special advantages. The simplest of these is the algebraic method which merely depends upon the multiplication of the two series together as illustrated in the following general example for two factor differences. Female gametes AB + Ab + aB + ab Male gametes AB-+ Ab+aB-+ ab F, zygotes: AABB + AABb+ AaBB-+ AaBb AABb + AaBb +AAbb + Aabb AaBB + AaBb + aaBB + aaBb AaBb + Aabb + aaBb + aabb F, genotypes: AABB + 2AABb + 2AaBB + 4AaBb + AAbb + 2Aabb + aaBB + 2aaBb + aabb Collecting these F, genotypes into their respective phenotypes we get the following results: 9AB 3Ab 3aB lab 1AABB 1A Abb laaBB laabb 2AABb 2Aabb 2aaBb 2AaBB 4AaBb This tabulation of the genotypes since it shows that the genotypes within a phenotype are in definite ratios to each other immediately sug- gests the method of progression of writing down the F2 phenotypic and genotypic distributions on the basis of the symmetrical relations displayed by them. The ratio of phenotypes in F2 in a cross involving n pairs of factors is conveniently obtained in cases of complete dominance by the expansion of the expression (3 + 1)” or by continuously dividing the terms of a simpler ratio in the ratio 3:1 until the number of pairs of factor differences involved is satisfied. In the following table the phenotypic ratios obtained by the expansion of (3 + 1)” for values of n up to five have been given in condensed form. 7 98 GENETICS IN RELATION TO AGRICULTURE TABLE XV.—PHENOTYPIC RATIOS OBTAINED BY EXPANSION OF THE BINOMIAL 1 (3,+ 1)*. Pairs of # : . Number of mnctors (3 + 1) Phenotypic ratio combinakione 1 (3+1)1 |3 +1 4 2 (3 +1)? | 3?+2341 16 3 (3 + 1)3 | 334 3.324+3341 64 4 (3 + 1)4 | 34+ 4.33 + 63244341 256 5 (3+ 195 | 35 + 5.34 + 10.33 + 10.32+ 5.341 1,024 — jl 5 = = n (3 ab 1)" 3” + n.g"—1 oe n—* + a a a Skil 4n For three pairs of factors, therefore, we interpret this table to mean that the distribution with respect to phenotypes is as follows: 27ABC:9ABc:9AbC:9aBC:3Abc:3aBc:3abC:1abe If it is desired now to write down the numbers of each particular geno- type in a given phenotype, the procedure according to the method of pro- gression is very simple. Let us select the class 27A BC the genotypes of which are as follows: 1AABBCC 4AABbCe 2AABBCc 4AaBBCe 2A A BbCC 4AaBbCC 2AaBBCC 8AabbCe It may be noted that there is one phenotype in each class homozygous for all its factors. In this class starting with this phenotype, we double the number of individuals each time an additional pair of factors becomes heterozygous. Thus there are three genotypes possible with only one heterozygous factor, and there will be two individuals of each of these, there will be three different genotypes having two heterozygous factors, and each of these will be represented by four individuals, and finally there is only one genotype -with three heterozygous factors and it is represented by eight individuals. The method of progression is based upon the symmetrical relations which exist in the phenotypic ratios and in the ratios of genotypes within a phenotype and is a very convenient method for general use. | For illustrative purposes when it is desired to bring out relations graphically the checkerboard method of Punnett is much used. This method has already been employed in this book and needs no extended discussion here. The accompanying general checkerboard for three pairs of factors will illustrate the relations obtaining when this method is employed consistently. As shown in Fig. 48 the gametic series is written INDEPENDENT MENDELIAN INHERITANCE 99 down at one side and at the top of the checkerboard and any square is filled out by writing down the genetic formula of the gamete at the top of its column and the one at the end of its row. If the series are written in the order shown the diagonal 1-3 will pass through all homozygous combinations. The number of these is evidently equal to the number of possible combinations in the gametic series. The diagonal 2-4 passes through all those combinations in which all three pairs of factors 1 ABC ABc AbC Abe aBC aBe abC abe 2 ABC ABe AbC Abc aBC aBc abC abe 3 Fia. 48.—Checkerboard method of analyzing expected results in F2 from a cross involving three pairs of allelomorphs. The ‘‘x” zygotes belong to phenotype ABC. Cf. p. 98. are heterozygous, and the number of these is also equal to the number of kinds of gametes. The bottom row gives the ratio which would be ob- tained by crossing the F'; back to the triple recessive form. The student will be able to determine other relations existing in checkerboards of this type. From a mathematical standpoint, students of genetics are interested in two things, the number and proportion of various types of individuals, 100 GENETICS IN RELATION TO AGRICULTURE and in methods of testing the mathematical validity of segregation ratios. Table XVI gives the mathematical relations which obtain in the pro- duction of gametes in F; individuals and in their union to form the F» zygotes. It is assumed throughout that one factor of each pair of allelomorphs is dominant. TaBLeE XVI.—Proportions EXIsTING IN MENDELIAN EXPERIMENTS INVOLVING Various NumMBERS OF Factor DIFFERENCES Number of pairs of factors 1 2 3 4 5 6 n | Number of different kinds of gametes....| 2 4); 8] 16 32 64 2” Number of combinations of gametes...... 4 | 16 64 )256 |1,024 |4,096 4 Number of homozygotes in Fy........... 2 4) 8] 16 32 64 Qn Number of heterozygotes in F2.......... 2 | 12 | 56 |240 | 992 |4,032 |4" — 2” Number of kinds of genotypes in F2...... 3 | 9D) 27%) Sie, - 24d i 729 3” Number of kinds of homozygous genotypes) 2 4 8 | 16 32 64 2" Number of kinds of heterozygous geno- ! : (IA/] (25 PRP te einer heh a NOI a ae 1 5 | 19} 65] 211] 665 |3" — 2s From this table it is clearly apparent how rapidly Mendelian problems increase in complexity with increases in the number of factor differences. With only five pairs of factors the number of individuals necessary to represent the F, population is 1024 and in order to be sure to have all classes represented it would be necessary to grow four or five times as many individuals as this. In such an experiment there would be 243 different genotypes distributed among thirty-two phenotypes. Natu- rally the chances of selecting a homozygous individual would vary ac- cording to the phenotype within which such selection was made, but the average chance of selecting a homozygote would be one in thirty-two, and the chance of selecting such an individual in the class displaying all five dominant characters would be only one in 243. The practical diffi- culties of dealing with large numbers of factor differences are there- fore of considerable importance in planning and carrying out Mendelian experiments. Methods of testing the ‘‘ goodness of fit’’ of Mendelian ratios depend upon the application of the mathematical theory of probabilities. It . is beyond the province of this book to enter into any exhaustive treat- ment of this subject, the present discussion is intended merely to point out the mathematical requirements which must be fulfilled, if no factors are present which tend to disturb the ratio constantly in a given direc- tion. For most problems of this kind it is sufficiently accurate to con- sider the standard deviation of a Mendelian ratio = ++»/N(K —N where N represents a particular term of a Mendelian ratio and K repre- INDEPENDENT MENDELIAN INHERITANCE 101 sents the sum of all the terms of such aratio. This gives for the probable error E,, of a given term N of a Mendelian ratio the value E, = £ 0.6745 Js. In this formula » = the total number of individuals classified. The actual application of this formula may be illustrated by the use of data from East and Hayes givenin Table XI. The totals in this table give observed frequencies as shown in Table XVII. TaBLE XVII.—Goopness or Fir In A MENDELIAN EXPERIMENT Phenotypes Observed PeeeENee ||, Bret eueet | BE Probability Purple starchy....| 1,861 9.190 9 +0.104 | 1:3.45 Purple sweet...... 614 3.032 3 +0.074 11 White starchy.....| 548 2.706 3 +0.074 1:142.26 . White sweet....... 217 OZ 1 +0.046 Zea otailss. set). 2h 3,240 16.000 16 The results are expected to be in agreement with a 9:3:3:1 ratio; therefore these observed results are first reduced to the form of a ratio . per 16 by dividing each term by 1g of the total number of individuals, or by —.. = 202.5. By this method the observed ratio in Table XVII was calculated. To obtain the probable error for the purple starchy class values are substituted in the above formula as follows: ni 9116 — 9) _ Hy = + 0.67454)° TTS + 0.104 The observed deviation 0.19 is approximately twice the value of the probable error. For practical purposes a deviation less than three or four times the probable error is not considered significant. A deviation of the above magnitude in comparison to the probable error occurs about once in four times. In Table XVII the values of the probable error have been calculated for all four of the terms of this ratio. One term lies considerably within the probable error and its probability has been put down as 1:1. This is not strictly correct but serves the purposes of these calculations. It will be noted that there is one serious deviation, that of the white starchy class which could occur only once in 142 times. This deviation is not serious enough, however, to lead us to reject the hypothesis of two factor differences for this case, but it may indicate that other dis- turbing forces are in operation in this experiment. 102 GENETICS IN RELATION TO AGRICULTURE A better method of testing goodness of fit has been suggested by Harris. The formula employed is ENO Xe a In this formula o = the observed frequency of any class; c, the cal- culated frequency of that class; and 2 indicates that all values of the cf 2 type —_ we are added together. When this formula is applied to the case treated above the values obtained are as given in Table XVIII. The value of X2is 8.14. The number of phenotypic classes is four. To deter- mine the significance of this value it is necessary to refer to Elderton’s tables for calculating goodness of fit. The value for P, the probability, for this case derived from such a table is 0.0437. The chances that the deviations shown in this ratio are merely due to random sampling are ‘about one in twenty-three, again confirming our previous statement that some unknown slightly disturbing forces may be operating in this case. The deviation, however, is not enough to establish this certainly, for such a deviation might be expected to occur in about 4 per cent. of cases. TasLeE XVIII.—Goopness oF Fit In A MENDELIAN EXPERIMENT Phenotypes Observed Calculated emo 0 C Purplewbarchy: ty see cs oes oe are ee 1,861 1,822.5 0.81 Burpletaweet: as. os aia we ec ste see 614 607.5 0.07 White starchy e.c..t 5 cclew onset tore 548 607.5 5.83 WVAIGORB WGC bss cies 5 cuteearreisitiewhaveas he 217 202.5 1.43 3,240 3,240.0 8.14 = xX? Mathematically the method suggested by Harris is preferable. It has also the advantage that it gives a measure of the goodness of fit of the ratio as a whole; which particular terms are most seriously at variance = 2 Lesa on For determining the significance of X?, it is necessary to have available Elderton’s table for test of goodness of fit. These are given in Pearson’s tables for statisticians and biometricians. It must ever be held in mind that forces which tend to disturb Mendelian ratios may not neces- sarily be of significance as bearing upon the essential feature of the analy- sis, namely, that a given number of independent factors are concerned may be determined by simple inspection of the values of INDEPENDENT MENDELIAN INHERITANCE 103 in a certain experiment. There is always a chance that biological conditions of necessity may disturb a ratio, for after all a ratio is only the end point of a series of phenomena which we pretend to describe step by step. Unless constantly guarded against, such biological con- ditions as differences in viability, variations in phenotypic expression, etc., may result in selective elimination of a certain number of zygotes at some time previous to classification, or in error in the classification of some individuals. CHAPTER VI LINKAGE RELATIONS IN MENDELISM Thus far Mendelian experiments have been considered in which the different pairs of factors segregate independently, and it has been shown that such cases may be explained very simply on the assumption that different pairs of chromosomes carry independent factors. However, there are several different species of plants and animals in which the number of known factor differences exceeds the number of pairs of chromosomes. Since it is reasonable to believe that only a small proportion of the possible number of factorial differences in any species has been analyzed, the conclusion appears justifiable that the number of factors in any species of plant or animal greatly exceeds the number of pairs of chromosomes; in fact our present evidence leads us to believe that the number of hereditary units in any organism must reach into the thousands. If the chromosome view of heredity is valid, therefore, each chromosome must carry a very great number of factors. In the present chapter it is proposed to discuss that class of Mendelian phe- nomena which depend upon factors which tend to remain together during segregation rather than to undergo independent assortment. Assuming that such factors are borne by the same chromosome, it will be shown how the chromosome mechanism provides an adequate physical basis for all the relations exhibited by such factors. Linkage and factor coupling are terms applied to that type of inheritance in which the factors tend to remain together in segregation. Linkage of factors is definitely an exception to one of the principles which Mendel laid down, namely, that of independent character segregation. Nevertheless by common consent the term Mendelism has been extended to include all phenomena of inheritance based on the unit factor hypothesis. For a long time only a few cases of linkage were known, and these were regarded in effect as anomalies. But the advocates of the chromosome theory of heredity have zealously prosecuted the study of linkage because of the many ways in which linkage relations parallel chromosome behavior. Moreover as the number of definitely recognizable factors within a species increases it becomes more and more important to determine the relations which the factors display among themselves. Linkage relations among factors, therefore, are of primary importance, and have been the direct means of giving us a clear and illuminating picture of the consti- 104 LINKAGE RELATIONS IN MENDELISM 105 tution of the hereditary material and of the operation of the chromosome mechanism in the distribution of the hereditary units. Purple Aleurone and Waxy Endosperm in Maize.—A typical example of the relations which obtain for linked factors is given in the experiments which involve purple aleurone color and waxy endosperm inmaize. We have shown in Chapter V that aleurone color in maize in certain cases depends on a single factor difference, so that in F', segregation is in the ratio 3 purple:1 white. For waxy endosperm, when contrasted with starchy endosperm, Collins has shown that starchiness is dominant and that in F2 segregation is in accordance with the normal monohybrid | ratio, 3 starchy:1 waxy. The factors involved in these two cases are C for aleurone coloration and its recessive allelomorph c¢ for colorless aleurone, and W for starchy endosperm and its recessive allelomorph w for waxy endosperm. Collins found that when purple starchy corn, CCWW, is crossed with white waxy, ccww, Ff; is purple starchy, CeWw; but Ff. does not segregate in the expected dihybrid ratio 9 purple starchy: 3 purple waxy :3 white starchy:1 white waxy. The data which he actually obtained from six ears are given in Table XIX. The calculated TasBLe XIX.—F, SEGREGATION OF Cross PuRPLE STARCHY X WHITE Waxy (After Collins) Number of Purpl 1 aie hi Ear number “grains starchy rane soe cee 152 183 112 20 22 29 301 579 372 62 63 82 302 536 343 52 53 88 303 627 409 57 62 99 325 650 434 55 61 100 380 16] 104 17 18 22 Observed totals. 2,736 1,774 263 | 279 420 Calculated....... Lo Ee ee ea 1,539 SL =| 513 | Le [ASE boo Perea Powe eft D-day los Calculated 22.6 crossing-over...) 1,775 276 | 276 409 ratio based on independent segregation evidently falls far short of agree- ment with the observed results, even though, when each pair of characters is considered separately, the agreement with the monohybrid ratio is very satisfactory. Thus for purple and white the observed totals are 2037:699 giving a ratio of 2.98 :1.02, and for starchy and waxy the observed totals are 2053:683 giving a ratio of 3.00:1.00. The latter ratio is so close that it would be perfect if only one kernel were shifted from the starchy to the waxy class. Taking each pair separately, there- 106 GENETICS IN RELATION TO AGRICULTURE fore, the factors evidently segregate in the normal Mendelian fashion, but the excess of purple starchy and white waxy kernels indicates that the factors C and W which came from one parent and c and w which came from the other have been distributed to the same gametes more often than would occur on the basis of independent segregation. The ordinary gametic ratio for independent segregation in a hybrid of the genetic constitution CeWw is LCW :1 CwrlewW : lew. In this particular case, however, the gametes were produced in about the ratio 3.4CW :1Cw:1cW:3.4cw. The factors, therefore, display partial linkage, 7.e., the parental com- binations of factors tend to remain together more frequently than they tend to form new combinations. The factor W breaks away from C to form a new combination with c only once in about 4.4 times, instead of once in two times as is the case for independent segregation. Neces- sarily whenever W breaks away from C to form a new combination with c, w forms a new combination with C. This accounts for the symmetrical relations displayed in the gameticratio. In order to show that the two fac- tors are linked,.in this case we represent the genetic constitutions of the parents as (CW)(CW), purple starchy, and (cw)(cw), white waxy; not CCWW and ccww respectively, which is the form used to indicate inde- pendent relations between the factors. Correspondingly the Fi is (CW) (cw), not CcWw, and the series of gametes which it forms is written 3.4(CW):1(Cw):1(cW) :3.4(cw). The method of deriving an F», ratio from such a gametic series is shown in the checkerboard in Fig. 49. Here it is necessary to take into account not only the genetic constitutions of the gametes, but also the coefficients which represent their relative frequency. Summing up the totals for like phenotypes from this checkerboard, we find the F’, grains are distributed in the following ratio: 50.28 with purple aleurone and starchy endosperm 7.8 with purple aleurone and waxy endosperm 7.8 with white aleurone and starchy endosperm 11.56 with white aleurone and waxy endosperm. The calculated results based on this ratio are given in Table XIX. They show very close agreement with numbers actually observed, but in judging the significance of this agreement it must be borne in mind that a gametic ratio was arbitrarily selected which would give the closest possible agreement with the observed results. LINKAGE RELATIONS IN MENDELISM 107 When the factors enter the hybrid in different relations, the segre- gation ratio is different. Thus when purple waxy, (Cw) (Cw), is crossed with white starchy, (cW)(cW), the F, is purple starchy as in the previous ease. The resemblance, however, is not complete except as to phenotypic 3.4 (CW) 11.56 3.4 (CW) (CW)(CW) 3.4 1 (Cw) (Cw)(CW) 3.4 1 (cW) (cW)(CW) 11.56 (cw) (CW) purple starchy 3.4 (cw) purple starchy purple starchy | purple starchy 1 (Cw) 3.4 (CW) (Cw) 1 (Cw) (Cw) purple waxy | (cW) (Cw) purple starchy purple starchy 3.4 (cw) (Cw) purple waxy white starchy 1 (cW) 3.4 (cw) Le 3.4 11.56 (CW) (cW) (CW) (cw) purple starchy | purple starchy 1 3.4 (Cw) (cW) (Cw) (cw) purple starchy purple waxy 1 3.4 (cW)(cW) (cW) (cw) white starchy white starchy 3.4 11.56 (cw) (eW) (cw) (cw) white waxy i ee EU EUEIE Ua ESSE SESE SEE REESE Fig. 49.—F»2 checkerboard of cross between purple starchy and white waxy maize. ‘ 1 (CW) 3.4 (Cw) 3.4 (cW) 1 (cw) a 1 1 (CW) (CW)(CW) purple starchy 3.4 3.4 (Cw) (Cw) (CW) purple starchy 3.4 3.4 (cW) (cW)(CW) purple starchy 1 (cw)(CW) purple starchy 1 (cw) 3.4 (CW) (Cw) 11.56 (Cw) (Cw) purple waxy 11.56 (cW) (Cw) 3.4 (cw) (Cw) purple waxy purple starchy purple starchy white starchy 3.4 (CW) (cW) purple starchy 11.56 (Cw) (cW) purple starchy 11.56 (cW)(cW) 3.4 (cw) (cW) white starchy 1 (CW) (cw) purple starchy 3.4 (Cw) (cw) purple waxy 3.4 (cW) (cw) white starchy 1 (cw) (cw) white waxy Fic. 50.—F» checkerboard of cross between purple waxy and white starchy maize. expression, for its genetic constitution is (Cw) (cW), instead of (CW) (cw) as in the first cross. 1(CW) :3.4(Cw) :3.4(cW) : 1(cw). In this ratio the numerical proportions of the gametes are reversed. This is due to the fact that here the original factor combinations, C and It produces a series of gametes in the ratio 108 GENETICS IN RELATION TO AGRICULTURE w, and c and W, although the reverse of those in the former case, tend to remain together in the same ratio. When F plants of the genetic constitution, (Cw)(cW), are selfed segregation occurs in F's as shown in the checkerboard in Fig. 50. When like phenotypes are collected into classes, the following distribution is obtained: 39.72 with purple aleurone and starchy endosperm 18.36 with purple aleurone and waxy endosperm 18.36 with white aleurone and starchy endosperm 1.00 with white aleurone and waxy endosperm. This ratio is strikingly different from that obtained for the former cross, although exactly the same characters are involved. Unfortunately data supporting this part of the analysis have not yet been presented in a satisfactory manner, but the results so far as reported do show a positive linkage between the factors. Moreover other cases which we shall discuss in this chapter demonstrate beyond doubt that the relations described above hold rigidly for cases of factor linkage. The different results obtained when factors enter a cross in different combinations are, there- fore, simply due to the fact that the original combinations tend to be preserved in segregation in a definite fixed proportion of gametes. To give a chromosome interpretation of linkage we assume that the factors linked are borne in the same chromosome. ‘Thus the factor for purple aleurone color is one of the chromomeres occupying a definite locus in a particular pair of chromosomes in a purple starchy race of corn and the factor W for starchy endosperm occupies a different locus in these same chromosomes. In Fig. 51 the chromosome behavior in linkage is shown graphically. In the hybrid one member of a pair of chromosomes bears the factors C and W, the other member c and w. During synapsis these chromosomes conjugate, and when the threads representing the two chromosomes separate after conjugation they may in consequence of their twisted condition break at certain points and, reuniting, the free ends of different threads may join together. In a cer- tain percentage of cases this breaking of the filaments may occur between C and W, so that the chromosomes afterward reconstituted will contain the factors C and w, and c and W rather than the original combinations. More frequently the chromosomes will untwist without exchanging chro- matin material or after having exchanged it in such a way as not to dis- turb the original factor combinations. Exchange of chromatin material between homologous chromosomes is called crossing-over. This term is also applied to the formation of new combinations of linked factors, and these new combinations are called cross-overs. In this particular case the end result is that for the factors C and W and their allelomorphs LINKAGE RELATIONS IN MENDELISM 109 crossing-over occurs in 22.6 per cent. of cases. Accordingly the gametes are formed in the ratio: 38.7 per cent. (CW) : 38.7 per cent. (cw) : 11.3 per cent. (Cw) :11.3 per cent. (CW) = 77.4 per cent. non-cross-overs 22.6 per cent. cross-overs. It follows, therefore, that linkage may be interpreted as due to as- sociation of factors within the same chromosomes and that crossing-over or breaking apart of linked factors may be regarded as a consequence of Fig. 51.—Diagrammatic representation of crossing-over and results. At the left, the two original chromosomes. In the middle, the twisted condition of the chromosomes in synapsis and their subsequent separation. At the right, the four types of chromosomes which result and their proportions. Fig. 52.—Diagrammatic representation of crossing-over and its results when the factors enter in the opposite combination from that shown in Fig. 51. W chromatin exchange between homologous chromosomes during synapsis. The factors may be thought of as the purely passive objects with which the chromosome mechanism deals, they are linked together because they are borne in the same chromosome, they show breaks in linkage in a certain percentage of cases because in synapsis breaks occur between the loci which they occupy in the chromosome such that new combinations of the factors are formed. The chromosome relations are the same even when chromatin interchange results in no new combinations of factors, 110 GENETICS IN RELATION TO AGRICULTURE it is only when there are factor differences between the homologous chro- mosomes that the operation of the mechanism can be detected and some conception gained of its mode of operation. Linkage in Drosophilaw~—To Morgan and his associates through their investigations with mutations of Drosophila ampeloplila we owe directly practically our entire conception of the linkage relations dis- played by factors. No other single species has provided such a wealth of data or proved so favorable for genetic investigations. This body of data is still growing very rapidly and is adding new conceptions all the time, but even at this time it is no exaggeration to say that the Zodlogical Laboratories of Columbia University, like the old garden of the Kénigs- kloster at Briinn, have yielded results which will be accounted among the epochal advances in genetics. Mendel’s work showed that the char- acters of the organism were dissociable elements of its makeup which could be recombined and shuffled about in genetic experiments. From this starting point the factor conception of heredity, which assumes that characters of the individual may be referred to the action of definite factors in the hereditary material, was developed by a host of investi- gators. Morgan’s work has also furnished an overwhelming body of evidence supporting the factor conception of heredity, but its most im- portant contribution to genetics has been in the establishment of the relations existing between the factors of heredity and the chromosome mechanism of the cell. The Four Groups of Factors in Drosophila.—According to the chro- mosome theory of heredity a factor is located at a particular locus in the chromosome mechanism. Consequently since linkage depends upon . factor relations within the same chromosome it follows that the factors should display linkage relations such that they would be thrown into groups corresponding to the number of pairs of chromosomes. In Droso- phila the linkage relations existing among over a hundred factor mu- tations have been studied. The factors fall into four groups correspond- ing to the four pairs of chromosomes in Drosophila, and furthermore the relative sizes of these groups corresponds roughly to the relative sizes of the different pairs of chromosomes. There is a large group of sex- linked factors all of which display the same type of inheritance as white- eye color, which has already been described. This group corresponds to the X-chromosomes. There are two large groups of factors which correspond to the two large pairs of autosomes, and finally there is a small group, consisting as yet of only two factors, which corresponds to the small pair of autosomes. The following list of the groups of factors in Drosophila, although incomplete, gives some idea of the number and kinds of factors which have been studied in this species (Table XX). The type of behavior shown in linkage in Drospohila may be illus- 111 LINKAGE RELATIONS IN MENDELISM i , rojoo-Apog |*°°°° "7" * + MOTIAx qojoo-ahm | ttt sts" aim. TOTOO-O AG |e mae UOT[UIIO A SSUIM |TogIsuezuUl ayvounry, sojoo-Apog |°"***°""°*" ‘UR, 10]00-Apog FOO HORE CCRC yodg SSULA ey uoodg sur |oo6 866 qa0qg esate se [etarys0 A uojeuoa-Suty [°° 6° paytyg aur |e ayeounsy, iojou-Apog |:«* "ss" 7" atqrg XULOU Ty | te. eee [tojor ssutM |° °° * “Atequeurpnyy soko ojdutg |*** TJ200 OTT AA xeiouilee ee ee yeoryg er fl Aqny XB LOU a ee au ie peeq o7t A BSULAA lino” = * deg sfeT [°° ‘ peyeordnpeyy SsuIM JOYIsuejUTa}BoOUNIT, FSF YESS MD Ga C * yoodg Bao COD AAS oe ce ee *** TOION xeson st quopity, xofo0-84iT ee a aiding eguty [oot amnqeruryyy saura |S * ++ prodg uotyeuaa-Buty | snxo[gq ey [tn CL ‘s[eqe'T saurdeiieo ssojautdg TOpopeApOdale yea ae" aATlO qojoo-Apog |****** +++ -qoUIeT Iojo0-Apog er ary £y00g SoA as 01.8) .s\lbl'b: elaine e[NIOWN SBUILM see eee IT Ayuner 10]00-aAq y eae oy eidag IOAO-SUISSOID |“ “I9AOSSOION 9[94V'T rojoo-Apog |**"""""* each eS) I0[00-0Aq Be dre uluBijeg | spueq [vulmOopqy |"° "°°" peqmary IO[OO-oAG | °° 7" *-qouley pokm io" 9° °° =" Gano Seat ee ee Aqunep uoryeueA-Bury [oo pasnyg TojOmeke ee wee es yurg Chine OOD Soo ook t posumy sex | “> pomoam gy. TO;Oo-akiapee re” ee youeg woryeucA-Buryy [°° * SUIDA BI}XT sopeug joo ‘payso,q qojoo-eag |**** "+ ** woos saan lee eer syed CX a aout I9AO-BUISSOID |** °° IAAOSSOIO-MO'T BAULANC pa oe ee ae peamny Iojoo-0A [°° *** 5" *uIsony eneysotai a ars soupryy qojoo-ofg |*° °° °°" I] weeig xvioyy, [°° °°" tere a0 azis-Apog |* ae qyuBly UOTJVUBA-BULMA | °° yuenpguoy SOUT AACR og se pesserdeq TOWOOsepOE eo) ae ee Au0qny xwioyg |'° 0° 0' 7" BUIUIOD asuryy [oc nig ezts-Apog |"**"*"** “yea esura [ooo poreqstq TO[oO-A DDE a acne autoryO | SSUIM pues satystig |*"°"°"*"** ayoBqoIq TOTOO= APO Cie leuitas cleeui ov LOTOO=OAG |e 2” Se oe AYO Eye il Ceo ae pourr0jaq esuryy |°° °° 3° °° WOOTTEg eur | smog qojoo-eAm | 7" °° ** IIT weer9 esutyy |°°°*** renee eon sduty |" Foes pag soAW eee ewes sso[aAq SSUTMA Cty 6 bie vie « poeprog SOUL gis) eo «) 6 8 5 0 6 snoiojdy odeys-oA © a) 6 © due) ee) Was. of whe Ivgq SUM stew eee eee queg xe1oy see i sew eee purg SBULM Bun} sole «ape = polepuy usmIOpqy sees [eariouqy wena ; eae ceieay 10J0B} JO OWIBN Toma) 10408} JO 9UIBN iisenag 10408} JO 9UIBNY AI dnoiy III dnory J] dnory J dnoiy i a ae ee 8 ee ee Se ee ee en nn ‘MOLOV, HOVY AG aaLodaay UALOVUVHD ‘IvdIONIUG FHL ONIMOHY vpydopdwn vpydosoig NI SHOLOV,, JO SaNOUy) UNO AHL AO ASTT IWILAVG—~XX AAV], 112 GENETICS IN RELATION TO AGRICULTURE trated by the cross yellow white female by gray red male. The genetic formula of a yellow white female is (ywX)(ywX), and of a gray red male (YWX)Y. In these formule y stands for the factor for yellow body-color and w for white eye-color, and the capital letters for the corre- sponding dominant allelomorphs present in the wild type. When two such flies are bred together the F; consists of females of the genetic con- stitution (YWX)(ywX) and males of the genetic constitution (ywX)Y. The F, females, therefore, have gray bodies and red eyes and the males have yellow bodies and white eyes. Gray red females of the genetic constitution (YWX)(ywX) produce four kinds of eggs in the following proportions: (YWX) 49.45 per cent. (ywX) 49.45 per cent. (YwX) 0.55 per cent. (yWX) 0.55 per cent. 98.9 per cent. non-cross-over gametes. 1.1 per cent. cross-over gametes. When such a female is bred to a yellow white male, genetic constitu- tion (ywX)Y, which produces only two kinds of sperms, (ywX) and Y, the progeny in both sexes obviously will be in the ratio 49.45 gray red:49.45 yellow white:0.55 gray white:0.55 yellow red. Table X XI shows the results which have been secured, mostly from matings of this type. The relations shown when the factors enter in the reverse combinations may be determined by mating a gray white female, genetic constitution (YwX)(YwX) to a yellow red male, genetic constitution (yWX)Y. This gives F; gray red females of the constitution (YwX)(yWX), and gray white males of the genetic constitution (YwX)Y. In this case the F,; females produce four kinds of eggs in the following proportions: (YwX) 49.45 per cent. :(yWX) 49.45 per cent. z (YWX) 0.55 per cent.:(ywX) 0.55 per cent. 98.9 per cent. non-cross-ever gametes. 1.1 per cent. cross-over gametes. Taste XXI.—Factror LINKAGE IN GRAY ReD FeMALE DROSOPHILAS OF THE TYPE (YWX) (ywX) Non-cross-overs Cross-overs Number |_ Percentage Reported by of flies of cross- classified Gray Yellow Gray Yellow ing-over | red white white red Weer cet, Serer koe 14,939 | 8,093 | 6,672 93 81 1.16 Morgan and Cattell.....| 1,818 | 1,075 729 14 0 OFnT Morgan and Cattell..... 854 513 334 2 5 0.82 Morgan and Bridges.... | 3,424 1,807 1,600 is 10 0.50 IROGALS As. at cos ne se puoi 21,035 11,488 9,335 116 96 1.01 LINKAGE RELATIONS IN MENDELISM 113 When, therefore, the F; females are mated to yellow white males of the genetic constitution (ywX)Y the progeny will give directly in its phenotypic ratio the proportions in which the gametes are produced as follows: 49.45 gray white:49.45 yellow red:0.55 gray red:0.55 yellow white. The actual experimental results from this type of mating are summarized inTableX XII. Theactual linkage value obtained is 1.13 which is substan- tially the same as that shown in the previous table. On the basis of sum- marized data of counts of 81,299 flies, Morgan and Bridges fix the value for crossing-over between these two loci at 1.1 per cent. This is the value we have used in deriving the above gametic ratios. Other factors have been studied in the same way and give different percentages of crossing- over. Thus Morgan and Bridges report the value for crossing-over between white and miniature based on counts of 110,701 flies at 33.2 per cent., between white and vermilion from 27,962 flies at 30.5, between vermilion and bar from 23,522 flies at 23.9, and so on for the whole series of factors in the first group. TaBLeE XXII.—Factor LINKAGE IN GRAY-RED FEMALE DROSOPHILAS OF THE TYPE (YwX)(yWX) a Non-cross-overs Cross-overs Number Percentage Reported by of flies of cross- classified Gray Yellow Gray Yellow ing-over | white red red white DWExterier te ei eir es. 1,348 440 889 16 3 1.41 Morgan and Cattell..... 3,258 1,841 1,412 4 1 0.15 Morgan and Cattell..... | 9,027 4,292 4,605 86 44 1.44 LON OREN? 13,633 | 6,573 | 6,906 Factors in the second and third groups display the same type of linkage relations as those in the first group. As an example we may take the recessive factors black and curved which lie in the second group. A black curved female of the genetic constitution (bc,)(bc.) crossed with a gray normal male (BC,)(BC,) gives in F, gray long females and males of the genetic constitution (BC,)(bc,). When such F,; females are crossed back to black curved males the results as reported by Sturtevant and Bridges are given in Table XXIII. The observed percentage of crossing-over between the loci B and C, in the second chromosome in this experiment amounts to 24.04 per cent. When gray curved females (Bc,)(Bc,) are mated to black normal males (bC,)(bC,) the F, flies are gray normal as in the previous case, but genetically they are of the constitution (Bc,)(bC,). Such females 8 114 GENETICS IN RELATION TO AGRICULTURE mated to black curved males, according to the same investigators, gave the results tabulated in the last two columns in Table XXIII. Here the percentage of crossing-over amounted to 22.74 per cent., a value substantially in agreement with the results of the reverse factor tests. ~TasBLE XXIII.—Crossinc-ovER BETWEEN B anv C, IN DROSOPHILA Black curved o&@ (bev) (bcv) mated to Phenotype Gray normal 92 (BC») (bev) Gray normal 2 (Bey) (bCv) Non- Nonk iden eel Cross-overs fet eae Cross-overs Gray sori neat ben nae: Ser 610 ecg guateallits ste e 644 Gray CUIEVCU se 6 ae eee ee a h 184 2,292 Blak TORMI A oho sccle oie ars, x euee eee 226 2,148 Blackcunvedeeereeereeieeeen: 652 Ae aedalaae We rs 663 PO GaSe SRK tihs Deed tetls ere 1,262 410 4,440 1,307 Percentage of crossing-over..... 24.04 22.74 No Crossing-over in the Male.—The above results show clearly that crossing-over in the female between the loci B and C, of the second chro- mosome results in the production of approximately 23 per cent. of cross- over gametes irrespective of the particular combination of the factors concerned. Sturtevant and Bridges, however, have shown that there is no crossing-over in the male so that males of the genetic constitution (BC,)(bc,) produce only two types of sperm in the ratio 1(BC,) :1(be,) and males of the genetic constitution (Bc,)(bC,) produce sperms in the HAMMOe I sC,) sd (DC): It is not known just what the absence of crossing-over in the male depends upon. In the case of factors in the X-chromosome or first group, crossing-over would involve exchange of chromatin material be- tween the X- and Y-chromosomes. Since these differ strikingly it is not surprising that interchange of chromatin does not take place between these chromosomes for it is difficult to see how the difference could be preserved if crossing-over should occur. But the other chromosomes are alike in both sexes, nevertheless no matter how high the percentage of crossing-over in the female, none whatever has been observed in the male. This has been found true for factors lying in the third group as well as for those lying in the second group, and it is without doubt a general phenomenon. . The knowledge that no crossing-over occurs in the male has often been turned to advantage in experimental work. When gray curved flies, (Be,)(Bc,) are mated to black normal, (6C,)(6C,), the F1 flies are LINKAGE RELATIONS IN MENDELISM 115 gray normal and of the genetic constitution (Bcy)(bC,). When these are interbred to obtain the /’2 generation the results are shown in the checker- board in Fig. 53. According to this checkerboard, the F2 will consist of flies in the ratio 2 gray normal :1 gray curved :1 black normal. No black curved flies are obtained in this cross in F's, and it is of interest to note that no matter what the amount of crossing-over in the female the flies in F', will always be in the ratio 2:1:1. In F; black curved flies may be obtained from a certain percentage of matings of black normal or of gray curved flies. The failure of the double recessive class to appear in F’, has been much used by Morgan in determining the factor group to which new mutations (Bey) (bC») 11.5 11.5 ia (BG,) (BC,) (Bev) (BC,)(bC>) gray normal gray normal 38.5 38.5 Soom sCn) (Be,) (Bey) (Bey) (bC») gray curved gray normal 38.5 38.5 38.5 (bC») (bC.) (Bev) (bCy) (Cv) gray normal black normal es EAS iL (oe) (bey) (Bev) (bc,) (bC») gray curved black normal Fic. 53.—F>2 obtained by crossing gray curved and black normal flies. belong. For this purpose black-pink flies are crossed with the new mutant type. Since black lies in the second group and pink in the third, if the new factor belongs to either of these groups it will fail to show the corre- sponding double recessive form in F2. Whether it belongs to the sex- linked group is of course readily determined from the sex relations ob- tained in such an experiment, and if the test shows that the factor in question belongs to none of these three groups, by exclusion it must be- long to the fourth. Linear Arrangement of Factors.—It was an old idea of Roux brought forward to explain the division of the chromatin while in the form of along thin thread that the individual elements of the chromatin are arranged in a linear series in the chromosomes. Later Janssens developed the idea that in synapsis homologous chromosomes twist about each other 116 GENETICS IN RELATION TO AGRICULTURE and in separating tend to break apart at places, and in reuniting exchange chromatin material. Morgan has taken these two ideas and applied them to the results of the Drosophila investigations. The twisting of the 0.0 | Yellow, Spot ee k 0.0 | Sepia 1.1} White, Cherry, Eosin ue a oR ' 2.4; Abnormal Bent 6.3 + Bifid j Eyciees 12.5 * Lethal II 14.6 j Lethal sb we 16.7* Club 16.6 + Dachs ‘eel Lethal IIIa 23.7+ Lethal sa 26.5% Lethal III 25.0 - Pink, Peach 33.0; Vermilion 34.7 5 Black 36.1: Miniature 35.0 } Jaunty 38.0 + Furrowed 3 40.0 + Purple 40.0 : Kidney 43.0 + Sable 52.0: Vestigial 55.1- Rudimentary 55.0 « Ebony, Sooty 57.0; Bar 59.5°F teed 60.4: Curved 66.2* Lethal se 73.0 : Beaded 84.1- Arc 85.0 + Rough 90.0+ Speck 91.9 | Morula Fig. 54.—Plotted relations of factors in Drosophila ampelophila. chromosomes in synapsis is held to be the physical evidence of the known interchange of factors between chromosomes which takes place in crossing- over. Moreover, if the factors are the individual elements of the chro- LINKAGE RELATIONS IN MENDELISM 117 matin threads which twist about each other and these elements are held to occupy invariable loci: in the chromatin thread, then the percentage of crossing-over between any two loci may be taken as an indication of the distance between the factors. For obviously if the chromatin thread is as likely to break between any two chromomeres as between any other two, then the farther apart two factors lie in the chromatin threads repre- senting homologous chromosomes, the greater is the chance that crossing- over will occur between them. The results of the application of this idea to the linkage relations existing in Drosophila are shown in Fig. 54. In this chromosome map of Drosophila the factors have been plotted in a linear series according to their relative position in the chromosomes as determined by linkage rela- tions. The evidence as yet is not sufficient to give an accurate picture of the arrangement of all the factors, but the number of factors plotted and the relations which they display provide further evidence of the corre- spondence between the chromosomes and the factor groups. Morgan has taken 1 per cent. of crossing-over as the unit for expressing linkage relations. Expressed in such units the first chromosome, which contains all the sex-linked factors, has a length of 66.2. The second and third groups, as far as determined, have lengths of 91.9 and 85.0, respectively. These lengths in general correspond fairly well to the known relative sizes of the two large pairs of autosomes when compared with each other and with the X-chromosomes. In the fourth group but two factors are known and their loci are so close together that thus far no crossing-over has been observed between them. Accordingly no definite value can be fixed for their linkage relations. From a knowledge of the small relative size of the third autosome Muller, at the time he an- nounced the discovery of the first factor in the fourth group, predicted that factors in this group would show very close linkage values. This prediction has been upheld satisfactorily and it is further evidence that the chromosome theory of heredity works. The demonstration that factors lie in a linear series in each group provides a unique method of predicting the results of factor behavior. Obviously if a factor is known to belong to a particular group, it is possible to predict confidently that it will display independent segre- gation with factors belonging to other groups. But further than this when the loci of a number of factors in a given group have been plotted accurately, with a new factor it is only necessary to determine the linkage relations with two of the plotted loci in order to determine its locus. When its locus has been determined, its linkage values with any other members in the group may be predicted from its distance in units from those factors. To.illustrate, in Group I, if the position of miniature were unknown, it might be tested with vermilion and sable. It would 118 GENETICS IN RELATION TO AGRICULTURE give about 3 per cent. of crossing-over with vermilion and about 7 per cent. with sable. Knowing the position of the vermilion locus at 33.0 and the sable locus at 43.0, we would be able from these data to fix the locus position of miniature at about 36.0. With this value determined we could confidently predict for example that miniature and white would show somewhat less than 35 per cent. of crossing-over or miniature and bar about 21 per cent. The ability to make such predictions is a unique product of recent investigations in heredity. How experimental results support the hypothesis of linear arrange- ment of factors may be illustrated by what Morgan calls a three-point experiment, 7.e., an experiment involving three different factors in the same chromosome. We may take three factors which are in widely separated loci in the chromosome, white at locus about 1.0, miniature at about 36.0, and bar at about 57.0. The summaries which Morgan and Bridges have given of the data involving these three loci are in- cluded in Table XXIV. White and miniature give directly 33.2 per cent. of crossing-over, and miniature and bar 20.5 per cent. Since the distance between white and miniature plus that between miniature and bar is equal to 53.7, this latter value should represent the distance between white and bar. But direct experimental determinations of the per- centage of crossing-over between white and bar give a value of 43.6 per cent., which is 10.1 per cent. short of the calculated value. aapee XXIV.—CROSSING-OVER FOR THE Loct W, M, and B’ 1n DrosopuHi.a. Data OsTaIneD BY Mating FEMALES OF THE CONSTITUTION (wmB'X)(WMb’X) Wits Tripte RecesstvE Mares (wmb’X)Y. Number Number | Per cent. Character combinations! of flies of cross- |of crossing- classified overs over White miniature....) 110,701 | 31,071 33.2 Miniature bar...... 3,112 636 | 20.5 Wihite bark e302. 5,955 2,601 | 43.6 The reason for this should be plain from a consideration of Fig. 55 which shows diagrammatically how the chromosomes behave in a three- point experiment. On the left in the two upper groups are represented the two chromosomes with the factors in the original positions in which they were derived from the parents. On the right the homologus chromo- somes are shown twisted about each other, and at A, B, C, and D the types of chromosomes which are obtained after chromatin interchange in synapsis. The numbers below refer to the relative frequency of pro- duction of the four types of chromosome pairs in this three-point experi- ment based on the data of Table XXIV. In A (Fig. 55) no exchanges LINKAGE RELATIONS IN MENDELISM 119 of chromatin have occurred which affect the relations of the factors to ach other, so that this type of separation after synapsis gives the non- cross-over gametes (wmB’X) and (WMb’/X). Types B and C involve single breaks in the chromosomes followed by chromatin interchange in reunion. They are the single cross-overs and give the cross-over gametes | ‘ a a OSS W wt Ay M M b 46.3 28.15 15.45 10.1 Fig. 55.—Diagram to show crossing-over in a three-factor experiment. (wMb/X) and (WmB’X) and (wmb/X) and (WMB’X) respectively. Finally in type D the chromosomes have broken and exchanged material at two points. This type is called double crossing-over and results in the production of gametes of the genetic constitutions (wM5’X) and (Wmb’X). In this last case, although chromatin interchange has occurred between the two chromosomes, the relations between the loci W and B’ remain unchanged. 120 GENETICS IN RELATION TO AGRICULTURE The occurrence of double crossing-over accounts for the low per- centage of crossing-over between white and bar as compared with the sum of the values given by white and miniature and miniature and bar. The value for crossing-over between W and M is given by B+ - = 28.15 + 5.05 = 38.2 per cent. and similarly between M and B’ C+ . = 15.45 + 5.05 = 20.5 per cent. consequently the distance between W and B’ as measured by adding together the values for W and M and M and B’ gives the equation BC + D =153.7. Since double crossing-over of the type D does not involve a rearrange- ment of the loci, W and B’, however, the actual crossing-over obtained experimentally must fall short of the computed distance by a value equal to D as given by the equation B+ C = 43.6 per cent. The lowering of the percentage of crossing-over when extreme distances are involved is, therefore, a logical consequence of the relations existing between linked factors. Obviously double crossing-over occurs much less frequently in short distances than in long ones. Consequently since a factor map is designed to give the total values for crossing-over between the different loci, such a map is prepared so far as possible from experi- ments involving short factor distances. If such data are not at hand simple methods of interpolation are used to locate the loci. It should be noted in passing that variations in linkage values some- times occur among members of a given set of factors. Bridges has pointed out that in some cases at least the percentage of crossing-over depends somewhat on the age of the female, and Plough has detected definite effects of extremely high or low temperatures on the percentage of crossing-over between factors of the second chromosome in Drosophila, although crossing-over in the first and third chromosomes was not in- fluenced by the changes in temperature. Besides such variations, how- ever, definite factors have been discovered (Sturtevant) which lower the percentage of crossing-over. Muller has shown that such a factor exerts a particularly disturbing action in the third chromosome in which it is located. But even in cases of variation in linkage values the order of the factors in the chromosome is not disturbed. The relations shown, there- fore, in cases involving variations in linkage are in harmony with the conception of linear arrangement of factors in the chromosomes. LINKAGE RELATIONS IN MENDELISM 121 The most striking confirmation of the hypothesis of linear arrange- ment is found in the case of “deficiency” in the X-chromosome, which was investigated by Bridges (see p. 155) and in which the location of forked spines within the deficient region “was detected and proved as a result of deliberate search among those genes which had previously been mapped closest to bar!” The Mode of Interchange in Crossing-over.—F actor interchange conceivably might take place by interchange of isolated factors here and there along adjacent threads or it might follow as a consequence of inter- change of relatively large sections of chromatin between chromosomes. The sectional mode of chromatin interchange appears to have more cyto- logical evidence in its support and Plough’s recent studies on the effect of temperature on crossing-over corroborate Muller and Bridges’ inference that crossing-over takes place in the fine thread stage of synapsis, which would be the most favorable stage for sectional interchange. But breed- ing investigations of themselves clearly establish this hypothesis. Thus Muller made up females which contained twelve sex-linked mutant factors. These females received from one parent the factors for yellow body color, white eye color, abnormal abdomen, bifid wings, vermilion eye color, miniature wings, sable body color, rudimentary wings, forked spines, and from the other parent the mutant factors cherry eye color, club wings and bar eyes. Using the system of writing the genetic formulz which has been followed in this text, these females were of the genetic constitution (ywA'b; Cwmsrfb’X) (Yw'a'B;uVMSRFB’X). Muller found in tests of 712 individuals arising from gametes from such females, that the proportions of crossing-over between factor loci in the formation of gametes occurred according to the figures given in Table XXV. The results show that in this experiment there was no crossing-over in 54.4 per cent. of cases; single crossing-over in 41.7 per cent., and double crossing-over in 4.2 per cent. No example of triple crossing-over was found among these flies, but a few such cases have been observed. The values agree satisfactorily with those calculated from the three-point experiments involving the loci W, M, and B’ in this same chromosome. If we consider the double cross-overs which were obtained in this experiment we find abundant evidence in support of the sectional mode of chromatin interchange. It is difficult to visualize the relations from the numerical data, consequently Fig. 56 has been prepared to illustrate diagrammatically the types of double crossing-over obtained in these experiments. In all but one case the points of crossing-over are far re- moved from each other, and even in the exceptional case the distance between the points of crossing-over may have been as great as nineteen units distance. 122 GENETICS IN RELATION TO AGRICULTURE TasBLE XX V.—CLASSIFICATION OF FAcTOR COMBINATIONS TRANSMITTED BY FEMALES oF DROSOPHILA HAVING THE GENETIC CONSTITUTION (ywA'b:Cwmsrfb’X)(Yw'a’B;c VM SRFB’'X) No crossing-over 186 | 200 386 Crossing-over between the loci Ae nice | pnae ee Totals Wellowsardewilibete seve sielele eck ae oes 2 5 of Wihiteyancdeabnonmealees rei werent 3 5 8 /Nomo sal aniye wort Sedo eoacuoddacadsosodos 4 11 15 Bifidvandecluloe cater-(s oe costo omieet ee eter = it7/ Path 44 Clubyandavermilioni. eases eee 46 51 97 Vermilionvand miniature. pees oeeie ce ene 7 9 16 Mimiatumerandesanles sm) cicieiciee cecisien senior 18 19 37 Sable and rudimenttatynac). seiieceeise cite 28 38 66 Rudimentanyand torkedvers: creer seis oe 5 5 Horkedhandebarseemrerece otter: oem % 1 1 MNotal’sinleteross-Overse sense eee ee 296 Double crossing-over Totals Vaan Bah GianGiVes hombre tetas steels aeeettrs 1 SF i Yeand We: dVeandtS Araceae seen see ae 1 1 EMG ASS Ehite lie ean ak Gomes on eae OE ooo. 1 1 2 Se etree US] SNE P eM o¥o 4 Ge ee ee hs cee ee 1 1 Weandval (Gand Wer scar cc eroecs of anterret gc 1 1 Waan dec vie andre saan. eau ce cc cm meet ae 1 ae 1 MAE oY bac! Syne OargarsWoXG by VVC etanicney oitaud Cheers GioNS Sieh alc Le 1 1 VAP Cali deeeASenaOl Vite dime Gg Seat Giokecerosd G 1 oF 1 FS SAN Ce Opp VI ATIO US ncveus chore eon eee 1 1 2 Jeet navel (Oh Sasi alte lid Pal hy ieee Nis eA ie a Suge ale ne 4 3 a CID VAT Vipin. earns seeks eee store = af 1 1 CrandavVecrSy and Reni. eeind scan eee aek ek 7 1 8 Grand avai anGelie pee ice oe nee 2 2, Cand eVeE. ANnGeB. Pacer. Acie oto sab Ht 1 Total double cross-overs..............- 30 Interference.—Interference is merely a consequence of the sectional mode of chromatin interchange between homologous chromosomes. The term is used to designate the observed fact that when crossing-over takes place at a particular point in the chromosome the regions for some dis- tance on both sides are protected from coincident crossing-over. The operation of interference is well illustrated in Muller’s data, although the numbers are not sufficient to warrant a quantitative determination of its effect. With long distances interference decreases, which is in accord- ance with expectation. Even for relatively long distances, however, as for the loci W, M and B’ which we have already considered in detail LINKAGE RELATIONS IN MENDELISM 123 there is still some evidence of interference. Based purely on the laws of chance, if crossing-over occurs between W and M in 33.1 per cent. of cases and between M and B’ in 20.5 per cent., then the chance of coinci- dent crossing-over is equal to the product of the independent chances of crossing-over. This gives a value of 6.8 per cent. which is slightly greater than the value 5.05 per cent. calculated from the experimental data. A three-point experiment involving shorter distances, however, gives a clearer idea as to the extent of interference. Morgan and Bridges 0.0 Y 2 Ww N \ 35 A N N N 5.5 B, : N N \ N \ ai ' : NY . N \ - IN 13 .C; \ ‘ N : \ N N \ \ N N \ N VF N AS ‘ N : N , ‘ N vate ai 2a-v | N N N N N AS 3i-M fY N N N N \ f ve TR a a | ae ain VN as a N N N N \ N N N wae vain vee N N NN N N N \ va nisin Tn oe) WN VN vn 36 UO U s \ JG UG L 1 1 1 1 1 1 2 7 1 8 Fie. 56.—Diagram showing types of double crossing-over in females of Drosophila heterozygous for twelve sex-linked factors. The figures below indicate the number of times the type occurred in 712 cases. (The loci indicated in the ‘“‘map” at the left are only approx- imately correct according to recent data of Morgan and Bridges, but they are sufficiently accurate for the purpose of this diagram.) have reported such an experiment involving the loci for vermilion, sable, and bar with the results given in Table XX VI. From this table the total percentage of crossing-over between vermilion and sable is 9.8 per cent. and between sable and bar 13.8 per cent. The expected percentage of double crossing-over for these values obtained by taking 9.8 per cent. of 13.8 per cent. would be 1.35 per cent. The observed amount of double crossing-over, 0.25 per cent., is only about one-fifth of this value. That interference is normally to be expected from the method of chromatin interchange in synapsis may be seen clearly by a consideration of Fig. 57. Thus if the chromosomes have a modal length in loop twisting about each other in synapsis, then a crossing-over at point B 124 GENETICS IN RELATION TO AGRICULTURE Taste XXVI.—LINKAGE OF VERMILION, SABLE, AND Bar IN DROSOPHILA Single cross-overs between Non- Doubl Chareoters | cross-overs Vermilion and| Sable and cross-overs sable bar Gray red mormiale cents... 2 c.5 755 110 140 4 Gray vermilion normal......... 734 92 151 li Sablered normal... /7....202- 724 97 131 4 Gravanedioalerreece his ence 845 | 87 126 + Gray vermilion sable...........| 608 | 80 123 3 Gray vermilion bar............ | 800 95 129 1 Sablewed) Wena... ler). eer). 20.0 ae esnao < 9 TOs eo OST 12 22.5 ppt wale 0's x 10 9.0:1:1: 9.0 | 725.0 | 25.0 | 25.0 | 225.0 = 20 4.0:1:1: 4.0 | 700.0 50.0 50.0 | 200.0 30 2a sles 243 675.0 75.0 75.0 175.0 40 WS sakcsihe alee 650.0 100.0 100.0 150.0 50 Peed et 625.0 | 125.0 | 125.0 | 125.0 ieee EEE Ee eee CHAPTER VII THE NATURE AND EXPRESSION OF MENDELIAN FACTORS In previous chapters the formal relations which exist in the trans- mission of factors from parent to offspring have been discussed. It has been shown that these relations may be ascribed to the locus positions which factors occupy in the chromosomes. This single assumption taken together with the known behavior of the chromosome mechanism in its cycles explains very simply the two great categories of inheritance with respect to distribution of factors, namely independent segregation and linkage. Obviously, however, these are merely formal considerations, it is of considerable importance to know something about the factors themselves and the physiological interactions which they display with one another in the development of characters in the individual. It is to this problem that this and following chapters are addressed. It is true that as yet we know next to nothing about the factors themselves with respect to their physical and chemical constitution, we know them merely by their actions. We regard them as the loci ar- ranged in a linear series in the chromosome, we know they have certain characteristic effects in development and by these effects we recognize them. It is important to note that our knowledge of their behavior even is based on factor differences, not on a study of the factors them- selves. Thus we know that a certain locus in the germinal substance in Drosophila is concerned with the production of red eye color because when it is changed in a particular fashion, the eye color developed is no longer red, but white. We have no means of knowing how profound the relation of this factor to the other factors in the system is, nor can we judge as to the nature of the change in the locus by which the course of development was shifted from red to white in the production of eye color in Drosophila. Nevertheless a few things at least are known con- cerning the effects of factors in development and even in this vague field more and more facts are being discovered all the time. Factors are the genetic representatives of certain characters. Thus if a fly has a genetic constitution containing, among other factors for eye color, the factor w, then it will develop white eyes. In this particular case the eye color is practically the only character affected. Similarly in corn, if a mutation occurs in one of the basic aleurone color factors, for example, a change in the chromogen factor C to c then that corn thus 9 129 130 GENETICS IN RELATION TO AGRICULTURE developing is white as respects aleurone color. Here apparently only aleurone color is concerned. Similarly in other cases much more insig- nificant changes may be connected with definite factor differences. Thus a forked condition of the spines in Drosophila is dependent upon a definite factor difference, a recessive factor in this case. One could go on and recount indefinitely factors which cause only very slight character changes. Any character change, therefore, however slight, may be based on genetic factor differences. The only valid genetic test is the pedigree breeding method, at the same time giving due con- sideration to environmental influences which may obscure or temporarily cover entirely the underlying genetic differences. Very great somatic differences may also be dependent upon differences in single factors in individuals. Perhaps the most striking of these are large size differences such as are found in beans, peas, and even in animals at times. Thus in beans the main difference between pole and bush beans is dependent upon a single factor difference. The difference between tall and dwarf varieties of peas is of a similar nature and has been fully discussed above. Certain types of dwarfing in man appear to depend upon single factor differences and in Drosophila there are factors which determine the production of giant races and others of dwarf races. Moreover, factor differences show striking relations to one another. Thus in Drosophila there are factors for eye color which change the shade of red in the eye, some resulting in a darker and many in lighter shades, but there is also a single factor difference which results in white eyes or in other words in the entire loss of color in the eyes, and even further there is a factor for an eyeless condition, which when a part of the genetic constitution of a fly results in the production of mere rudiments of eyes or even none at all. Very frequently single factors may cause such profound changes as to alter the entire appearance of the individual and interfere more or less with all its functions. Such, for example, is the case with fasciated forms in plants, some of which at least are dependent upon simple factor differences. A striking case of this type has been reported by O. E. White in tobacco. In this fasciated variety the number of leaves is greatly increased, from 24 to as high as 80, the stem is flattened and exhibits a characteristic fasciated condition, and the flowers are very abnormal. The abnormality of the flowers extends to every part, the numbers of sepals, petals, stamens, and ovary locules are increased, and striking deformities of these parts give evidence of the disturbing effect of the factor. The abnormal effects of the factor are not confined to external characters, but cytological studies show that the division figures, par- ticularly in reduction, show marked irregularities which may be expressed in an increase in the number of chromosomes, or in a breaking down of THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 131 cells during division, or in various other peculiar phenomena. The ab- normal variety also displays a certain degree of sterility, probably asso- ciated with abnormal cell division. In spite of all the differences both external and internal which this mutation displays when compared with the normal variety from which it arose, its behavior in inheritance shows clearly that only a single factor difference is involved. When crossed with the normal type, the Ff, is intermediate, and in F, segregation is in approximately the ratio 1 abnormal:2 intermediate:1 normal. The F»2 homozygous segregants are exact duplications of the original pure forms, the normal segregants are in every respect as normal as the normal parent and the abnormal segregants are no less abnormal than those of pure abnormal races. The heterozygous forms are throughout clearly dis- tinguishable from abnormal homozygotes on the one hand and normal homozygotes on the other. Taken as a whole it would be difficult to find a better example of the profound effects which may result from a single factor difference. Lethal factors also exist which affect vital organs and result in the death of individuals homozygous for them. Excellent examples of such disturbing factors are those which affect the production of chlorophyll in plants. A number of species of plants at times produce races in which under experimental conditions approximately one-fourth of the seedlings are yellow or white instead of green and hence die soon after germination. Such strains are particularly common in cereals, and in maize in almost any variety when a large number of self-fertilized ears are tested, a number of strains may be found which produce seedlings about one-fourth of which die as soon as the food supply of the endosperm is exhausted on account of deficiency in chlorophyll production. Since the homozygous recessive forms of albino strains die soon after germination, it follows that such strains must be propagated by means of the heterozygous individuals. The operation of such a scheme is illustrated in the following case. The original self-fertilized ear gave on germination 3 fully green seedlings to one which was pure white and which died shortly after germination. If we call the albino factor g in this case and its normal allelomorph present in the green plants G, we may assume that this ear was produced by a heterozygous green plant of the constitution Gg. Half the pollen grains of such a plant carry the factor G and half the factor g; and likewise in the ovules half bear the factor G, and half g. By self-pollination of such a plant, random fertili- zation of the ovules by the pollen grains results in grains in the ratio 1GG:2Gg:lgg. Although grains of these different genotypes are indis- tinguishable in appearance, those of the genetic constitution GG and Gg produce fully green plants, while those which are gg produce albino seedlings which are incapable of independent existence on account 132 GENETICS IN RELATION TO AGRICULTURE of their lack of chlorophyll. Those green plants of the genetic constitu- tion Gg when self-fertilized produce grains one-fourth of which again give albino seedlings as in the previous generation. The green plants of the genetic constitution GG, however, since they are homozygous for the factor G produce nothing but green plants in succeeding generations. \ ie \ AMS ' 2 W ha ced Boo / oo ae ia ey DP Se 6 Fie. 58.—Disturbance of phenotypic ratio by a recessive sex-linked lethal factor. Com- pare with Fig. 36. Morgan has demonstrated the existence of a number of lethal factors in Drosophila. These factors result in the death of the individuals at some time before they reach the adult, stage. They are particularly found among sex-linked factors, because sex-linked recessive factors have no normal allelomorphs in the male. The results of the presence of sex-linked lethal factors is shown diagrammatically in Fig. 58. As in corn, the strains are propagated by means of heterozygous individuals. Since such individuals can only be females in the case of sex-linked | THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 133 factors in Drosophila, it follows that the strains must be propagated by means of females heterozygous for the factors. The diagram shows how such lines are maintained. A heterozygous female produces eggs half of which bear the normal factor Z, and half bear the lethal factor 1. When mated to a normal male the X-chromosome of which bears the normal factor L, half the daughters are normal homozygotes and half are heterozygous for /. Half the males receive an X-chromosome bear- ing the factor Z, and consequently are normal and half receive an X- chromosome bearing the factor /. These latter die before reaching the adult stage, consequently a heterozygous female produces flies two- thirds of which are females and one-third males. The unusual sex- ratio provides a convenient test for heterozygous females and by this means the strain may be continued. Some of the consequences of the presence of lethal factors when linked with other factors are of importance because of the disturbances to which they give rise in Mendelian experiments. An illustration of such effects may be taken from Lethal III in Drosophila which is located at about the locus 26.5 in the X-chromosome. It is about 25 units distance from the locus for white eyes. If now a white-eyed female heterozygous for Lethal III be crossed with a red-eyed male, as shown diagrammatically in Fig. 58 all the females will be red-eyed but only half will be homozygous for the normal factor L;. These females, homozygous for L;, produce flies in the ratio of 1 red 2:1 red &:1 white 2:1 white & when mated to their brothers. The other half of the F, females, on the other hand, will be heterozygous for L3 and conse- quently, since crossing-over takes place in 25 per cent. of cases, they produce gametes in the ratio 3(wlsX):3(WL3X):1(W1;X):1(wL3X). When such a female is mated to an F; male fly the ratio is distinctly different from that obtained with the other females, in this case 4 Red 9:3 Red o1:4 White 9:1 White @. The ratio of sexes in this latter case 1s 2 female:1 male and the same is true in F;. The sex ratio gives an immediate clue to the disturbing factor and leads to a true explanation of the cause of the disturbance. Manifold Effects of Factors.—In a preceding section of this chapter it has been shown how far reaching may be the effects of single Mendelian factors, and in the present account it is intended to deal specifically with what Morgan has termed the manifold effects of single factors. Careful study has revealed the fact that although factors are restricted in their conspicuous results to certain characters, nevertheless they may have other less noticeable results which are none the less definite and constant. Baur has observed for example in Antirrhinum that the factor which produces pure white blossoms also yields plants which are distinctly weaker in growth and are smaller than those which possess 134 TENETICS IN RELATION TO AGRICULTURE the normal allelomorph for this factor. Plants possessing the recessive factor may be recognized in the seedling stages by a peculiar coloration of the edges of the leaves and even better by the characteristic epidermis of the leaf blades. Manifold effects of factors are probably very common but very little definite work has been reported along this line. Morgan, however, has called attention to some cases in Drosophila. Thus there is a factor for club wings, and in strains of this type flies appear the wing pads of which fail to unfold after emergence. But this character is not constant, in fact about 80 per cent. of the flies in a pure strain have normal wings. Subsequent study, however, has shown that in such stocks the absence of spines on the side of the thorax is a constant differential test. These differences are shown in the accompanying figure (59). By employing the absence of spines as the differential test it is possible to classify mixed populations of “normal” and “club” flies accurately without paying any attention to wing characters. ; The Variability of Factor Expressions.—F actors also vary in the effects which they produce. We have pointed out that in pure strains of club- winged Drosophila (Fig. 59) only about 20 per cent. of the flies exhibit the unfolded wing pad characteristic of the club mutation. On the other hand, the absence of spines on the side of the thorax determined by the same factor appears to be an invariable characteristic of the club-winged flies. Sometimes this variability in factor expression may be traced to a defi- nite environmental condition. This is certainly true of the red Primula which produces red flowers under ordinary temperature conditions, but which when placed under abnormally high temperatures produces white flowers. The production of chlorophyll in some strains of corn, likewise, depends on generally favorable environmental conditions. This has been demonstrated by Miles for the yellow-green type of chlorophyll reduction. Plants heterozygous for this factor produce grains three-fourths of which produce fully green plants on germina- tion, but the other one-fourth produce pale yellowish seedlings with a tinge of green. The yellowish seedlings die under ordinary conditions, but in particularly favorable surroundings they continue to live and soon develop thenormal chlorophyll coloration. If self-fertilized, they produce only yellowish plants which must again be given very favorable condi- tions for the production of the normal green leaf color. In Drosophila a number of environmental relations have been de- scribed. Thus Morgan has studied in considerable detail the influence of environment on the development of abnormal abdomen. Fles with the dominant factor for abnormal abdomen should all exhibit the char- acteristic type of deformed abdomen shown in Fig.-60; but this is not the THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 135 case, for pure mutant stocks constantly show a high percentage of flies with normal abdomens. This variability in abdomen characters has Fic. 59.—Club-winged Drosophila. At a characteristic unfolded wing pads. At c the absence of spines on the side of the thorax is shown in comparison with the normal con- ditions, b. (From Morgan.) Fic. 60. Mutant type of Drosophila ampelophila called abnormal abdomen (the wings have been cut off); a, female; b, male; c, female that approaches the normal type. Development of this character is dependent upon moisture. (From Morgan.) been found to depend upon the condition of the food. When the food is moist a high percentage of flies have abnormal abdomens, but when the larve are raised on dry food nearly all of them have normal abdomens, 136 GENETICS IN RELATION TO AGRICULTURE On account of these relations the expected Mendelian behavior of this factor in crosses with normal flies is obscured in cultures grown on dry food, but with moist food Mendelian expectations are completely fulfilled. Moreover, the variability in the expression of the abnormal condition of the abdomen is not connected with any variability in the factor itself but is merely an expression of a variable reaction of the factor to the environment. Normal flies possessing the factor for abnormal abdomen when given moist food produce offspring just as abnormal as those from abnormal flies. The factor itself is invariable just as in a chemical system the elements which are in the system are invariable but may produce different results according to the dilution, temperature, and other conditions under which the reaction is going on. The reduplicated stock in Drosophila shows similar relations to en- vironmental conditions. The characteristic feature of this mutation is the production of extra legs or parts of legs. At normal temperatures very few flies show this condition, but when strains are grown at 10°C. a high percentage of them show supernumerary legs. As with ab- normal abdomen and moist food, so Miss Hoge has shown that with temperatures below 10° these flies satisfy Mendelian expectations when crossed with normal strains, but at ordinary temperatures of cultivation the phenomena are entirely obscured. Duplicate Factors—A number of cases are known where similar or identical effects are produced by factors located in different loci inthe germinal substance. A case in point which has been subjected to excel- lent analysis is that for capsule form in the common shepherd’s purse (Bursa). When the form having flattened triangular capsules is crossed with that having top-shaped seed pods, the F; plants produce triangular capsules. When the Ff, is grown approximately 15 produce triangular capsules to one which produces top-shaped capsules. Such a result may be explained by assuming that two recessive factors, c and d, combine to produce the top-shaped capsule. The top-shaped race then is of the genetic constitution ccdd, and the contrasted tri- angular-shaped race is CCDD. The factors C and D are fully dominant and produce identical results, namely plants bearing the typical tri- angular-shaped seed pods. ' Consequently selfing F'; plants of the genetic constitution CcDd gives F2, 15 plants with triangular pods to 1 with top-shaped pods. The checkerboard for this case is shown in Fig. 61. If this analysis is valid for the inheritance of capsule form the F3 and subsequent generations should display a characteristic type of behavior as shown in the checkerboard. In each square is given the ratio in which the particular genotype should segregate in Fs3. Thus it will be seen that THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 1387 7 families should breed true for triangular capsules. 4 families should give 15 triangular: 1 top-shaped. 4 families should give 3 triangular: 1 top-shaped. 1 family should breed true for top-shaped capsules. Shull applied this test to his cultures and obtained substantial agreement with theory throughout. Fig. 62 gives a graphic summary of his experi- mental results. oop cD Fic. 61.—Checkerboard diagram to visualize the genetic relations in a dihybrid F2 family of Bursa bursa-pastoris Heegeri, in respect to the capsule-characters. The capsules figured in each square indicate by their outline their phenotype, and by their oblique ruling their genotype, the gene C being represented by lines from upper right to lower left, and D from upper left to lower right. Homozygotes are densely lined, heterozygotes more sparsely. The ratios indicate the expectation in Ff; when a plant having the genotypic constitution indicated in the same square, is self-fertilized. (After Shull.) When three duplicate factors are concerned in a hybrid the ratio in F, is 63:1, with four factors, 255:1,andsoon. The first case of duplicate factors was that described by Nilsson-Ehle in wheat. Here the red color of certain races of wheat depends on the presence of three dominant Mendelian factors so that such races are to be represented by the genetic formula RRSSTT and the contrasted white race by rrsstt. The Fi of a cross between two such races is of a pale red color intermediate between the parental red and white, and in F» all shades of red are found from very pale to about the same depth of color as the parent 138 ~ GENETICS IN RELATION TO AGRICULTURE race. In the actual experiment among seven families comprising a total of 440 plants only one produced white grains, but the F's; generation demonstrated the adequacy of the three-factor analysis. The inter- ct HI aero +—t H+ 1 ttt | Wr | al : aes Coe ne el a HI +H 1 4 La FEA po i | + te +H | EEE HEE + t | rat eer zl it EH ae tt mai | t+} | aloo | N I NS ! 4 TT a ar balk tT Hf Coe LTT | I | i | {1 | . j=l BES oi iI Ly al 1 | Fs f | T T —s ne —— | | ial ea a lal Fs I NSNNEB Se | T t Ge Petes EEE Meee oth Bees EI | Eos 1 i { 1 ie 42 : [ear aati o il ttt | pap) 1 . ais mel | 1 | f t = sl fa a al a P| eal Ve | Li See 1 1 {4+ * i T | i | iste coy N i t | t ct ie it i owoosoevsosonset so DD G@S2SRR BSS ERE geesgsgegs85 88 8 Piper Cie irew Cp SC Ryd eee tee Fe oo om~oownene woes & x Ss BR e88ee8 938 Gs SrEBBRSSESRSSSS Fic. 62.—Resumé of ratios found in 142 families in the first five generations following the cross;between Bursa bursa-pastoris and B. Heegeri. Each square represents a possible family, the position of a family being determined by the percentage of plants with tri- angular capsules as indicated at the base of the figure. (After Shull.) mediate shade of red produced in F,; and the varying shades produced in segregation depend on the cumulative effect of the color factors. In- THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 139 stead of displaying complete dominance for any one member of the factor system as Shull found for the triangular capsule factors in Bursa, the factors here have a certain effect in color production which is additive, rrssTt rrssTt rrSstt rrSstt Rrsstt | rrsslt Rrsstt 0 1 rrSsTt rrSsTt rrSsTt rrSsTt RrssTt RrssTt RrssTt RrssTt RrSstt RrSstt RrSstt — RrSstt rrssTT rrSStt RRsstt 2 RrSsTt | RrSsTt RrSsTt rrSSTt | KRRssTt RRssTt RrssTT RrsstT | RrSsTt RrSsTt RrSsTt RrSsTt RrSsTt rrSsTT rrSsTT rrSSTt RrSStt RrSStt RRSstt RRSstt 3 RrSSTt RrSSTt RrSSTt RrSSTt RrSsTT RrSsTT RrSsTT RrSsTT RRSsTt RRSsTt RRSsTt RRSsTt rrSSTT RrSSTT RrSSTT RRSsTT RRSsTT RRssTT RRSStt 4 RRSSTt RRSSTt | RRSSTT | 5 6 Fie. 63.—-F2 squares of the checkerboard of a cross of red (RRSSTT) X white (rrsstt) wheat arranged in classes according to the depth of color displayed by the phenotypes. v.e., two factors produce twice the depth of red coloration in the grain that one produces and all six are necessary for the production of the 140 GENETICS IN RELATION TO AGRICULTURE full color of the parent red wheat. Consequently there are six shades of red in an F, population possessing various frequencies with respect to the proportionate number of individuals which display a particular shade of color as shown in the foregoing diagram (Fig. 63). Factors which display summation effects have been conveniently called cumu- lative factors. Besides dominant factors which produce similar or identical somatic effects a large number of recessive factors are Known which display the same phenomena. The first example of this type which was worked out was that in sweet peas described by Bateson. In sweet peas there are a number of different whites which phenotypically cannot be distin- guished from one another. The fact that they are genetically different is shown when they are crossed together, for then instead of producing white sweet peas the Ff; plants bear colored flowers, the particular color depending upon the genetic constitutions of the whites which were crossed. Since the simultaneous action of two dominant factors, neither one of which by itself can produce any color, is necessary for color pro- duction, Bateson has proposed to call such factors complementary factors. The same relations have been found to exist in the production of aleurone color in grains of corn. Certain white varieties of corn are known which when crossed together give red or purple corn according to the genetic constitutions of the races which were crossed. As with dominant duplicate factors this sort of phenomenon gives peculiar Mendelian ratios in F’, because of the fact that many of the genotypes are indistinguishable phenotypically. Thus for example we may repre- sent a purple corn by the formula CCPP, these factors being particularly concerned in the production of aleurone color. A mutation in the locus C would give a white corn of the genetic constitution ccPP, and likewise a mutation in the locus P would give a white corn of the genetic constitution CCpp. Phenotypically these two varieties of white corn are indistinguishable, but from a genotypic standpoint the factors for white are located in different chromosomes in the two varieties. Accord- ingly when two such white varieties are crossed, the F; is of the genetic constitution CcPp. Since C and P are both completely dominant over their allelomorphs c and p such a corn will be purple because the com- plete set of factors necessary for the production of purple aleurone color has been brought together by crossing these two genetically different whites. The checkerboard for the #2 of such a cross is shown in Fig. 64. It will be observed that the phenotypic ratio in Ff, is 9 purple:7 white. This is merely a modification of the typical 9:3:3:1 F»2 ratio, for in this cross the last three classes are phenotypically alike, although geno- THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 141 typically different. Of the nine purples, only one breeds true in Fs, and of the remaining eight purples, four give families which segregate in the ratio of 3 purple :1 white, and four give families showing segrega- tion in the ratio of 9 purple:7 white. All the whites, although of different genotypes, produce entirely white families. All these relationships are shown clearly in the checkerboard. In Drosophila a large number of similar cases of like somatic effect have been found to be dependent upon different factors. Here the linkage values of the different factors with other factors have been determined very precisely, and moreover the mutants have for the most part arisen directly from the cultures, so that the relationships have been established much better than in any other form. Cr Cp cP cp CCEP CCP p CePP. CcPp CP Purple Purple Purple Purple 1:0 Sil 3:1 eT CCP p CCpp CcPp Cepp Cp Purple White Purple White Se 0:1 9:7 0:1 CePP CcPp ccPP ccP p cP. Purple Purple White White 3:1 9:7 0:1 0:1 CcPp Ccpp ccPp ccpp cp Purple White White White ay, Ore 0:1 0:1 Fig. 64.—-Checkerboard of F2 of cross white (ccPP) X white (CCpp) maize, showing phenotypes and F’; segregation as well as genotypes. For body color at least three similar mutant factors result in almost identical darker forms. The first of these to be discovered was the black factor which is located in the second group of factors. The factor for ebony body color is in the third group, and sable is a sex-linked factor. Although so nearly alike that a mixed population could not be certainly classified these particular races do show slight differences in coloration. Similarly nearly identical results are obtained from three different jaunty factors which cause the wings to turn up at the ends. Morgan has also pointed out other such similarities in effect of different factors which affect eye and wing characters, color, ete. Sometimes a dominant and a recessive factor give identical pheno- typic results. For an illustration of this we may again turn to aleurone 142 GENETICS IN RELATION TO AGRICULTURE color in corn. Taking into account the white dominant factor for aleurone coloration, the following genotypes may be obtained: WWecPP = white wwCCPP = purple wwecPP = white wwCCpp = white wwecpp = white WCP WCp WcP Wep wCP wCp weP wep WCP| WWCCPP | WWCCPp| WWCcPP | WWCcPp | WwCCPP | WwCCPp | WwCcPP | WwCcPp White | White | White White White “White White White 0:1 | 0:1 0:1 ()gal 1:3 3:13 3:13 9:55 WCp| WWCCPp WWCCopp | WWCcPp | WWCcpp | WwCCPp | WwCCpp | WwCcPp | WwCcpp White | White White White | White White White White (a eit > 0:1 0:1 | 3:13 0:1 93155 Ore WcP| WWCcPP | WWCcPp | WWecPP | WWecPp | WwCcPP | WwCcPp | WweePP WwecPp White | White White White White White White White 0:1 0:1 0:1 Osa 3:13 9:55 Onn 0:1 Wep WWCcPp | WWCcpp | WWecPp | WWeepp | WwCcPp | WwCcpp | WwecPp Wwecepp White | White White | White White White White White 0:1 | 0:1 O:1 Ort 9:55 0:1 0:1 0:1 wCP WwCCPP | WwCCPp | WwCcPP | WwCcPp | wwCCPP | wwCCPp | wwCcPP wwCcPp White White White White Purple Purple Purple Purple 1:3 3:13 | 3:13 9:55 1:0 ail 3:1 9:7 wep | WwCCPp | WwCCpp | WwCcPp | WwCcpp | wwCCPp | wwCCpp | wwCcPp | wwCcpp White White White | White | Purple | White | Purple White Sts | BOs Sian MRD T S21 | 0:1 i Os7 0:1 weP | WwCcPP | WwCcPp WwecPP WwecPp wwCcPP wwCcPp | wweePP wwecP-p White White White White Purple Purple White White 3:13 9:55 | 0:1 0:1 3:1+ ORT, 0:1 0:1 weP | WwCcPp | WwCcpp | WwecPp Wwecpp | wwCcPp | wwCcpp | wwecPp wwecpp White White White White | Purple White | White White 9:55 0:1 0:1 0:1 9:7 0:1 0:1 0:1 Fia. 65.—F.2 checkerboard for cross of white (WWCCPP) X white (wwecpp) corn. In the F3 segregation ratio the purple is given first as in Fig. 64. The student will be able to figure out many different relations which exist when such races are crossed. In this section only one will be con- sidered as an illustration of the working of such a system. If a white corn, WWCCPP, is crossed with a white corn, wwecpp, the F; is of the genetic constitution, WwCcPp, and is white on account of the action of W. The F., however, shows some purple grains as will become apparent from a study of the accompanying checkerboard, Fig. 65. In F2 such a hybrid segregates in the ratio 55 white : 9 purple, and in F’; the families show the segregation ratios indicated in the proper squares of the checker- THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 143 board. As in the previous instance these ratios are merely modi- fications of the typical Mendelian dihybrid and trihybrid ratios due to the fact that many of the classes are white and hence are merged into one. It should be apparent from the discussion in this chapter that many complex relations exist as respects the nature and expression of factors. Only some of the best established and most conspicuous cases have been discussed and some of these in rather incomplete fashion, but the material presented is sufficient to establish several facts concerning factors, namely that some factors have very minute, others very far reaching effects, that factors may affect many characters in the individual, that factors may vary in their expression in individuals, that sometimes this variability in factor expression is dependent upon definite environmental conditions and sometimes on obscure or unknown causes, and that at times different factors may have similar somatic expressions. It is difficult to treat such various matters in any systematic fashion, con- sequently this chapter must be regarded merely as an introduction to the general topic of factor interactions. CHAPTER VIII ALLELOMORPHIC RELATIONSHIPS IN MENDELISM The present chapter is designed to deal with those relationships in which a single locus in the hereditary system is involved. Mendel worked with seven pairs of contrasted characters and he ob- served that in all of these one member of the pair controlled the expres- sion of the character when the individual was heterozygous. When tall peas are crossed with dwarf the hybrid is tall, in fact slightly taller even than the tall parent. Similarly yellow cotyledons are dominant over green, and smooth over wrinkled seed. The same is true for the other four pairs of characters. So important did this fact of dominance appear to investigators that for some time after the rediscovery of Mendelism reference was very: generally made to the law of dominance, and great significance was attached to any failure to observe dominance in genetic investigations. But subsequent investigations have shown that domi- nance, far from being a general rule, is merely a special condition met with in certain cases of inheritance. That it is by no means universal must be conceded. How far it obtains and what other conditions are met with in its absence, we shall endeavor to show in what follows. Dominance is a relation existing between a factor and its allelo- morph such that in plants heterozygous for the factor in question the character expression is the same or approximately the same as that when the factor is homozygous. Dominance, therefore, applies only to rela- tions existing between a pair of factors. .That two contrasted characters show an intermediate condition is no evidence in itself that dominance is lacking. It must further be demonstrated that this condition is due to the fact that the character expression of a genotype Aa lies between that of AA and aa. Otherwise the intermediate expression of the hybrid character may be the expression merely of the action of several pairs of factors each displaying dominance for one member of each pair, but together giving an intermediate expression. The Extent of Dominance.—Off hand it would appear that com- plete dominance is a very common phenomenon in genetic investigations. The seven pairs of contrasted characters in peas could hardly have dis- played it in all the pairs unless it were a condition of wide occurrence and considerable significance. Otherwise we should have to consider this a remarkable case of coincidence. Likewise the oft-cited investi- 144 ALLELOMORPHIC. RELATIONSHIPS IN MENDELISM 145 gations with Drosophila indicate that usually a normal allelomorph is dom- inant to a mutant factor, and in fact, often to the eye completely dominant. More precise investigations indicate, however, that al- though for all practical purposes dominance often is so complete as to closely approximate the expres- sion of the homozygous character due to the duplex ‘condition of the dominant factor, still the completeness of dominance is often more apparent than real. Darbishire has attacked this problem in the case of the cross smooth as con- trasted with wrinkled peas. Mendel’s experiments showed that smooth or round shape is dominant over the wrinkled shape in peas and as in other cases the dominance appears to the eye complete. Darbi- shire investigated the cause of the difference between round and wrinkled peas and found it associated with a difference in starch content. Thus during the development of the seed in those races possessing round seeds the sugar is almost wholly converted into starch so that when the seed is ripe and drying it retains water rather 10 (After Darbishire.) Magnified about 300 times. peas. Fic. 66.—At the left, starch grains of round pea; at the right, of the wrinkled pea; and in the middle, of a hybrid between round and wrinkled 146 GENETICS IN RELATION TO AGRICULTURE firmly and shrinks uniformly to form a round seed. Like the seeds of round races those of wrinkled peas are also round at the height of development, but in peas of such varieties the sugar is very incompletely transformed into starch. Consequently in ripening and drying they give up more water proportionally. than round races and do not shrink uniformly... As a result they become very much wrinkled at maturity. This difference in the starch grains of the wrinkled pea is not only a matter of less complete trans- formation of sugar into starch, but is also associated with less perfect production of starch grains as shown in Fig. 66. Thus in the round races the starch grains are numerous and are large and entire. They show practically no subdivision of the grains. But in the wrinkled peas the grains are not only less numerous, but they show fissures which give them an appearance like that of the compound starch grains of some species of plants. This appearance is probably due to the fact that actual breaking down of starch grains occurs in wrinkled peas during ripening so that the grains remaining are in a partial stage of disintegration. In the hybrid between a round and a wrinkled pea, however, the condition of the starch grains is intermediate between that of the two parents. The grains are intermediate not only in number and shape but also in the degree of disintegration they display. In the contrasted pair of characters, round vs. wrinkled seed in peas, the dominance of round is, therefore, merely a superficial character expression. Actually the basic phenomena involved, 7.e., the transformation of sugar into starch, show an intermediate condition in the hybrid. The superficial character ex- pression of this intermediate condition happens to be the same as that of the strict parental round condition, so that dominance here is merely dependent on superficial resemblance. We may well hesitate, therefore, in our judgment as to the completeness of dominance in any case until it has been examined with considerable care. Sometimes the application of more precise character measurements will suffice to detect a difference between the homozygous and hetero- zygous character expression. This is shown for the case of miniature vs. long wings in Drosophila. In miniature-winged flies the wings reach about to the tip of the abdomen, whereas in the long-winged flies they extend considerably beyond the abdomen. The long-winged condition is dominant, to the eye completely, and there is absolutely no difficulty in segregating the long-winged flies of an F, population from those which have miniature wings. Nevertheless Lutz has shown that when biomet- rical methods are employed the length of wings of heterozygous flies com- pared with the length of legs is shorter than that for flies homozygous for the long-winged factor. The difference in character expression in this case is slight but it can be demonstrated by the employment of precise methods of measurement. ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 147 Intermediate Expression in the Hybrid.—F rom those cases in which dominance is nearly or quite complete we may next pass to those in which the character expression of Aa is intermediate to that of AA and aa. There are numberless instances of this kind, and they are of interest because the heterozygous class may be distinguished in F2, so that the typical ratio obtained is 1A:2Aa:1a, instead of 3A:1la as in cases where dominance occurs. For a concrete example we may turn to Baur’s case in the snap- dragon. Baur and Miss Wheldale have independently conducted very extensive investigations of Mendelian inheritance in Antirrhinum. For most cases one member of a pair of contrasted characters is dominant, but when ivory is crossed with red the F; is intermediate in color, it is pale red or pink. When F»2 is grown it is found to consist of 1 red: 2 pink: 1 ivory. In one case among 97 plants, Baur obtained 22 red, RR x rr Red Ivory Rr Pink 1RR 2 Rr lor Red Pink Ivory RR Pik 2hr ler rr Red Red Pink Ivory Ivory Fig. 67.—Results of crossing snapdragons with red and ivory colored flowers. 52 pink, and 23 ivory, a satisfactory agreement with Mendelian ex- pectations. The actual proof for this case comes out in growing F’. When this is done it is found that the red plants and the ivory plants give progeny which are entirely red and ivory, respectively. The pink plants on the other hand are all heterozygous and they give in Ff; and in all succeeding generations plants in the proportion of 1 red:2 pink:1 ivory. The case is very evidently one in which a single factor difference is concerned. If the factor responsible for the production of red in Antirrhinum be designated by R, then we may designate its allelomorph present in the ivory race by r. The case then works out according to the diagram in Fig. 67. In the Four o’clock, Mirabilis jalapa, it appears to be the rule that _ heterozygous plants present visible differences from plants homozygous for color factors. For this reason in breeding experiments this plant gives a rather remarkable diversity of colors with relatively few factors involved. Thus we may start with the primary assumption that in one series of colors we have involved two pairs of factors as follows: Y = factor for yellow colored sap. R = factor which turns yellow sap red. 148 GENETICS IN RELATION TO AGRICULTURE The various homozygous combinations of these two factors give four primary races which breed true as follows: YYRR = crimson. YYrr = yellow. yyRR = white. yyrr = white. By hybridizing these races four heterozygous forms may be produced which are of the colors given below: YYRR (crimson) X YYrr (yellow) gives YY Rr = orange red. YYRR (crimson) X yyRR (white) gives YyRR = magenta. YYRR (crimson) X yyrr (white) gives YyRr = magenta-rose. YYrr (yellow) X yyrr (white) gives Yyrr = pale yellow. ot WEP AP yR yr fe) YVR War YyRR YyRr Wake Crimson Orange red Magenta Magenta rose WARP Wap YyRr Yyrr Yr Orange red Yellow Magenta rose Pale yellow YyRR YyRr yyRR yyRr yk Magenta | Magenta rose White White Yykr | Yyrr yyRr yyrr yr | Magenta rose Pale yellow White White Fic. 68.—Checkerboard analysis of the progeny of a magenta-rose Mirabilis of the genetic constitution YyRr. We thus have seven distinct color classes as a result of various com- binations of two pairs of color factors. Moreover, this species gives a very good example of the diversity which may be obtained in an F, population. Thus Miss Marryat has shown that when magenta-rose, YyRr, is selfed, the progeny fulfil the conditions indicated by the accompanying checkerboard analysis in Fig. 68. . TABLE XXIX.—F, PHENOTYPES AND Ff; PHENoTYPIC RaTIoS DERIVED FROM THE ORIGINAL Cross, Crimson, YYRR X Waite, yyrr Color of parent E arasae | pa Color of offspring Ballon rs tee 2 | 26 All yellow. CHIGBOM ye joo eho 5 6 5 | 2 23 All crimson. Orange red?: 2. 4. 2..| 3 61 17 crimson : 31 orange red : 15 white. Magenta so 8. Yeo 2 Ls | 4 64. 18 crimson : 32 magenta : 14 white. Pale yellow.........| 3 46 9 yellow : 25 pale yellow : 12 white. Magenta-rose....... | 5 70 5 crimson : 9 magenta : 6 orange red: | 19 magenta-rose: 3 yellow:7 pale | yellow : 21 white. ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 149 When the 7/3 was grown from such an F’2 population Miss Marryat obtained excellent agreement with this analysis as is shown by the data in Table XXIX. | Variable Character Expression in the Hybrid.—Sometimes the character expression in F', while intermediate displays a range of varia- tion extending almost from one parent to the other. This is shown rather strikingly in the case of bar eyes in Drosophila (Fig. 69). The bar eye factor is a sex-linked mutant factor which is responsible for the pro- duction of flies with long narrow eyes instead of the round eyes normal for the species. When a female with bar eyes is crossed to a normal male the F; all have bar eyes. In the males especially the eyes are Fic. 69.—Normal (a, a’) and bar eye (b, b’) of Drosophila; shown in side view and as seen from above. (After Morgan.) just as narrow as in homozygous races, but among the females: some may be found which have eyes nearly as narrow as those characteristic of homozygous bar eye flies and others which have eyes nearly as round as those characteristic of the normal fly. Most of them, however, have eyes which display an intermediate effect of the factor. This case readily admits of explanation, if the genetic phenomena involved are considered. Since the factor for bar eyes is sex-linked we may represent the bar-eyed female as (B’X)(b’X), following Morgan in employing the primed symbol to indicate a dominant mutant factor. The male with normal eyes is then (b/X)Y. When a bar-eyed female is mated to a normal male, bar-eyed females and males are obtained in F, as shown in the diagram in Fig. 70. The F bar-eyed male obtains his only X-chromosome from the female and this chromosome contains the factor for bar eyes. He has exactly 150 GENETICS IN RELATION TO AGRICULTURE the same genetic constitution, therefore, as a male of a pure bar-eyed race, and it is to be expected that he will display the character to the same extent as a male from a pure race. On the other hand the female has one X-chromosome which bears the normal recessive allelomorph of the bar-eye factor. This factor may be considered as exerting a competitive influence against the bar-eye factor of the other X-chromo- Bar-eyed @ Normal Pi (B’X)(B’X) x (xX) Y Gametes (B’X) _~ WX) b 1% Fy SEKBES ee Z Bar-eyed Q Bar-eyed Fie. 70.—Results of mating bar-eyed Q with normal-eyed co Drosophila. some, so that the character expression in a sense depends upon a variable equilibrium reached between the two factors. Since they appear to be nearly equal in potency it is possible apparently for this equilibrium to be thrown so much to one side or the other that at times the character expression approaches that of the typical bar-eyed strains and at times that of the normal round-eyed flies. Fic. 71.—Longitudinal sections of corn grains showing differences in character of starch; left, floury; right, flinty. An interesting case which throws considerable light on the competi- tive action of factors in determining character expression has been reported by Hayes and East in maize. Flint races of maize are char- acterized by the production of a very small amount of soft starch in the center of the seed and a large amount of hard corneous starch sur- rounding it. Flour corns on the other hand produce grains the endo- sperm of which is almost wholly made up of soft starch with occasionally a very thin layer of corneous starch at the exterior of the endosperm. These differences are shown in Fig. 71. . ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 151 When a floury corn is pollinated by a flinty corn the grains which result show no effect of the flinty pollination, they are floury grains of the same character as those of a pure floury race. Similarly when a flinty corn is pollinated by a floury corn, the grains are flinty. Again they are of the same character as the maternal parent. Thematernal type of grains is always produced in such reciprocal crosses. Following up this experiment, when F; corneous grains of the cross corneous @ XX floury o are grown and selfed, the ears produced show distinct segre- gation into flinty and floury corn in the ratio 1 flinty: 1 floury. /F floury grains from floury @ %X flinty o when grown and selfed likewise pro- duce ears showing distinct segregation into 1 flinty :1 floury. Evidently the F; grains although different phenotypically display the same genetic phenomena. Cytological research has shown that in the fertilization of maize and other plants there is a double fertilization, one fertilization giving rise to the embryo and the other to the endosperm. In the case of the embryo, an egg nucleus unites with a nucleus from the pollen grain and from this fusion the embryo develops. In the fertilization which gives rise to the endosperm two nuclei from the female unite with one from the male, so that the cells of the endosperm contain 3x chromo- somes rather than the duplex number characteristic of the cells of the embryo. If the flinty factor be represented by Ff, and the contrasted factor for floury by f, the zygote of a flinty corn is FF, but the endosperm connected with it is FFF. Correspondingly for the floury race the zygote is ff, and its endosperm fff. In the fertilization of flinty by floury corn, the egg nucleus proper, the genetic constitution of which is F, is fertil- ized by an f pollen grain, giving a hybrid zygote of the constitution Ff. The endosperm which surrounds this embryo, however, arises from the fusion of the two endosperm nuclei, FF, with a single nucleus from the pollen grain, giving a zygote of the constitution FFf. This endosperm is flinty because two doses of / are apparently dominant to one dose of f. On the other hand, when floury corn is pollinated by flinty, the embryo has the same genetic constitution, namely Ff, but the endosperm sur- rounding it arose by union of two endosperm nuclei ff with a pollen nucleus bearing the factor F. It, therefore, has the genetic constitution ffF and it is floury because the two doses of f determine the phenotypic expression to the exclusion of the single dose of F. In Fy», the hybrid flinty grains from the cross flinty 9 xX floury & give exactly the same results as the hybrid floury grains from the cross floury @ X flinty . Here the ratio is 1 flinty: 1 floury in each case, and half the members of each class are heterozygous and will reproduce the same ratios in the succeeding generation. It would be difficult to conceive of a more beautiful illustration of 152 GENETICS IN RELATION TO AGRICULTURE the quantitative relations obtaining in the determination of dominance. Apparently the relations are about the same as those shown in the case of bar eye in Drosophila, for conceivably, if such a thing could be obtained, an endosperm arising from an Ff cell might show the same variation between flinty and floury that is shown in the bar-eye character of flies of the genetic constitution (B’X) (b’X). Mosaic Expression of the Hybrid Character.—Another type of hybrid condition is that in which the Aa individuals are a mosaic of the char- acters of the two parents. This condition is very strikingly illustrated in Blue Andalusian fowls. Andalusian fowls are of three types: black, splashed white, and the so-called blue. Of these types the black and splashed white breed true, but the blue is a hybrid and constantly segregates in the ratio lblack: 2blue:1splashed white. When black and splashed white are mated, the progeny are all blue. The Blue Andalusian fowl of the Poultry Standard of Perfection is, there- fore, a heterozygous form and for that reason all attempts to establish it as a pure breeding race have failed. The case, however, is of interest here because the Blue Andalusian is a peculiar mosaic of the characters exhibited by the black and splashed white. Its “blue” color is simply due to a fine but uneven sprinkling of black pigment through the feathers; and on some portions as for instance the feathers of the breast, the black is present as a distinct edging or lacing of the feathers. Similar mosiac hybrids which represent a simple heterozygous con- dition have been reported by Nabours in grouse locusts of the genus Parattetix. Nabours found nine distinct races which bred true for particular color patterns. Hybrids, however, between any two of these species display the entire color pattern of both parents, the color patterns being merely superimposed one upon another and in such a manner that the entire pigmentation of both parents is present in the hybrid and is distributed in the same fashion. If then two races of Parattetix A and B be crossed, the hybrid AB will be a mosiac of the two parents, and it is possible by simple inspection of such a hybrid form to determine what races entered into it. Such a hybrid will give a population consisting of 1A:2AB:1B, thus demonstrating that the case rests on a simple factor basis and that the mosaic pattern is simply an expression of a heterozygous condition in which both A and a, if we designate them thus, work out their full possibility in the development of the hybrid. In certain cases which did not appear to conform to this simple interpreta- tion, a microscopic examination was resorted to. This examination dem- onstrated that the lack of agreement was apparent rather than real. Thus in Fig. 72 the superficial characters of the hybrid (BJ) between P. leuconotus (BB) and P. nigronotatus (II) are for the most part those of P. leuconotus except for the broad black band across the pronotum which is ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 153 clearly derived from P. nigronotatus. In the posterior part of. the pronotum particularly the characters of P. lewconotus, appear to be dominant but the microscopic study showed clearly that this was due to differences in distribution in the two parents, and that the characters of P. nigronotatus, although obscured were as much present as those of lewconotus. Fic. 72.—Three types of Paratettix, BB, CC, II, and two of the hybrids between them. (After Nabours.) The Presence and Absence Hypothesis.—The foregoing accounts of the relations existing in the expression of the hybrid characters as compared with the two parental characters serves as an adequate introduction for a brief consideration of the presence and absence hypothesis. Accord- ing to the presence and absence hypothesis as advanced by Bateson and Punnett, the only relations which can exist with respect to a certain fac- tor depend on its presence or absence from the hereditary material. Thus if we consider the factor R for round shape in peas, and its allelo- morph r for wrinkled shape, according to the presence and absence hypothesis the r of the genetic formula of the wrinkled pea is not itself a factor as we have assumed throughout the discussion in this text, but merely represents the absence of the factor R. The wrinkled character, therefore, is merely an expression of the action of the set of genetic fac- tors in peas when the factor R has been taken away from the system. In this text we have throughout assumed that the recessive symbols stand for factors just as truly as do the dominant ones, and we have regarded the difference between a recessive factor and its corresponding dominant allelomorph as dependent upon some change in a dominant factor sometimes profound and sometimes less profound so that all 154 GENETICS IN RELATION TO AGRICULTURE variations from complete dominance to a strict intermediacy may be ob- tained among hybrids. For cases of complete dominance, the presence and absence idea satisfies conditions very satisfactorily as far as formal relations are concerned, and intermediacy and even other conditions of the hybrid expression may be assumed to depend upon the quantitative difference in the amount of the factor present in the hybrid race as contrasted with the parent races. Difficulties, however, begin to arise when attempts are made to explain the origin of dominant mutations in terms of this hypothesis, for in such cases it is almost necessary to assume that a factor has been added to the hereditary material. It is usually considered easy enough to account for a recessive mutation as due to the dropping out of a factor from the hereditary material, but when a factor is added to that material, we must ask from whence it came, what its nature, etc. If we regard mutations as simply due to changes in a fac- tor this difficulty vanishes for then dominance or recessiveness of the mutations depends merely on the relations between the mutated factor and its unchanged condition and there is no particular reason for as- suming that all mutations should be of the nature of “loss”’ mutations, 7.e., mutations depending upon the loss of a factor from the hereditary material and resulting in the absence of some dominant character in the individuals concerned. There is no difficulty therefore, in account- ing for the four or five dominant mutations which have been observed in Drosophila, if we regard mutation as a change in a locus, for these par- ticular mutations simply happened to involve changes of such a type that the mutated locus was dominant to the unmutated condition. Obviously, also, such a view conforms more closely with the facts ob- served in cases of the competitive action of factors such as is seen in bar eyes in Drosophila or in the factors for flinty and floury endosperm in maize. But there are more serious objections than these which can be raised against the presence and absence hypothesis. In Drosophila, for in- stance, a number of cases of return mutations have been observed, many of them in cultures so controlled that the possibility of explaining them by chance contamination is practically precluded. Thus in stock so controlled by the presence of other factors that it would practically have been impossible to have a contamination go unnoticed on account of the introduction of other factors, the bar-eyed race of Drosophila has been known to produce normal-eyed mutants (May) and eosin-eyed flies have been observed to give white-eyed flies on several occasions; while on the other hand eosin, although dominant to white, originally arose as a mutant in a stock of white-eyed flies. If we assume that the change from eosin to white involves a relatively unessential change in the W factor in Drosophila, in chemical terms perhaps a slight rearrangement in ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 155 the molecule or a change in an end radical, then it is not difficult to imagine how a reverse mutation might arise. Reverse mutations, there- fore, support the idea that the recessive member of an allelomorphic system is just as truly a factor as the dominant member. Never- theless these considerations do not in themselves confute the argument of presence and absence, although they tend to throw the weight of evidence strongly against it. It is, however, perhaps not amiss to point out that much of the weight of authority of the presence and absence hypothesis depends on the fact that it was advanced at the psycho- logical moment, and that, as Morgan points out, in the light: of our present knowledge of the relation between factors and characters it assumes a knowledge far beyond that which we have at present attained. But the really serious objections to the hypothesis are those based on the evidence furnished by multiple allelomorphism. Since the foregoing was written Bridges has published results of his investigation of a case of loss or inactivation of a portion of the X-chro- mosome in Drosophila. The deficient section involved the factor for bar eye. As Bridges points out this constitutes the first valid evidence upon the question of presence and absence. According to the presence and absence hypothesis the original appearance of the dominant bar character was due to the loss from the chromosome of an inhibitor, thereby allowing the normal narrowing effect of the remaining complex to assert itself. It should make no difference whether this inhibitor were lost by a special loss involving only the inhibitor or whether it were lost because of being situated in a particular section which became lost. In other words, the chromosome which is deficient for the region carrying the inhibitor should allow the occurrence of the same narrowing effect that is allowed by the simple loss of the inhibitor. In point of fact, the deficiency of the region in which the inhibitor must be hypoth- ecated does not produce an effect like that of the mutation responsible for bar. For, the female carrying one deficient X and one normal X shows no narrowing of the eye shape, and likewise the female carrying one deficient X and one bar X is no narrower in eye shape than a normal heterozygous bar. Thus, in the only case which has a direct bearing on the presence and absence hypothesis, it is seen that the ex- pedient of the loss of inhibitors to explain the origin of a dominant mutation is of no avail. Multiple Allelomorphism in General.—Multiple allelomorphism is the term applied to those cases which seem to depend on a series of changes in a given factor locus. Cuenot advanced such an explanation for the inheritance of certain color patterns in mice, and Morgan has since described several cases which occur in Drosophila. Since these later cases are simpler and have been worked out in more detail they will be treated first. 156 GENETICS IN RELATION TO AGRICULTURE Multiple Allelomorphism in Drosophila.—A typical case is that centering around the locus for eye color in Drosophila which we have called W. ‘This locus is situated in the X-chromosome at a distance of one unit from the locus Y for body color. The first mutations in Dro- sophila involved a change in W such that white eyes were produced, a mutation recessive to the normal red-eyed condition. This factor is called w and its inheritance has been dealt with in previous chapters. Later some flies arose in a white-eyed culture which had eosin eyes. When a white o is mated to an eosin 2 the F; 1s eosin! and F» consists of 3 eosin:1 white. When a red-eyed 9 is mated to an eosin-eyed o’, F, is red, and F»2 segregates in the ratio 3 red:1 eosin. The facts are explainable on the assumption that the factor W has been changed in a different fashion to produce the factor for eosin which we designated as we. On this basis the analysis of the genetic constitutions of these different races is as follows: (WX)(WX) =red 9 (WX)Y =redo (weX )(w?X) = eosin 9 (weX)Y = eosin (wX)(wX) = white 9 (wX)Y = white @%. A change in the same locus has occurred in the mutation to white and to eosin, but the change has been different in each case. Later four other changes in this locus occurred giving eye colors which have been named cherry, tinged, blood and buff, and these fulfil the same conditions as those pointed out for eosin. The factors are designated w‘’, w‘, w® and w respectively. These seven factors therefore display a particular type of behavior depending upon the fact that they occupy the same locus in the X-chromosome. They form together a system of septuple allelomorphs. In Drosophila there are at least three other such systems of multiple allelomorphs. One of these centers around the Y locus in the X-chromo- some which may change to y giving a yellow-bodied fly in place of the normal gray body or may change to y* when a spot-bodied fly is produced. Another system of triple allelomorphs for eye color is located in the third chromosome; it consists of the factors for pink and peach eye color, and the normal allelomorph of these which is concerned in the production of red eyes. A fourth such series of allelomorphs is that of the factors for ebony and sooty body color and their normal allelo- morph concerned in the production of gray body color. This series is also located in the third chromosome. ~ Assuming that more than two factors may occupy identical loci in homologous chromosomes there are several simple relations which | must be fulfilled in order to establish the case experimentally. The 1The F,; Qs actually have an intermediate eye-color, ‘““white-eosin compound”. ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 157 linkage values of such a series of allelomorphs when tested with other members of the group to which they belong should be identical. The factor for yellow body color is located at the locus 0.0 in the X-chromo- Fic. 73.—Forms and hybrids of Paratettix. AA, teranus; BB, leucorotus; CC, leuco- thorax; II, nigro notatus. (After Nabours.) somes, and displays definite linkage values when tested with any other factor belonging in this chromosome. The factor for spot gives exactly the same values with all factors with which it has been tested. The factors for eosin and white eye color both give one unit of crossing over 158 GENETICS IN RELATION TO AGRICULTURE with the factor for yellow body color and they give identical linkage values with the other factors in this group. Since the factors occupy identical loci in the homologus chromosomes not more than two can occur in the same individual at the same time. This fact was demon- strated in the breeding tests applied above. Other cases of multiple allelomorphism are known to occur in a large variety of species. In the silkworm there is apparently a series of Expectation 3 6 3 Actual Numbers 5 4 3 21 e e e e F; B BC Cc Cc Expectation 28:25 56.5 28.25 | 8 29 53 31 Actual Numbers é 3 § 4 F; 6 B BC C Cc Expectation | 7.5 15 7:5 | Actual Numbers 3 21 10 i 3 9 vy) e t e t t t e ) ® Fy A AB B B BC Cc Cc AC A Tape es a Expectation 231.25 462.5 231.35 Actual Numbers 107 977 251 452 222 454 29 | ——| e ® ® tC) e e t t ) C ) F A AB B B BC Cc (@ AC A Expectation 20 60 65 130 65 111.75 37.25 Actual Numbers 21 59 66 136 58 ily 32 bt tee ry e r r e r e e e F2 a 284 8s B "{BCs 2G) uC 9) emer 5 6 6 12 3 p e ® ® e 6 Fy AB BC AC A AD ® x e Parents AB AC 1 J from 208 from the field the field not virgin A= texanus C= leucothorax B= leuconotus D= punctofemorata Fig. 74.—-Chart showing results of a continued series of pedigree experiments with Paratettix involving types A, B, and C. (After Nabours.) multiple allelomorphs concerned in the production of larval patterns. As Tanaka has shown there are four larval patterns, moricaud, striped, normal, and plain. Each of these colors is allelomorphic to the other three and moreover they all apparently display the same linkage values with the pair of factors for yellow and white cocoon color. Like the multiple systems in Drosophila they give no new types by recombination when crossed. Multiple Allelomorphs in the Grouse Locust.—A very striking series of ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 159 multiple allelomorphs is that concerned with color pattern in Parattetix. Nabours has investigated the inheritance of pattern in fourteen races of this insect, the grouse locust. Some of these are shown in Fig. 73 and also the hybrids between them. It was pointed out in a previous sec- tion in this chapter that these races when hybridized give intermediate forms in F,, intermediate in the sense that they display the type patterns of both hybrids superimposed one upon the other. In F» they segregate into three types, the two parent types, and the hybrid form in the, ratio 1:2:1. Nabours has prepared a chart from the data of an extensive breeding experiment with some of these forms. It illustrates so admirably the type of behavior displayed by multiple allelomorphs that it is given in full in Fig. 74. In these experiments separation of B from AB and C from AC has not been attempted because the type A exerts very little influence on the color pattern of the hybrid. In this chart expected results are indicated wherever the ratio of types actually observed is of significance. The observed results show excellent agreement with expectations. The multiple allelomorphs in Parattetix appear to affect the entire color pattern of the body and to cause different colors to develop in different parts of the body. This, however, is merely another instance of the manifold effects of single factors, and furnishes no sound argu- ment against the conception of multiple allelomorphs. Furthermore, Nabours has discovered at least one modifying factor which can exist only with, and in addition to, any of the fourteen multiple allelomorphs or their hybrids. Multiple Allelomorphs in Maize.—In maize there is apparently a remarkable series of multiple allelomorphs concerned in the development of red color in the husks, silk, pericarp, and cob. Practically all com- binations of these are known in various different varieties of maize, so that it is possible to have varieties with red grain, silk, cob, and husk; red grain, white silk, white cob, and white husk; or any other com- bination whatsoever. When, however, such types are crossed the F, displays a superimposed set of characters, red being dominant; and in F, but three forms appear in the ratio 1:2:1 as with Nabours’ locusts, namely the two parental types and, if it is different from either of them, the hybrid form. This indicates that the F; hybrids form gametes bearing factors determining only the conditions represented in the parents. This fact Emerson subjected to direct test by crossing F1 hybrids back to varieties lacking the red color in all these parts. In one case an Ff, plant produced ears which had red cobs and variegated red grains. When such a plant was crossed back to a race having white cobs and grains, the next generation consisted only of plants which bore 160 GENETICS IN RELATION TO AGRICULTURE ears with white cobs and variegated grains and ears with red cobs and white grains. None were produced which bore ears having the F, combination, red cob and variegated grains, and on the other hand none were produced showing the reverse recombination, white ears and white grains. This series of multiple allelomorphs is perhaps the most striking one known and displays just as unique relations as does that series in Parat- tetix. For considering only red vs. white alone in these characters there are sixteen possible combinations which would give pure breeding races. Besides this, however, the red, particularly of the pericarp, may be modified in many different ways with respect to shade and distribution, apparently without altering the relations of the factors involved to the allelomorphic system, so that the number of possible combinations is considerably greater. Emerson has studied the in- heritance of a large number of these types and so far they all may be consistently explained on the hypothesis of multiple allelomorphs but the data are not as yet extensive enough to establish this interpretation beyond any doubt. The general nature of multiple allelomorphism is attested to by its occurrence in widely separated species of animals and plants. Its occurrence in Drosophila, the silkworm, Parattetix, and maize has been noted above. Besides these Morgan has pointed out that cases are known in rabbits and mice among animals, and in Aquilegia, Lychnis, and the bean among plants. In rabbits the factors concerned are three, those for self-color, Himalayan pattern, and albinism. In the mouse apparently four factors make up a similar system, namely those for yellow, black, gray, and gray with white belly. In Aquilegia the system has to do with leaf color and three factors are involved, those for green, variegated, and yellow leaf color. Shull’s case in Lychnis has to do with sex-determining factors. In the bean the case is somewhat like that in corn but the series is less extensive. The system there as re- ported by Emerson is green leaves, green pods; green leaves, yellow pods; yellow leaves, yellow pods. Morgan has brought together the arguments in favor of multiple allelomorphism and the following discussion is based for the most part upon his presentation. This discussion will serve in a sense as a summary of the material dealing with multiple allelomorphism. 1. Systems of multiple allelomorphs appear always to affect the same character. This fact is readily apparent from a consideration of the cases which have been cited above. Beyond this the cases often give a series of diminishing intensities with respect to the character affected as for example, black, Himalayan, and white in rabbits. On this basis, Pun- nett has sought to disprove the validity of the hypothesis of multiple ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 161 allelomorphs as applied to the case in rabbits, for although the homo- zygous forms give such a series of diminishing intensity of melanic pig- ment, nevertheless the heterozygous forms give inconsistences. Black by agouti gives agouti-black, but black by yellow gives full black, in spite of the fact that yellow is regarded as a lower intensity of pigmenta- tion than agouti. The argument does not appear to be valid, however, for specific relations may still exist among the factors of a system of multiple allelo- morphs. Bridges has pointed this out in the case of the eye color series red, white, cherry, eosin, tinged, blood and buff eye-color in Drosophila. He has discovered a number of factors which modify eosin, one in partic- ular called whiting changes eosin to pure white, but does not produce any visible effect on the other members of the series. The conception of diminishing intensity as applied to multiple allelomorphs is clearly not fundamental to the hypothesis. 2. The behavior in inheritance is different from that which would be expected in case different loci in the hereditary system were involved. When different loci are involved, each of two different mutant types will contain besides its own mutant factor the normal allelomorph of the mutant factor of the contrasted type. Consequently on crossing they will unite the series of factors present in the original type and give a character expression corresponding to that of the original form. Such is normally the case in undoubted instances of mutations affecting differ- ent loci, but in the case of multiple allelomorphs one or the other of the mutant types or an intermediate is produced in F;. When identical loci are concerned in two mutations, the hybrid between them will not reconstitute the original system, but will contain only the two mutant factors at that locus. The character expression of the hybrid therefore will depend on the interrelations existing between the mutant factors and the rest of the hereditary system rather than on the reuniting of the normal allelomorphs of the mutant factors. 3. There are difficulties in explaining the origin of some of the forms on the basis of complete linkage between factors, which disappear on the adoption of the hypothesis of multiple allelomorphism. The difficulty may be illustrated by a specific case, that of the series red, white, cherry, eosin, tinged, blood and buff eye-color in Drosophila. Considering two specific instances, cherry and white, both of which arose from red immedi- ately, it must follow on the basis of complete linkage that one differs from red by one factor and the other by two factors. If red be (CE)(CE), then cherry, which is recessive to red would be (cH#)(cH), and white, which is’ recessive to both red and cherry would be (ce)(ce). This involves the assumption that white arose as a result of simultaneous mutations in two completely linked factors affecting the same character, a practically 11 162 GENETICS IN RELATION TO AGRICULTURE inconceivable thing, if viewed from a purely mathematical standpoint, unless a special biological mechanism exists which favors such mutations. The same difficulties are met with in the case of other systems of multiple allelomorphs the origin of which have been observed in pedigree cultures, consequently the situation in the above system is not unique. 4. If a curve of linkage values be plotted in Drosophila for a consider- able number of known factors it will be found that the frequencies of different values correspond with one another until those displaying multiple allelomorphism (or complete linkage) are met with and these are far in excess of the number normally to be expected from purely mathematical considerations. They are not, therefore, merely the ex- tremes of ordinary cases of linkage. 5. There are no very good reasons why only one sort of change should be possible in a given locus in the hereditary material. It is true the presence and absence hypothesis does hold that the only difference with respect to a given factor is its presence in the hereditary material or its absence from it, but there are many reasons why this view at present appears untenable. oR eS eese -- INHERITANCE OF SEX AND RELATED PHENOMENA 199 to disjoin from each other. As a result eggs are occasionally produced which contain two X-chromosomes instead of one as is normally the case. In Fig. 90 are illustrated in diagram the consequences of such aberrant reduction divisions in the female. If the X-chromosomes fail to disjoin in the reduction divisions, they may be included in the egg, in which case an egg with two X-chromosomes is produced, or they may both be thrown out into the polar body, in which case an egg with no X-chromo- some is produced. This phenomenon Bridges calls primary non-dis- junction. An egg (vX)(vX) fertilized by a Y sperm gives a (vX)(vX)Y zygote, and it develops into an exceptional vermilion female. An O Fic. 90.—Diagram of the production of exceptional individuals, vermilion females and red males, through primary non-disjunction from matings of vermilion female by red male. (Adapted from Bridges.) egg (one which contains no sex chromosome) fertilized by a (VX) sperm gives a (VX)O zygote, and it develops into an exceptional red male. Zygotes of the constitution (VX)(vX)(vX) and YO are, also, possible as a consequence of such non-disjunction but it is certain that they die, consequently nothing definite can be determined as to their characters. The proof that non-disjunction is the correct interpretation of these exceptional cases in the transmission of sex-linked characters has been established by breeding tests and by actual cytological examination of exceptional individuals. Assuming that homologous chromosomes pair in synapsis, in an XX Y exceptional female two types of reduction divisions are possible. If the two X-chromosomes pair, then in reduction they disjoin and one goes to each pole. The free Y-chromosome then passes as often to one pole as to the other, and as a consequence, two kinds of eggs, X Y and X, are produced in equal numbers. On the other hand, when the 200 GENETICS IN RELATION TO AGRICULTURE Vermilion XXY Female V XY Synapsis 16% (6) (Oe Synapsis 84% 8% iy ns 8%, Ne Reduction Divisions \ 1 (0 08 0 4% 4 4+42% 4+42% Js (00) 6) Fertilization by X Sperm \: y | of Wild Male Dies (1) Wild Type Male(2) Wild Type Female (3) Wild Type Female (4) Exception Fertilization by Y Sperm y y of Wild Male Vermilion Female (5) Dies (6) Vermilion Male (7) Vermilion Male (8) Exception Fig. 91.—Secondary non-disjunction in the female. Diagram showing the constitu- tion of an exceptional vermilion female, the two types of synapsis, reduction, and the four classes of eggs produced. Each kind of egg may be fertilized by either of the two (X and Y) kinds of sperm of the wild male, giving the eight classes of zygotes shown. (After Bridges.) INHERITANCE OF SEX AND RELATED PHENOMENA 201 Y-chromosome pairs with an X-chromosome, the free X-chromosome then goes as often to one pole as the other and this results in the pro- duction of equal numbers of X, XX, XY, and Y eggs. This set of re- lations is shown in diagram in Fig. 91, which illustrates the phenomena exhibited in the production of gametes by a vermilion non-disjunctional female. From experimental evidence it has been determined that homosynapsis, 7.e., pairing of the two X-chromosomes, takes place in 84 per cent. of cases in non-disjunctional females and heterosynapsis, pairing of an X- with a Y-chromosome, in 16 per cent. of cases. A non- disjunctional female, therefore, will produce four types of: eggs in the following proportions A(vX)(vX) :4Y :46(vX) :46(0X) Y. When a vermilion non-disjunctional female is mated to a red male, the F, consists of about 46 per cent. each of red females and vermilion males and about 2 per cent. each of further exceptions, vermilion females and red males. Non-disjunctional females are, therefore, characterized by the production of further exceptional offspring to the extent of about 4 per cent. This type of non-disjunction consequent upon the presence of an extra Y-chromosome is styled secondary non-disjunction. Two additional types of zygotes are produced as a result of secondary non- disjunction, those of thé constitution YY which die, and those of the constitution XYY, which make up half of the males and are not ex- ceptional with respect to their characters but which can transmit non- disjunction to a certain proportion of their offspring. It will also be noted that of the regular daughters half are of the constitution XXY. They possess the power of producing exceptions on account of the presence of the extra Y-chromosome, but they can only be distinguished from their normal sisters by breeding tests or less conveniently by cytological examination. It is evident that an F, population such as this from the mating of a vermilion female to a red male is very different from that which is normally obtained. Bridges has followed out very skilfully many of the consequences of the assumption that these exceptional individuals are actually due to non-disjunction of the sex-chromosomes and consequent production of various types of abnormal chromosome constitution. Thus if we con- sider the exceptions produced by a non-disjunctional female, it is clear that they are a consequence of heterosynapsis in the female. Now when the X-chromosome pairs with a Y-chromosome in synapsis, it very evidently has no opportunity to exchange chromatin material with the free X-chromosome. Accordingly all the XX eggs and con- sequently all the exceptional daughters from such a female will belong to non-cross-over classes. A consideration of an actual experiment 202 GENETICS IN RELATION TO AGRICULTURE will make this matter clearer. Bridges took non-disjunctional females known from the type of mating involved in their production to be of the genetic constitution (WVFb’/X )(w*vxfb’‘X)Y and mated them to bar- eyed males (WVFB’X)Y. Obviously the regular daughters of such a mating will be bar-eyed, because they receive from the father an X- chromosome bearing the dominant factor for bar eyes, but the excep- tional daughters will not be bar-eyed since both their X=chromosomes are derived from the mother. The question concerning these excep- tional daughters is as to whether they are invariably of the genetic constitution (WVFb'X)(wvfb/X)Y or whether they may occasionally be cross-overs, for example (WVfb’X)(wvF'b’X)Y or (Wofb’X) (w°VFb'- X)Y. Since the loci involved in this case are W = 1.1, V = 33.0, and F = 56.5, normal crossing-over should give about 50 per cent. of cross-overs. By testing the exceptional females again with bar males of the above genetic constitution, the distribution of the males into phenotypes serves as an accurate indication of the genetic constitution of the mother. In every case in tests of thirty-seven exceptional daugh- ters, wild type males (WVFb’X)Y and eosin vermilion forked males (wvfb’X) made up the largest classes. This indicated that the females were all of the genetic constitution (WVFb’X)(wvfb’X)Y, and, there- fore, were non-cross-overs. The above facts are to be taken in conjunction with the fact that crossing-over actually may occur in non-disjunctional females in homo- synapsis. We have pointed out in another place that crossing-over does not occur in males. Now in non-disjunctional females the occur- rence of heterosynapsis might well set up a condition like that which is responsible for non-crossing-over in the male for we would have duplicated the exact type of reductional divisions which occur in the male aside from the presence of an unpaired X-chromosome in the reduction spindle. But as a matter of fact the presence of the Y-chromosome does not appear to affect crossing-over between the X-chromosomes in homosynapsis. Thus Bridges has summarized the data for crossing-over in non-dis- junctional X XY cultures and compared them with the data for crossing- over in normal XX cultures with the results given in Table XXXVI. Far from resulting in no crossing-over the presence of the Y-chromosome actually appears to have increased the per cent. of crossing-over be- tween loci in the X-chromosomes. No reason can be readily assigned for this increase in crossing-over, but it is of interest to note that the presence of a Y-chromosome does not preclude the occurrence of cross- ing-over. In Fig. 91 it is shown that half of the regular sons of a non-dis- junctional female are of the type X YY instead of XY as normally. The hereditary behavior of such males as determined by experiment is shown INHERITANCE OF SEX AND RELATED PHENOMENA 203 TaBLE XXX VI.—A ComPaRISON OF CROSS-OVER VALUES FROM NORMAL AND NON- DissUNCTIONAL CuLTURES IN DrosopHina (Data from Bridges) ] XX cultures | XXY cultures Loci | | Increase | pt zi | - Cross-over | mm | Cross-over een le | Total | Total a value value WLI (AS RS & ea aa 2,600 | 24.4 | 2,436 26.0 6 6.6 RARE. ol LS £77 29)..5 12,817 | Sout | 4.2 14.2 ee ie oe | 6,262 | 43.1 | 3,651 49.8 GEgiey t TLb5 Os a | 1,699 43.6 257 B3..0- sii. 494 21.6 LN SRS ae ee 2,600 | ao | 2,436 5.9 Oe 4.6 | ) Pea. ....| 6262 | 22.4 | 3,651 26.0 4.4 19.6 Vermilion XYY Male XY Synapsis 67% Reduction| Sperm Fertilization of Eggs of Sable Female . V Y) at May V ~(Q08 5 Sable Male (1) Wild Type Female (2) Wild Type Female(3) Sable Male (4) ~” ie) ” e-4 Fic. 92.—Diagram of secondary non-disjunction in the male. Four kinds of sperm are produced, but pone of these lead to the production of phenotypic exceptions in F1. (After Bridges.) 204 GENETICS IN RELATION TO AGRICULTURE in diagram in Fig. 92. There are two possible types of synapsis in non-disjunctional males, the ordinary type of heterosynapsis in the male in which Y is paired with X, in which case one Y is free, or the YY type of homosynapsis in which the X-chromosome is free. Obviously, if these two forms of synapsis take place according to the laws of chance homosynapsis will occur twice as often as heterosynapsis. Assuming this to be true the gametic series of a non-disjunctional vermilion male will be as follows: VOD. Gy OO) GIA P.O Pn le) 2) 6 When such males are mated to sable females, all the males in F; are sable and all the females are of the wild type. No exceptions, therefore, are produced in F,, but two-thirds of the daughters are non-disjunctional and should give exceptions in F,. Bridges showed that among fifty- four females only fifteen gave no exceptions in F,. Consequently 72 per cent. of the females must have been non-disjunctional, and this may be regarded as an insignificant deviation from the expected value of 67 per cent. We cannot go into detail concerning any other of the numerous points which have been investigated with respect to non-disjunction and its attendant phenomena. That non-disjunction is not due to the pres- ence of a sex-linked factor was proven by two lines of experimental evidence. In the first place such a factor should have shown linkage relations with the sex-linked factors and consequent crossing-over in definite percentages with different loci. An extensive series of matings showed, however, that non-disjunction was entirely independent of linkage relations. The other line of evidence related to attempts to establish pure stock of non-disjunction. These attempts failed com- pletely, a fact readily explainable on the basis of non-disjunction, but reconciled with considerable difficulty to the factor idea. If this were not sufficient evidence, the results of cytological examination are cer- tainly conclusive. Examination of a number of exceptional females showed them to be of the chromosome constitution XX Y, and examina- tion of regular females from non-disjunctional mothers demonstrated that about half of them were X YY, as was to be expected from theory. In brief the entire series of investigations give unique support to the chromosome theory of heredity, for throughout in this exceptional behavior of the hereditary mechanism, the factor distribution exactly parallels the unusual history of the X-chromosomes. From the standpoint of the inheritance of sex the investigations on non-disjunction throw interesting sidelights on the relations of chromo- some constitution to sex. Thus females may be of the constitutions XX or XXY orevenXXYY. Evidently, therefore, the presence of the INHERITANCE OF SEX AND RELATED PHENOMENA 205 extra Y-chromosome has no influence on the determination of. sex, although it does give rise to unusual relations in the production of gametes. Zygotes of the constitution XXX would presumably be females, but they die and consequently nothing can be determined as to their behavior. Males can be either normal XY or exceptional XYY and XO. The last, although normal males in appearance, are always sterile. The Y-chromosome, therefore, must play some definite, positive rdle in gametogenesis, although we are at present unable to state just what its function is. Along with the preceding eases of female constitutions, these different types of males indicate that the determina- tion of sex depends upon the number of X-chromosomes present. If two be present, a female is produced and the presence of one or two super- numerary Y-chromosomes does not alter this fact. If only one X-chromo- some is present a male is produced, and it is immaterial whether no Y is present or whether one or two such chromosomes are present. Throughout, the inert nature of the Y-chromosome is emphasized, the only evidence we have of its positive action being the sterility of XO males. It is important also to note that the derivation of the chromosomes, whether from the female or from the male, does not influence the sex of the offspring. Ordinarily a male is produced when a gamete from the female bearing an X-chromosome is fertilized by a gamete from a male which bears a Y-chromosome. In non-disjunctional strains, however, some males are produced from the union of a Y-bearing egg with an X-bearing sperm, exactly the reverse of the usual procedure. Also in such strains some females are produced by the union of an egg containing two X-chromosomes with a Y-bearing, or ordinarily male-producing, sperm. Non-disjunction, therefore, establishes firmly the intimate relation between chromosome constitution and sex determination. The WZ Type of Sex-inheritance.—A method of sex-inheritance exactly the reverse of the XY type is that which Morgan has styled the WZ type of sex-inheritance. In this type of sex inheritance the females are heterozygous for a sex-determiner and the males homozygous. If we diagram the relations which exist here, they will be as follows: WZ x ZZ W. 2 oe hip 2 leila WZ 27, The classical example of this type of sex-inheritance is Abraxas grossulariata, and, as in the XY type, the evidence for the relations obtaining in the inheritance of sex was given by the behavior of a sex- 206 GENETICS IN RELATION TO AGRICULTURE linked character. As it occurs in the wild, the currant moth is usually of the typical form which is characterized by dark markings on the wings which although highly variable are of characteristic shape and arranged in a definite pattern. This is the form styled grossulariata. Occasionally in nature, however, a female is discovered which is much lighter than Fic. 93.—Diagram illustrating the inheritance of lacticolor type in Abraxas. ..7.% 160 is 10 80 iets 0 (Eo SGNE isubrobustassese te eee. (aaelGOle ae 3 85 ae 12 (ia XANG) subrobusta 2 eno lees) | is 34 52 Sire) vl (CN. SCR) subrobusta sae a O30 || 21 70 9 UiS < IN NN SURO a5 baal) Bye a) 73 12 Lamarckiana X R. nanella.......| 152 | Shea esate 77 20 R. nanella X lamarckiana........ ee a 25 be 32 43 (N.X R.) lamarckiana X nanella| 266 86 xf * 14 (N. X R.) lamarckiana X nanella 70 | 80 oh 4s 20 (R. X N.) lamarckiana X nanella| 112 | 76 ae a 24 Nanella X (R. X N.) lamarckiana 68 62 ae or 38 Nanella X (N. X R.) lamarckiana’ = 27 55 = ne 45 (R. X N.) lamarckiana X R.| MATICUTR. RR et ae se et ae 84 Steel 87 a 9 1 Nanella < (N. X R.) subrobusta..| 45 | 33 16 ve 51 R. nanella X (R. X N.) subrobustal 204 ba 33 af 67 (N. X R.) subrobusta X nanella...| 138 | 51 20 ay 29 (NV. X R.) subrobusta X R. nanella 246 bys 75 ee 25 (N. X R.) subrobusta X R. nanella| 214 Lk 72 a 28 (R. X N.) subrobusta X R. nanella| 289 one (2 ese 28 parent that the phenomena exhibited, although complex, are very orderly; but no very consistent Mendelian interpretation has been advanced to account for all of them. The hypothesis of de Vries while ingenious does violence to many of our most cherished conceptions of the general nature of hereditary phenomena. De Vries assumes that pangens exist in three forms; active, labile, and inactive. Two pangens are concerned in the above series of forms, the rubrinervis pangen for strengthening of the vascular bundles and the nanella pangen for stature. These pangens exist in lamarckiana in the labile condition in which they occasionally change to the inactive condition and thus produce the corresponding muta- tions rubrinervis and nanella. Labile pangen X inactive pangen then gives according to de Vries in F; the ascendency of either one or the other condition to the complete exclusion of the other form in later generations. Accordingly lamarckiana X nanella gives in F, lamarck- zana and nanella which breed true in further generations. Similarly SPECIES HYBRIDIZATION 247 when lamarckiana is crossed with rubrinervis, the rubrinervis pangen in lamarckiana is in the labile condition, but in rubrinervis it is in the in- active condition. Here, however, a difficulty is introduced by the fact that the form corresponding to rubrinervis in F, is intermediate between rubrinervis and lamarckiana, it is the form which de Vries calls sub- robusta. Must we assume a fourth condition for the pangens in this form? An additional difficulty is introduced when we consider crosses of rubrinervis and nanella. Rubrinervis has arisen from lamarckiana by mutation, by a change of the labile rubrinervis pangen in lamarckiana into the inactive condition. But when rubrinervis is crossed with nanella, F, consists entirely of lamarckiana and subrobusta plants. As we pointed out, crosses of nanella with lamarckiana show that the nanella pangen NN x nn rubrinervis nanella N'N' Nn lamarckiana subrobusta | hy A 8a a N'N’ NN Nn mn lamarckiana rubrinervis subrobusta nanella ‘ Fic. 105.—Results of crossing two “ mutants’”’ of Ginothera lamarckiana. in lamarckiana is in the labile condition. How, then, should this pangen have become inactive in rubrinervis which was supposedly de- rived from lamarckiana by a change in the rubrinervis pangen? For according to de Vries the behavior of the nanella pangen in such an experiment is illustrated in Fig. 105 in which the active pangen is designated by N, the labile pangen by N’, and the inactive pangen by n. Those who have attempted to apply a rigid Mendelian analysis to the (Enothera phenomena have failed to do so without making assumptions which thus far remain beyond the limits of experimental verification. Nevertheless the work of such investigators as Heribert-Nilsson, Renner, Davis, and others demonstrates that Mendelian analyses may be applied to particular cases and that when the difficulties which occur in Ginothera are considered, the facts thus far discovered do not preclude an ex- planation on an essentially Mendelian basis. Davis in particular has pointed out that thus far no species of Gnothera has been found which will stand trial as of strict genetic purity. In all species apparently 50 per cent. or more of the pollen grains are abortive and similar 248 GENETICS IN RELATION TO AGRICULTURE proportions of the ovules are non-functional. To this category of facts must be added the high percentage of seed sterility which is common in the genus. If any of this pollen, ovule, and seed sterility is selective, then obviously it will be impossible to analyze the progeny successfully, unless the exact nature of the non-functional gametes and zygotes may be determined. The importance of this point has been indicated in the explanation of the frequent occurrence of parental forms among the sesqui-hybrids of rye and wheat and of Nicotiana tabacum with N. sylvestris, and it has been definitely established for many cases of albinism in plants and for peculiar sex ratios and consequent disturbances of Mendelian ratios in Drosophila. Until, therefore, a satisfactory account can be given of the difficulties which have been enumerated above it will be impossible on the one hand to offer a satisfactory Mendelian interpretation of the Gnothera investigations and illogical on the other hand to advance the results of these investigations as evidence of non- Mendelian inheritance. Moreover, considerable success has attended the efforts to produce by species hybridization strains of Ginothera which behave like lamarck- vana. It is not without significance that Davis has been able to pro- duce forms by crossing O. biennis and O. franciscana so much like lamarckiana as to be indistinguishable from it taxonomically. Tower also has taken pure species of Leptinotarsa, the Colorado potato beetle, and by mating them has produced strains which breed approximately true, but which under the stress of unusual conditions may throw off small percentages of aberrant forms. In his species crosses in Anti- rrhinum, Lotsy has reported the occurrence of races which give small proportions of aberrant forms. Since at present we have no certain knowledge that lamarckiana is not a form of hybrid origin and that its so-called mutants are not really segregants from a race possessing a peculiar hybrid constitution, these analogous cases assume considerable importance as an indication of the line of attack which may be followed for an explanation of the Ginothera phenomena. Conclusions.—-If we attempt to outline the present status of our knowledge of the phenomena of species hybridization, we see thus far no clear evidence of non-conformance to an explanation which is essen- tially Mendelian. The strict Mendelian explanation must be modified to take into account the peculiar relations which obtain in species hy- bridization. For an explanation of such relations the reaction system conception has been advanced. According to this conception the total set of factors in any species forms a reaction system in which the factors display harmonious interrelations with one another. Variety hybridi- zation, since it is concerned only with isolated differences in systems which are fundamentally identical, usually produces no disturbances in SPECIES HYBRIDIZATION 249 the reaction system relations. Consequently strict Mendelian analyses may be applied to such phenomena, and the reaction system relations need not be considered. But when species are crossed we must look to reaction system relations to account for the fact that not every set of factors which can be obtained by recombination is capable of establish- ing the harmonious interrelations which are necessary for normal func- tioning in a reaction system. As a consequence species hybrids exhibit a peculiar set of phenomena including sterility, whether partial or com- plete, production of abnormal forms, and apparent lack of conformance to established principles of hybridization. Underlying all these surface phenomena, however, is a behavior essentially Mendelian, if we take Mendelism to include all those phenomena consequent upon the shuffling and recombination of factors which possess at least a relatively high degree of stability. Since any irregularities in the distribution of factors or chromosomes, which may be occasioned by the inharmonious re- lations within the hybrid reaction systems acting upon the chromosome mechanism, can hardly be considered to give rise to results which should not be included under the term Mendelism, it is very evident that simple assumptions such as we have outlined above will account for a con- siderable array of phenomena. CHAPTER XIII PURE LINES For half a century succeeding Darwin, it was assumed that by selecting a certain type of individual for propagation, the species or variety would be continually transformed in the direction of the selec- tion. Such a conception was a natural result of the widespread acceptance of Darwin’s theory of the method of evolution and later of Galton’s “law of inheritance” as applied to selection. Experience seemed to bear out this idea also, inasmuch as continual selection of the best plants for seed and the best animals for mating was found to be profitable. But it was not until Johannsen decided to test the power of selection by keeping the pedigrees of individual plants and their descend- ants that the truth concerning the composition of varieties of cultivated plants became known. Heterogeneity within single botanical species had already been discovered, but that horticultural varieties were also heterogeneous but with respect to less easily distinguishable characters had not been realized. Definite knowledge concerning the composition of horticultural varieties threw light on the problem of selection by ex- plaining why continuous selection within a variety is necessary in some crops while it has little or no effect in the case of certain other crops. This discovery was of tremendous significance to genetics, particularly to breeding. For this reason the following account of Johannsen’s classical experiments is based directly upon his own presentation of the matter. Discovery of Pure Lines.—Johannsen chose a certain brown variety of the common garden bean (Phaseolus vulgaris nana) known as the Princess bean. In 1901 he harvested 287 plants which had grown from selected seeds of very different sizes and of known weights. The har- vested beans from each plant were weighed separately. They were then divided into classes with an interval of 10 cg., the class center values ranging from 30 to 80 cg. Next he determined the mean weights of all the beans from the plants grown from mother beans falling in the first class (25-85 eg.) and similarly for the progeny of each cf the groups of the mother beans. The result is shown in the following table. Weight of mother beans............... | 30 40 | 50) | 70 | 80 — ——< — Mean weight of progeny.............-.| 37.1 | 38.8 | 40.0 | 43.4 | 44.6 | 45.7 250 PURE LINES 251 These two series may be expressed in terms of percentage by multiplying each series by a factor that will change the value of the middle class to 100. The mean weight of all the mother beans was very nearly 50 cg. while that of the progeny is approximately 40 cg. Thus the first series is multiplied by 2 and the second by 2.5 giving the following result. Weight of mother beans...........| 60 | 80 100 Mean weight of progeny........... | 93 | 97 100 Now the deviation of each progeny class can be compared directly with the deviation of the mother class. Deviation of mother beans............. | #40°|, =20:\, 0 20 | 40 | 60 Deviation of mean weights of progeny. | —7 | —3) 0 | Se a ey lil Thus the ratios of the minus deviations of the progeny classes to the minus deviations of the mother classes are 749 and 349, the mean of which s 13¢9 or 0.163. Similarly for the plus deviations, 849, 1149, 1469 x 14, 0.303. The average of these two values is 0.233 which is about 14 as compared with Galton’s observation of 24 inheritance in size of seed in the sweet pea and stature in man. During these preliminary experiments, however, Johannsen noticed that plants grown from similar sized beans produced beans of very differ- ent sizes. Thus, for example, the plants grown from the largest mother beans (about 80 eg. in weight) yielded seeds of strikingly different sizes. The average weight of the seeds of these individual plants varied between 35 and 60 cg. and when the weights of all the individual beans of this series were arranged in a frequency distribution it produced a series that differed considerably from the normal frequency distribution. The distribution of 598 seeds, all progeny of beans about 80 cg. in weight, when arranged in classes of 5-cg. intervals, was as follows: GlassesteeeeEe ernie LO mon20n2 on o0nso 40) 456 50055560865 70)-75) 80 Number of seeds............ 5 18 46 144 127 70 70 68 2815 8 4 Wheoretical mumbersis 2.2 eon le 2Onoous>) O09, 112° 91°59) 30 13 4.71 M = 45.44 + 0.48 cg.; o« = 10.40 cg. Clearly this distribution if plotted would produce a skew polygon with the mode to the left of the theoretical mode. This observation caused Johannsen to have serious doubt regarding the biological justification of Galton’s law. For such a distribution did not appear to be the expression of only one ‘“‘type’’; on the contrary, it seemed more likely that the material was mixed. 252 GENETICS IN RELATION TO AGRICULTURE This state of affairs was the starting point of further critical study. In order to take account of the effect of selection supposedly in the opposite direction, he next examined the progeny of the smallest mother beans (about 30 cg.) and found that they displayed no such striking irregularity as did the progeny of the largest beans. (Possibly this was due to the fact that about 20 plants were grown from the smallest beans while the progeny of the largest beans came from only 11 plants.) The progeny seeds from the smallest mother beans were weighed individually and the data put in the form of a frequency table as in the former case. Glasses. Sa ot, eee ae ee eae eee 1520525 30 35 407 45 50955760765 INfibaaloyerooreiserreleh Ac Ay ba Mk oceaogusancesneuue So ane) 7k alta ales are Sis Sg Theoretical numbers........ cde Le Gah WG WoO) LO2ulol him momen M = 36.68 + 0.30 cg:; o = 7.33 cg. This distribution does not indicate a mixture. Instead it suggests that a single original ‘‘ weight type” of bean was set apart by selection in the minus direction. The general result of this preliminary study was cer- tainly a sort of confirmation of Galtonian regression; but at the same time the doubt was aroused whether the original population was not a hetero- geneous mixture from which selection simply sorted out already existing “types”. Hence came the question: Will selection of plus or minus variants within pure lines bring about the isolation of types and cause Galtonian regression ? This question was answered the following year (1902). A series of 19 pure lines was used for this investigation. Each of these pure lines originated from a single bean from the crop of 1900. In the fall of 1901 each line was represented by the seeds of one plant. In 1902 he planted 524 seeds. Every seed was given a number and each plant was harvested separately. Each pure line, each plant and every single bean was sepa- rately numbered. Thus each individual could be compared with every other individual. Johannsen first compared his material as a whole with the results of his preliminary study. Having recorded the weight of each bean, he arranged the data in groups corresponding to the classes of mother beans as in the previous year. Weight of mother beans............... 20 30 40 50 60 70 Mean weight of progeny.............. 44.0 | 44.3 | 46.1 | 49.0} 51.9 | 56.1 Number of progeny seeds............. 180 835 |} 2,238 | 1,138 | 609 494 Again he found about 14 inheritance and 34 regression of progeny on mother beans. He next divided each of these six groups of progeny beans into classes according to”"weight as shown in Table XL. PURE LINES 253 TaBLE XL.—SHOWING VARIATION WITHIN CLASSES IN A POPULATION COMPOSED oF Pure Lines. (From Johannsen) Classes of progeny seeds in cg. Classes of mother M beans, cg. | oy £ 5/15] 25 35 45 55 | 65 |75 |85 195 15-25 i] 15| 90| 63/ 11 180 | 43.78+0.56| 7.47 25-35 15) 95) 322 | 310) 91) 2 | 835 | 44.47+0.31 9.03 35-45 5|17|175| 776 956/282/24 | 3] | 2,238 | 46.17+0.19 8.93 45-55 4| 57) 305 | 521)196/51 | 4) | 1,138 | 48.94+0.28| 9.34 55-65 1} 23) 130 | 2380)168/46 |11 609 | 51.87+0.42 | 10.42 65-75 ios 175)180/64 |15) 2 494 | 56.03+0.45| 10.02 Total 15-75 eg..| 5/38/3870) 1,676|2,255|/928)187/33) 2 5,494 | 47.92+0.13 9.87 ‘ It is true that each of the six progeny series corresponds closely to the normal frequency distribution. There is no distortion such as would be expected from mixed material. Nevertheless it becomes evident that the material is heterogeneous as soon as the data are arranged by pure lines as shown in Table XLI. Taste XLI.—Survny or THE EFFrect oF SELECTION IN PuRE LINES (The dark-faced figures indicate mean weights in eg.; the light-face figures designate respective numbers of seeds.) (From Johannsen) Weight in cg. of the mother beans Mean The pure weights lines of the 20 30 | 40 50 60 70 lines I 5 $s) 16 aeol | soba Gee eco oll SOEs whi aie So |e 6Seu 64.9 91 | 64.2 | 145 UE reel eaeesell Bee aial! Ills ocete IO aicias 86 | 54.9 | 195 | 56.5 120 | 55.5 74 | 55.8 | 475 III .. | 56.4 | 144 | 56.6 | 40 | 54.4 98 | 55.4 | 282 IV acapske ..2 | 64.2 32 | 53.6 |163 | 56.6 | 112 | 54.8 | 307 Vv 52.8 | 107 | 49.2 ZOOM ete 60.2 |} 119 | 51.2 | 255 VI soto loa tele IBCS Al PLO ACTORS I abil 42.5 | 10 we 50.6 | 141 VII | 45.9 | 16 eeretee lene ces l veo wi Obie om 48.2 | 27 49.2 | 305 VIII LSet 49.0 | 20 | 49.1 | 119 | 47.5 20 ae 48.9 | 159 IX 48.5 117 Ai cee | 47.9 | 124 48.2 | 241 x ae 42.1 | 28 | 46.7 | 412 | 46.9 93 46.5 | 533 XI a 45.2 |114 | 45.4 | 217 | 46.2 87 os 45.5 | 418 XII | 49.6 | 14 spe ae ere 45.1 42 | 44.0 | 27 | 45.5 83 XIII Waxecs 47.5 | 93 | 45.0 | 219 | 45.1 | 205 | 45.8 | 95 45.4 | 712 XIV er abcan at | 2659 51 c 42.8 | 34 45.3 | 106 XV | 46.9 18 FNS: Sos 44.6 | 131 | 45.0 | 39 45.0 | 188 XVI cians ...| 45.9 (147 | 44.1 90 | 41.0 36 44.6 | 273 SVS Aa On 7S. ce 42.4 | 217 ws 42.8 | 295 XVIII | 41.0 | 54 | 40.7 |203 | 40.8 | 100 40.8 | 357 XIX 35.8 | 72 | 34.8 | 147 | 36.1 | 219 I-XIX | 44.0 |180 | 44.3 |835 | 46.1 |2238 | 49.0 |1138 | 51.9 |609 | 56.1 | 494 | 47.9 [5494 | 254 GENETICS IN RELATION TO AGRICULTURE The above analysis not only demonstrates that Johannsen’s material was a mixture of different ‘‘ weight types”’ but it also gives striking proof that selection within a single pure line has no effect. Johannsen points out that in certain lines (J, X, XJ) there seems to be a slight effect but that in others (VJ, 1X, XII, etc.,) an opposite tendency appears; while still others (UJ, IJI, VIII).are irregular. Generally speaking then no effect of selection is seen for there is no significant difference between the means of the several groups in each pure line. The apparent indications of selection effects are merely fortuitous variations. In each of these lines, therefore, the offspring of plus and minus variants exhibit complete regression to the mean of the particular line. In short, individual varia- tions were not inherited, only the characteristic BETO LEE) Ui of the particu- lar line was inherited. Johannsen did not rest here but continued to test his pure lines of beans during successive years. He found a certain amount of seasonal fluctuation in the range of variation and in the variation constants, yet each pure line maintained its own individuality as indicated by the varia- tion in weight of beans produced. And this maintenance of entity was accomplished in spite of repeated selections of smallest and largest beans so that each year every pure line was represented by two lots of plants, a ‘plus strain”? grown from the largest beans and a “‘minus strain”’ grown from the smallest beans. Complete failure of such repeated selection to cause significant change in the mean weight of either strain was observed in each pure line. As illustrations the data on Lines I and XIX are presented in Tables XLIT and XLIII. From these data it is evident that six years of selection of plus and - minus strains within Line I produced no permanent departure in either direction. In fact the last column (B—A) actually shows an inverse effect during three of the six years. Moreover, if the average of the means for ° the six years in both strains be compared this conclusion is verified. TasBLeE XLIJJ.—SELEctTIoN-EFFECT Durine Six GENERATIONS IN LINE I OF THE Princess Beans. (From Johannsen) | | Mean weight of M heat Als. 445 inaels f ean weight o progeny seeds 0 Harvest emote Hee igen Differ- select strains Difference years 0 eras B- beans ee a—minus | b—plus A-—minus B-plus 1902 145 60 70 10’ | 63. 15+1.02 | 64.8540. 76 | --l 7021.27 1903 252 55 80 | 25 | 7519+1.01 | 70.88+0.89 | —4.31+1.35 1904 ane 50 87 37 | 54.59+0.44 | 56.6840.36 | +2.09+0.57 1905 654 43 73 40 | 68.55+0.56 | 63.64+0.41 | +0.09+0.69 1906 384 46 84 38 | 74.38+0.81 | 73.00+0.72 | —1.88+1.08 1907 379 56 $81 25 | 69.07+0.79 | 67.6640.75 | —1.4141.09 PURE LINES 255 Taste XLIII.—SELEcTION-EFFECT Durine Srx GENERATIONS OF Line XIX OF THE Princess Beans. (From Johannsen) | Mean weight of owe thar beans of Mean weight of progeny seeds of | Harvest ills | the select strains aes select strains Difference Cane of beans |—— —— |) = B-A a—minus | b—plus A-—minus B-plus 1902 219 30 40 10 | 35.8340.44 | 34.78+0.38 | —1.05+0.58 1903 200 25 42 17 | 40.21+0.65 | 41.02+0.43 | +0.81+0.78 1904 590 3l 43 12 | 31.39+0-29 | 32.6440.21 | +1.25+0.36 1905 1,657 | 27 39 12 | 38.26+0.16 | 39.15+0.17 | +0.89+0.23 1906 1,367 | 30 46 16 | 37.92+0.22 | 39.8740.16 | +1.95+0.27 1907 594 24 47 23 | 37.36+0.30 | 36.95+0.21 | —0.41+0.37 oe Te ee a The mean for the progeny of the plus strain is 66.12+0.28 and for the progeny of the minus strain, 66.66 +0.33. The difference is —0.54 +0.43 (the probable error of the difference in all cases being found by taking the square root of the sum of the squares of the two probable errors). In Line I, therefore, there is no positive effect of selection; on the con- trary there would appear to be a slight inverse effect! Line XIX was characterized by beans of the least weight. The data for the results of six years of selection in plus and minus directions, particu- larly the difference between the progeny means (B—A), reveal somewhat larger fluctuations in the plus direction than in Line I but it will be noted that the probable errors of the differences are smaller, hence the validity is the more certain. Comparing the means of the means of the progeny seeds as before, for the plus strain we have 37.40+0.11 and for the minus strain, 38.83+0.15, the difference being +0.57 +0.19, which is certainly small although in the plus direction. Now, if we compare the summaries of the data from these experiments, —0.54 and +0.57, we are forced to conclude that selection was without effect in these pure lines. Finally Johannsen conducted similar experiments with the Princess beans, using the characters, length and breadth. He came to the same general conclusion, to wit, that he found no trace whatever of selection effect within pure lines and that the variations in pure line individuals are merely fortuitous modifications and are not inherited. Conditions Necessary for the Existence of Pure Lines.—J ohannsen defined a pure line as the progeny of a single self-fertilized individual of homogeneous factorial composition. Unless mutation takes place none of the descendants of such an individual can differ from the parent in their genetic factors. Two important conditions are imposed by this definition, viz., homozygosity and self-fertilization. The latter of these is the more fundamental inasmuch as it is mathematically demonstrable that self-fertilization, if continued generation after generation, leads 256 GENETICS IN RELATION TO AGRICULTURE rapidly toward a homozygous condition in all descendants. Thus, Jennings shows that in the case of the original cross, AA by aa giving all Aa, if thereafter all breeding is by self-fertilization, then, after n genera- tions, the proportions of different genotypes in the population may be calculated by the following formule: 27 — 1 AAS Soret 1 Aa On? 2" — 1 A One t - Therefore, within six self-fertilized generations after a cross involving a single pair of factors, the proportion of homozygous individuals in the population for one or the other of the two factors will be 98.4 per cent. Hence it is clear that, even though many genetic factors are concerned, as is undoubtedly the case in any crop plant or domestic animal, yet in those species where self-fertilization is the method of reproduction, the fundamental condition necessary to the existence of pure lines is met. Although by definition every pure line is a genotype, yet every genotype is not a pure line, for any heterozygote belongs to some genotype whereas a pure line is necessarily homozygous. Upon the basis of Johannsen’s definition, it would be impossible to obtain pure lines from obligatory allogamous species, to which class belong all domestic animals and certain cultivated plants. However, it is clear that continual inbreed- ing in such organisms would tend to produce a homozygous genetic composition. Isolation of Pure Lines from Mixed Populations.—In order to obtain pure lines from mixed populations the method employed will de- pend upon the method of reproduction of the organism. In autogamous species the method adopted by Johannsen in working with beans is adequate. The individual plant being capable of reproducing the species through self-fertilization and incapable of natural cross-fertilization, it is only necessary to isolate the progeny of single individuals to establish pure lines. However, in supposedly autogamous species natural hybrids sometimes occur. Hence in critical work it is always advisable to pro- tect the flowers even of autogamous plants. In dealing with allogamous species, in which it is necessary to mate two individuals, when starting with a mixed population of unknown genetic factors the original selections must be made on the basis of phenotypic similarity. With domestic animals the repetition of such selection for a large number of generations has produced the ‘“‘pure”’ or pedigreed breeds, which approximate more or less closely to pure lines and hence should be expected to breed fairly PURE LINES 207 true to type. With plants the method of procedure depends upon the details of reproduction in the species under consideration. For example, corn is naturally cross-fertilized but is also self-fertile, while the common sunflower is self-sterile and so must always be cross-fertilized. With such plants as the sunflower, then, the procedure will be as with animals and the length of time required to produce approximately pure lines will depend upon three things: (1) the number of genetic factors for which each of the selected individuals is heterozygous; (2) the number of genetic factors with respect to which the two selected individuals differ; (3) the number of chromosomes in the species. The specific chromsome number is an important consideration because of its direct relation to the number of linked character groups or in other words to the possible number of freely assorting pairs of factors. Sufficient has been said concerning the comparative ease of isolating pure lines from populations of autogamous species and the relative difficulty of obtaining pure lines from allogamous species to make it clear that the material under consideration is of the highest. importance in all critical discussions of the effect of selection within pure lines. Finally, it is to be noted that a vegetatively pro- pagated phenotype may or may not be a pure line according to its genetic constitution. A group of individuals thus propagated is known as a clone. In strictly allogamous species a clone would hardly ever be homozygous. ; The Effect of Selection Within Pure Lines.—There is now con- siderable evidence in support of the theory that selection within a pure line is without effect. This evidence comes from the results of practical breeding as well as scientific investigations of certain autogamous species of plants, such as wheat, oats and barley; also from thoroughgoing re- search on a few allogamous species, especially on certain insects and pro- tozoa, particularly paramecia. The constant maintenance of head typein wheat is strikingly portrayed in Fig. 106, which shows two heads from each of four varieties which were first isolated by Louis de Vilmorin between 1886 and 1856. The plants according to Vilmorin were found to be identical in all respects ‘‘although separated by an interval of 50 years during which annual selection had been continued. This fixity is shown not only in the characters of the ear but also in all the other characters of the plant even that of precocity, which would appear to be most dependent on climate.’’ The use of this case as evidence in support of the pure-line theory has been criticised upon the ground that the selec- tion practised had for its purpose the preservation rather than the altera- tion of the type. But from the experience of many investigators and breeders we may safely conclude that within true pure lines selection is without effect on the type unless mutations occur. After subjecting a variety of barley known as Glorup to plus and minus selection for eight 17 258 GENETICS IN RELATION TO AGRICULTURE generations, the character under observation being degree of mealiness of the kernel (Schartigkeit), Johannsen concluded that the selection had produced no effect. Moreover the Swedish plant-breeding station at Svaléf has been guided for years by the knowledge that their pedigree cultures, ¢.e., pure lines, were not changed by selection. A similar con- clusion was reached by Tower after four to ten generations of rigorous selection of albinie individuals in three different attempts to establish an albinie race from a stable race (pedigree material) of the Colorado potato beetle (Leptinotarsa decemlineata). The history of these three Fie. 106.—Four pure lines of wheat which have been grown by Vilmorin for 50 years. The original specimen in the seed museum is shown on the left in each case. The close similarity of the pairs of heads indicates that pure lines remain constant indefinitely, (After Hagedoorn.) experiments are shown at A, B and C in Fig. 107. The small black polygons show for each generation the individuals selected to become the parents of the next generation. It will be noted that neither the range nor the mode of the population is permanently shifted in the direction of the selection. Thus we find that in races or varieties which are constant (homozygous) selection has no effect unless mutations occur. Various evidence has been brought forward to show that the principle does not hold for all organisms. But in all such cases among sexually propagated species we may assume that the material used was hetero- zygous for certain factors. Such has been shown already to be a satisfac- tory explanation of Castle’s results in selecting for plus and minus strains in the hooded rats which is one of the cases originally advanced as evidence against the pure line theory. Significance of the Pure Line Theory in Breeding..-The question thus arises: How does the pure line theory explain the fact that man has wrought profound changes in domesticated animals and plants by selec- “Normal” Range of Variation “Normal” Range of Variation Gots’ css Bascal LM A lanka cbse 7 Mode fi Mode sete. woghy - Generstions Generations Generations Fic. 107.—Diagrammatic representation of results of three experiments in selecting beetles in an effort to create an albinic strain from a pure strain. (From Tower.) tion? It is well known that as a rule a mixed population consists of a number (probably quite large) of distinct biotypes and that in autoga- mous species these biotypes are pure lines to begin with, while in alloga- mous species it is only by continued intensive selection that existing 260 GENETICS IN RELATION TO AGRICULTURE biotypes can be differentiated from one another so that they ‘‘breed true.’ How these distinct biotypes originate will be considered in the following chapter, the fact that they exist is the chief consideration here. The effect of ‘‘mass”’ selection in causing temporary changes in heteroge- neous varieties of plants and races of animals is easily understood by the aid of the diagram shown in Fig. 108. The area within the large curve represents a mixed population or phenotypically similar group containing a number of distinct genotypes indicated by the small curves A-Z. Every genotype has its own variation curve and is distinct from each of the others, but they intergrade with each other so completely that the population appears as an entity. Now if one should select individuals from either extreme of the population, say at 90 or 70, it is clear that such individuals might belong to any one of four or five geno- J061 62 63 64.65 66 6768697071727374 Fie. 108.—Schematic diagram showing the relation of a population to the biotypes composing it, or of a phenotype, to the genotypes or pure lines within it. (After Lang from Goldschmidt.) types. If selection in the same direction were continued a strain would be established with a mode distinct from the mode of the original popu- lation. These strains could be maintained by continual selection and in time a single genotype might be isolated when selection would be said to have changed the type permanently. But selection changed nothing—it only isolated a certain genotype or genotypes from the origi- nal mixture. Tower’s results in selecting for the purpose of creating albinic and melanic strains of beetles as illustrated in Fig. 109 may be explained in this way. The original population shown at A consisted of a number of distinct biotypes. By the isolation of several extreme variants Tower separated plus and minus strains which he was able to maintain for eight generations by practising intensive selection. In the eighth generation he divided each population in half, continuing in- tensive selection with one portion and stopping all selection in the other. By this method he was able to maintain the plus and minus strains and at the same time to observe that in the ninth generation the mode of the PURE LINES 261 progeny of the unselected eighth generation population lay much nearer to the mode of the original population. Within three generations the unselected strains had moved back to the mode of the species. Now it is to be remembered that Tower was dealing with an obligatory allo- gamous species. Moreover, what is now known concerning body pigmentation in Drosophila makes it altogether likely that quite a large Wilitaly th th = i xe —= 4 =A! ee al! IX 4 = Vo | suoljeI9oues) Ill Cor arr joa | I Fie. 109.—Diagrammatic representation of the results obtained by the creation of albinic and melanic strains from a mixed population of beetles. (From Tower.) number of genetic factors are concerned in the degree of pigmentation of these beetles. Hence selection of phenotypes for a number of genera- tions did not isolate genotypes, 7z.e., the plus and minus strains were not homozygous. As this is an allogamous species undoubtedly most of the individuals in the original population were heterozygous for many factors. Furthermore, Tower did not select single pairs but always took several 262 GENETICS IN RELATION TO AGRICULTURE pairs as parents for each generation. While the selection of similarly pigmented individuals would tend gradually toward a homozygous condition,with respect to the specific factors conditioning pigmentation, yet it is altogether likely that under the conditions of the experiment a considerable degree of heterozygosity was maintained. In other words the selection practised did not isolate pure lines, the plus and minus strains did not become homozygous. Much of the work done in the past in ameliorating animals and plants has been by this method of selecting phenotypes but not genotypes, which accounts in part for the frequent necessity of continuous selection in maintaining improved strains or breeds. In reviewing the development of plant breeding we shall note certain cases of early recognition of the effects of genotypic selection, a principle which is now accepted as fundamental in all breeding operations. CHAPTER XIV MUTATIONS Baur’s third category of variations comprises all inheritable changes due to causes other than segregation and recombination of genetic factors. Although comparatively little is known concerning the specific causes of mutations, yet it is possible to distinguish between two general - classes of such inheritable variations according to the nature of the genetic units involved. These classes are (1) alterations in genetic factors, and (2) deviations in the number of chromosomes. We designate the first group as factor mutations and the second as chromosome aber- rations. Since the first group is of vastly greater importance to agri- culture than the second, we shall consider the latter very briefly before engaging in discussion of the former, which we deem worthy of recognition as mutations in the strict sense. Chromosome Aberrations.—By the aid of cytology it has been demon- strated that inheritable changes are occasionally induced, in plants at least, by irregularities in the behavior of the chromosomes during mitosis or meiosis, such that certain germ cells contain fewer or more chromo- somes than the number typical of the species. Aberrant forms in several plant families are now known to differ from the parent species in chromosome number. Some have only a single chromosome more or less than the parent, while a few are known in which the original number is doubled. It is possible that aberrations occur involving all combina- tions of numbers between these two extremes. In various forms of La- marck’s evening primrose (Gnothera lamarckiana), whose typical number is 14, according to Gates the following aberrant numbers have been reported—15, 20, 21, 22, 23, 27, 28,29, 30. Aberrations involving the doubling of the number of chromosomes typical of the species is known as tetraploidy because there are four times the haploid number typical of the parent. Occasionally aberrations or hybridization between diploid and tetraploid forms result in triploidy. There is a limited amount of evidence which indicates that groups of species have arisen by progressive alterations in chromosome number. Thus in Drosophila, Metz has found ten species in which the chromosome numbers range from 6 to 12 and the larger numbers appear to have arisen by subdivision of the large dumbbell-shaped chromosomes found in the species having smaller numbers. Evidence 263 264 GENETICS IN RELATION TO AGRICULTURE that doubling of the chromosome number may occur during somato- genesis has been found by Farmer and Digby in the interesting hybrid, Primula kewensis. The original plant, which was sterile, “had 18 and 9 chromosomes in its premeiotic and. postmeiotic nuclei respectively,” but in the fertile plants which were propagated asexually from it, as well as in similar fertile hybrids which were produced in later experi- ments, the diploid and haploid numbers were 36 and 18 respectively. Having found by means of careful measurements of the chromosomes in the two forms that the nuclei in both forms contain the same volume of chromatin, the authors conclude that the increase in number may be attributed to transverse fission of the 18 larger chromosomes and not to the fusion of two nuclei. From a study of chromosomal dimensions in relation to phylogeny, Meek “‘arrived at the conclusion that the widths of chromosomes are successively greater in higher zoological phyla, and that this dimension is constant for very large groups of animals.” But Farmer and Digby have shown that such a conclusion is without foundation since “closely related forms may possess chromosomes differing widely in shape and size and character.’”’ Hence they conclude “that phylogenetic affinity is not, necessarily, correlated with chromosome width.” They also point out that ‘unfortunately we know practically nothing about the phylogeny of the chromosomes. No convincing hypothesis has been put forward to explain how these remarkable bodies have become organized, nor how their peculiarities have either been brought into existence or are kept so true for a given species.”” However, we are reminded by Glaser that chromatin is present in bacteria though not in the form of a nucleus and it may not be too much to hope that cytology may yet discover the principal stages in the development of the chromo- somes and establish such correlation as may exist between this develop- ment and organic evolution. Certainly extended investigations of chromosome numbers must be made before chromosome aberrations can be considered an important factor. in evolution. Except that certain chromosome aberrations, such as tetraploidy causing gigantism, might be of economic value, in general this class of mutations is of minor importance in breeding. Factor mutations, on the other hand, are of prime importance and of general occurrence. Factor mutations have appeared in controlled cultures of many animals and plants and the character differences con- ditioned by them are as a rule such as distinguish varieties of a single species. Moreover, varietal characters are Mendelizing characters in the narrow sense and the existence of simple Mendelian phenomena among all classes of sexually propagated organisms proves that factor mutations are of general occurrence. Although it is probable that every MUTATIONS 265 factor mutation has a certain effect upon every character in the organism, yet the visible effects of some factor differences are restricted to a single character. According to their visible effects, therefore, we recognize two classes of factor mutations: (1) those conditioning apparently only single characters; (2) those having a visible manifold effect on the soma.’ Cases Fig. 110.—A seedling of the oak-like walnut (left) and of the California black walnut, the parent species (right.) involving mutations of the second class are known in several species of animals and plants. An interesting example is the oak-like walnut, Juglans californica var. quercina, which appears to differ from the parent species by a single factor difference, Fig. 110. But this variety is distinct from the species type in nearly all gross morphological characters. 266 GENETICS IN RELATION TO AGRICULTURE The Nature and Causes of Factor Mutations.—Our knowledge of genetic factors is entirely of an inferential sort and it is probable that these ultimate hereditary units are no more likely to be objectively perceived than are the atoms of which all matter is generally believed to be composed. But our present understanding of biochemistry and the chromosome mechanism of heredity leaves no room for doubt concerning the theoretical nature of these factors. Living protoplasm is generally considered as composed of very complex organic compounds. The phenomena of stereochemistry, especially the substitutional or cyclic changes which occur within various compounds under proper con- ditions, suggest that similar compensatory relations exist between the substances composing the living cell. Yet cytological observations indicate that the chromatin is the only permanent constituent of the nucleus and that the chromosomes are unaffected by the regular physio- logical processes of metabolism, growth and reaction to stimuli even though they play a very definite rdle in all these activities. As was explained in Chapter IV, the chromosomes are linear series of loci whereat are located specific factors. According to the multiple allelomorph hypothesis more than one factor may exist at a given locus. Since the chromosomes appear to consist of the only permanent substance in the nucleus, it is conceivable that at each locus there exists a unique chemical system; yet it is not unreasonable to suppose that occasionally substi- tutional changes similar to those known to take place in less complex organic compounds may occur here. The contributions of Reichert on the specificity of proteins and carbohydrates as a basis for the classification of animals and plants are based on the fact that such substances as serum albumin, hemoglobin, glycogen and starch exist in stereoisomeric forms. That is, ‘each kind of substance may exist in a number of forms, all of which forms have the same molecular formula and the same fundamental properties in common, but each in accordance with: variations in intramolecular configuration has certain individualities which distinguish it from others. ... It has been found that the number of possible forms of each substance is de- pendent upon the possible number of variations of the arrangements of the molecular components in the three dimensions of space, or, in other words, of variations of molecular configuration, the possible number in case of each substance being capable of mathematical determination. Thus, we find that serum albumin may exist in as many as a thousand million forms. Hemoglobin, the red coloring matter of vertebrate blood, is a far more complex carbon compound than serum albumin, and theoret- ically may exist in forms whose number is beyond human conception, running into millions of millions. The same is true of starch.’ Having in mind this complex molecular structure of protoplasmic constituents MUTATIONS 267 and the phenomenon of substitutional changes of atoms or radicals by which such complex compounds are transformed, we can express a conception of the nature of factor mutations. To be specific let us suppose that some unusual condition occurs in a certain germ cell of a normal female Drosophila such that a single atom in each of the very complex molecules of the substance unique for the locus W in the X-chromosome changes place with a different atom in the surrounding nucleoplasm—the substance unique for the locus W is no longer capable of conditioning the laying down of red pigment in the eyes and, if the affected ovum is fertilized by a Y-bearing sperm, a white-eyed male appears, the result, as we say, of a factor mutation. This conception of factor mutations is useful as a basis for the multiple allelomorph hypothesis. In order to explain how two or more factors may have the same locus in a chromosome, it is only necessary to assume as possible the substitution of two or more different atoms or radicals in the molecule of the complex organic substance unique for the given locus by other atoms or radicals in the nucleoplasm. Factors are relatively stable entities however. It has been shown al- ready that any organism must possess thousands of factors, yet mutations are comparatively rare even in Drosophila. These facts are rather difficult to harmonize with our conception of the nature of factor muta- tions. If substitutions of atoms or radicals occur why do they not take place more frequently? Such questions must remain obscure until we know something about the chemical constitution of the hereditary factors. Only then can we expect to understand clearly the nature of the altera- tions which occasionally are made in them. In this connection the behavior of factor mutations in inheritance is of decided interest. As a rule they are recessive to their normal allelo- morphs and for some time they were thought to be due to the loss of factors, this idea being associated with the presence and absence hypothe- sis. But on rare occasions dominant mutations have appeared. Among 150 mutations from the normal type of Drosophila ampelophila several, such as bar eye, dark streak on thorax, abnormal abdomen and CIII, a factor which modifies eosin eye color, are dominant over their respective allelomorphs. A few other mutant characters have been found to be dominant, such as hornlessness in cattle and red buds in the evening prif- rose (Hinothera rubricalyx), but the great majority are recessive as 1s indicated by the ratio in Ff, from crosses between mutants and normal individuals. The condition in F; by no means always indicates complete dominance of the normal character. Hence it is clear that whatever the nature of the mutation-producing chemical change may be, as a rule it is either completely subordinate to the normal condition or else it merely modifies the effect of the normal state in heterozygous individuals, making 268 GENETICS IN RELATION TO AGRICULTURE its own distinctive manifestation in one-fourth of the progeny of such individuals. When we enquire as to what are the particular conditions or specific antecedent events that make possible or cause the assumed substitution of atoms or radicals, we find ourselves again confronted by an almost total lack of knowledge. One thing is certain however, namely, that factor mutations are not fortuitous in occurrence, because, if they were the outcome of wholly indeterminate series of events, they would be as likely to occur in one species or race as in another at a given time and with the same relative frequency under all conditions, but such is not the case. On the contrary, certain species appear to be much more prolific in factor mutations than others and, as stated in Chapter II, it would appear that inheritable variations can be induced under controlled environmental conditions in pedigree strains that have bred true for a number of generations. Furthermore, even though our knowledge of the occurrence of factor mutations were so meager as to furnish no basis for reasoning and even though future observations of the same might seem to indicate that they are fortuitous, we should still be justified in assuming the existence of specific causes for factor mutations. It has been clearly shown by Pearl that, while natural phenomena are the result of long series of antecedent events or con- ditions, yet these are not all of equal determinative value; but rather that there are always specific causes which are few in number, immediate in time and large in relative quantitative effect. It does not seem necessary to present here the course of reasoning on which this con- clusion rests. The important thing for agriculture is the fact that factor mutations are caused and the possibility that some of the deter- minative antecedent conditions are external to organisms, 7.e., that they exist in the environment and are controllable by man. The prob- lem of the exact nature of factor mutations is only a phase of the general problem of the nature of living protoplasm, the solution of which is one of the ultimate aims of biology. But it is possible at least that experimental research may reveal methods by which factor mutations can be induced in both plants and animals. Factor Mutations Both Germinal and Somatic.—F actor mutations appear to occur in undifferentiated cells, the germ plasm or embry- onic tissue in animals and either the germ cells or any meristematic tissue in plants. Occasional discontinuous variations are found in animals which might seem at first to be due to factor mutations in the developing soma. But most of these abnormalities are more satis- factorily explained in other ways. Thus, gynandromorphism, or the condition of having one side of the body male and the other female, has been reported in insects more than a thousand times according to MUTATIONS 269 Morgan. Without doubt it is caused by some irregularity in the proc- ess of fertilization. Homeosis, or the replacement of one organ by another, is known to have followed mutilation. Examples of the modi- fication of characters by environmental conditions are given in Chapter II. There are many similar variations in animals, none of which are hereditary. However, we shall again refer to the possibility of somatic mutations in animals. There is no direct evidence as to the cytological time of factor muta- tions, but the stage in the germ cell cycle of animals at which factor mutations are most likely to occur would seem to be shortly before or during the process of maturation. This is indicated by the sporadic appearance of mutants. The first observed mutation in Drosophila ampelophila was white eyes, which were found in a few males among several hundred individuals in a pedigreed red-eyed race. Similarly with other sex-linked mutant characters that have been observed in this species, they have appeared either singly or at most in a few individuals. Had these mutations occurred at an earlier stage in the germ cell cycle, more gametes would have been affected and more mutant individuals would have been found. Obviously the length of time that must elapse before a factor mutation can manifest its existence depends upon two things in addition to the stage in the germ cell cycle at which it occurred: (1) its relation to its normal allelomorph, 7.e., whether it is dominant or recessive; (2), its relation to sex determination, 7.e., whether it is sex- linked or not. A mutation from W to w in an X-chromosome of a normal male Drosophila would have produced a heterozygous red-eyed female in the next generation and no white-eyed flies whatever. One-fourth of the progeny of such a female would in turn be white-eyed if she mated with a normal male. Similarly with any non-sex-linked recessive character which upon its first appearance in pedigree culture is found in more than a single individual the probable order of events is as follows. A muta- tion occurred in a single germ cell of a single individual, which mated with a normal individual, thus giving rise to one heterozygote among its progeny. This heterozygous individual mated with a normal individual, producing heterozygotes among one-half of their progeny. Finally some of these heterozygotes mated together and one-fourth of their progeny bore the recessive mutant character. It would seem, therefore, that factor mutations in animals occur in the germ cells shortly before or during maturation and the time of appear- ance of a mutant character depends upon the relation of the mutant factor to its normal allelomorph and whether or not it is contained in the sex chromosome. In plants factor mutations may occur in any meristematic tissue as well as in the germ cells. Observations on the occurrence of mutant 270 GENETICS IN RELATION TO AGRICULTURE seedlings indicate that, as in animals, germinal mutations usually occur just before or during the maturation process. The strongest evidence for this conclusion is the fact that, so far as known, new dominant char- acters appear first in only one or two individuals. The following cases illustrate this point. The red-leaved evening primrose, Cnothera rubricalyx (Fig. 118) has been known to occur but once in all Ginothera cultures and then in a single plant. The red sunflower, Helianthus lenticularis coronatus, as reported by Cockerell, first appeared as a single plant which proved later to be a heterozygous dominant. 1718 white ee ge preaSS (cRBE or CrBE) : i CoS 1731 white Painted Lady OS Fi (=pink and 2 \ ~ white) CRbE XN SS Se =e 1793 white Painted Lady scarlet—CRbe black —S (= deeprose) (=dark violet) | 1806 white Old Painted NewPainted dark purple | blue Lady Lady or eae 7 Scarlet PS i 1817 striped (= purple with brown, lavender or white?) 1824 yellow white * (= primrose?) 1840 white Old Painted NewPainted dark purple fs dark bluish Lady Lady and deep H purple and violet | pale blue | | 1845-49 New Large New Striped Rear 1850 New Large : Dark Purple 1860 yellow (= primrose?) Blue Edged 1865 Scarlet In- vincible yellow, white Painted Lady, Scarlet In- black, Imper- purple Blue Edged Crown Prin- __vincible, ial Purple, striped 1870 cess of Prussia scarlet, scar- black with with white. List (pink and rose let striped light blue. pink). with white. yellow, white Painted Lady, New Painted Black, Black purplestriped Blue Edged? Crown Prin- Lady, scarlet, Invincible, with white, Butterfly’ cess of Prus- Searlet Invin- black with Invincible Captain sia, Fairy cible, scarlet light blue, Striped Violet Clarke‘ 1880 Queen (= striped with Large dark Queen. Hetero- List pink on white. purple, Im- sperma® white), Queen (= light pink and pink pur- ple). perial purple, Purple In- vincible. 1 In each ease the color of the standard or banner is given first and of the wings second; the descrip- tive terms and variety names are identical with those in the original descriptions. 2 Described by Bailey and Wyman as purple-lilac in color (= purple picotee). 3 Quite similar to Blue Edged according to Beal (=purple picotee). 4 = “white merging into pink and purple, wings white with purplish cast, wings edged with blue”’ (= purple picotee). 5 No description available; mottled seeds? 20 306 GENETICS IN RELATION TO AGRICULTURE the garden traces of the early peculiar form of the flower portrayed in Plate III. In the original form the standard was erect, narrow at the base, notched at the top, and reflexed or slightly rolled at the sides. From it have been derived three distinct flower types; the grandiflora, the hooded, and the popular waved Spencer forms. The origin of the first two named is in some doubt. The hooded character was found in some of the earlier varieties. It was sometimes associated with notches in the sides as in the Butterfly (Fig. 121), and this character is found also Fig. 121.—Forms of sweet pea flowers—the standard or banner. Open or grandiflora form (upper row left to right)—-Alba Magnifica, Shasta, Golden Rose. Hooded form (middle row)—Butterfly, Admiration, Dorothy Eckford. Waved form (lower row)—Elsie Herbert, Apple Blossom Spencer, White Spencer. (From Beal.) in some of the present day favorites. Bateson reports that hooded is recessive to grandiflora or erect type of standard. Some of the earliest rarieties of improved grandiflora form were Queen of England (1888), Blanche Ferry (1889) and Alba Magnifica (1891). The waved or Spencer form is of more recent origin, and authorities are agreed that it arose as a “sport”? from a beautiful, pink, hooded variety, Prima Donna. The pronounced waviness of standard and wings which characterizes this type had not appeared before in sweet peas. The two upper series in Fig. 121 indicate the more recent progress in enlarging flower size. Alba Magnifica and Butterfly were great acqui- ON VARIETIES IN PLANTS 307 sitions in their day and were doubtless considerably larger than the oldest varieties. The first definite reference to size is found in New Large Purple, listed in 1845. As this occurs in the darkest color group and 15 years before the hybrid origin of a new variety, Blue Edged, was even suggested, it probably represents a factor mutation. That such mutations actually occurred in the sweet pea is proved by the fact that Countess Spencer and Gladys Unwin were both decidedly larger than Prima Donna from the very first. The same is true as regards number of flowers in the cluster. Prima Donna, according to Beal’s description, bore two or three, usually three, flowers on a stalk, while Countess Spencer has three to four flowers in a cluster. Many of the recent Spencer Fria. 122.—On the left, Snapdragon sweet peas. On the right, double sweet pea, White Wonder. (From Beal.) varieties bear almost uniformly four-flowered clusters. The original form and earliest varieties had two flowers in the cluster. The oldest varieties definitely known to bear more than two flowers on a stalk are Invincible Scarlet (1865) and Crown Princess of Prussia (1868). As these antedate the era of hybridization it is probable that the increased number arose by mutation. Novelty forms have also arisen from time to time. In double sweet peas there are two standards instead of one. In some varieties this character has been fixed by selection so that most of the flowers come double. It gives the effect of increased size (Fig. 122). In the snap- dragon type of flower (Fig. 122) the standard is folded around the wings. It is recessive to erect standard and gives a simple Mendelian ratio of 3 erect to 1 snapdragon in F». 308 GENETICS IN RELATION TO AGRICULTURE Habit in Sweet Peas.—There are several distinct types of plant in the sweet pea the origin of which may be definitely ascribed to mutation. The first Cupid plant (Fig. 123a) appeared among plants of the tall, Fig. 123.—a, Cupid or prostrate, dwarf sweet pea; 6, bush or erect, tall form; c, Cupid X bush F;, the ordinary tall form (folded over in order to photograph). (From Bateson.) white-flowered variety, Emily Henderson, in 1893. The growers, C. C. Morse & Co. of San Francisco, raised seven acres of the new variety in 1895 and every plant was true to type. This mutation has since oc- Fie. 124.—Dwarf or Cupid sweet peas. J, ordinary or prostrate Cupid; JJ, erect Cupid, the F2 double recessive from bush X Cupid. (From Bateson.) curred in a number of widely separated localities. The bush type also originated as a mutation from the tall form. The investigations of the factor relations of bush and Cupid sweet peas have been described in ON VARIETIES IN PLANTS 309 a previous chapter. Semi-dwarf, early-flowering sports have appeared even more frequently than those of the Cupid type. They have been made the basis of the winter-flowering types of sweet peas. Ordinary sweet peas pass into a semi-dormant condition for atime after germination, growing very slowly until sideshoots have been developed. The winter- flowering sorts, however, promptly send up a central axis which begins blossoming as soon as it has attained a height of from two to four feet. The Blanche Ferry group of varieties apparently had their inception in a mutation of this sort which a woman in northern New York noticed among some plants of the Old Painted Lady. She selected them for about twenty-five years after which they passed into the hands of a seedsman. From this stock a series of early flowering mutations have arisen in the order shown below. Black-seeded varieties are indicated by (b) and white-seeded ones by (w). Old saa | Lady (b) Bright-flowered sport (b) (30 years later) Blanche Ferry (b) 4b oa a 7S gan ge Extra Early Blanche Ferry (6) Emily Henderson (white, w) Harliest of all (b) Mont Blane (early white, w) | Extreme Early Earliest of all (b) Karliest Sunbeams (primrose, w ) Earliest White (b) Fia. 125.—New varieties of sweet peas which originated by mutation among the progeny of Old Painted Lady. Hybridization and Selection in Sweet Peas.—The era of extensive hybridization in sweet peas dates from about the year 1880, consequently we can say but little of definiteness after that time with respect to the origin of new factors in the sweet pea save in a few particularly favorable eases. Laxton’s Invincible Carmine was the earliest recorded new variety which was produced by crossing, and its parents are reputed to have been Invincible Scarlet and Invincible Black. We can easily understand, therefore, how it originated, for it is apparently merely an improved form of Invincible Scarlet resulting from the inclusion of the factor for intense pigmentation of Invineible Black in the factor complex of Invincible Scar- let. Similarly by hybridization it has been found possible to establish families of varieties such as the Spencer, the hooded, the grandiflora, and the winter-flowering sorts. Hybridization has throughout been merely a means of fully utilizing germinal differences which have arisen by mutation. It is true that in most cases we cannot say just when the 310 GENETICS IN RELATION TO AGRICULTURE particular features of form, color, and habit have arisen but we know that there was only one original form, and fragments of the history (Beal and Hurst) are sufficiently clear to give us assurance in advanc- ing this explanation of the réle of hybridization in the creation of varieties of sweet peas. There is no authentic instance of a variety having origi- nated from hybridization of the sweet pea proper, Lathyrus odoratus, with any other species of Lathyrus, consequently the possibility of such germinal diversity is precluded. Similarly in the case of selection for more obscure characters such as number of blossoms in the cluster, size of flower, and vigor of growth, apparently the things that have been utilized in cases of improvement are mutations and new combinations of mutant factors. Fig. 126.—Four types of rose: a, typical modern Hybrid Tea rose, 6, typical Hybrid Perpetual rose; c, the Damask rose, which was popular in old gardens; d, the old single Rosa gallica. (Reproduced from The Garden Magazine by permission.) Creation of Varieties of the Rose.—No finer examples of the origin of horticultural varieties by means of hybridization could be found than the garden roses of today. The genus Rosa is widely distributed in the Northern Hemisphere and contains several hundred species of which, according to Wilson, twenty-six have been utilized in the production of our garden roses. But these twenty-six species fall into fifteen distinct groups, and in habitat they represent Asia, Europe, and North America. The most important group of modern roses are the Hybrid Teas for ON VARIETIES IN PLANTS oll they include garden and forcing varieties which combine marvellous beauty of form and color with vigor and hardiness (Fig. 1264). Four or possibly five distinct species enter into the ancestry of the group, as shown by the following pedigree. The Hybrid Perpetuals (Fig. 126b) are of mixed ancestry, all being hybrids of the Damask Rose (Fig. 126c) crossed either with Hybrid Bourbon or Hybrid Chinese varieties. The hardy, disease-resistant Japanese species, Rosa rugosa and R. wichuriana have entered into the ancestry of some of the best modern roses. Thus, the American Pillar variety is a hybrid between a red Hybrid Perpetual crossed with a hybrid between R. wichuriana and R. setigera, the Prairie Rose of America. Again, the Silver Moon variety is a result me SNES: tase et. derivatives of........... Rosa chinensis var. odoratissima oe Hybrid _ Rosa gallica ~\ Hybrid J, Chines (French or Provence Perpetuals Rose). See Fig. 126d. \™~ Hybrid Bourbons —1/ 2 |22-1 a heterozygotes where any num- ber, m, of pairs of heterozygous factors is involved in_ the original population. Thus starting out with a single plant having m pairs of heterozygous factors, or a population consisting wholly of such plants, the value for h, the proportion of heterozygous individuals, is given by the expression: - This expression is very useful for determining the degree of homo- geneity which a hybrid population may be expected to exhibit after a given number of generations of self-fertilization. Thus assuming that there are 10 pairs of factors in a given cross, what proportion of hetero- zygotes will there be after five generations of sowing? The formula is Spee 10 h=1—(“-)- THE COMPOSITION OF PLANT POPULATIONS 321 Solving we obtain h = 0.27; in other words, the chances are only about one in four that a plant selected from a population of this kind will be heterozygous. If there are 100 pairs of factors and ten generations of self-fertilization only 9 per cent. of the population will be heterozygous. Thus: we see how powerful is the tendency of self-fertilization to reduce the population to a homozygous condition. The number of homozygous genotypes to, which the population will be reduced, it should be remembered, is given by the expression, 2”, in which m again is the number of pairs of heterozygous factors. If there are 10 pairs of heterozygous factors in the original individual, then the population will ultimately be reduced to 1024 different homozygous genotypes; if there are 100 pairs of such factors, the number of different kinds of genotypes is approximately 1,267,666 10%". We should always remember in working with formule such as these that they are only valid for conditions postulated in the premises. For the above formule the following conditions are assumed: roughly equal viability of all genotypes, absence of any natural selection, and independent segregation of factors. Obviously none of these condi- tions is fulfilled in any even moderately complex population. We have already considered many examples of different viability in diverse geno- types, of which the many different Drosophila mutants provide the most conspicuous examples. Similarly natural selection of necessity enters in whenever any differences whatever exist in the ability of different geno- types to survive and reproduce themselves under a given set of condi- tions. In addition to these two obvious difficulties the universal occurrence of linkage also profoundly disturbs the mathematical rela- tions whenever any considerable number of factors is concerned in a given cross. It would be a very rare occurrence for even ten different pairs of factors to exhibit independent assortment in any plant species, impossible in a species like wheat which has but eight pairs of chromosomes. The biological significance of this mathematical discussion is merely this: that it demonstrates that populations in which self-fertilization is an invariable condition in seed formation must consist entirely of pure lines, if left undisturbed for a very few generations. Mathematic- ally the limiting condition is one in which all possible pure lines exist in constant proportions in the population, but biologically the limiting ‘condition is one in which the population is composed only cf the most vigorous and productive pure lines. Populations as Affected by Crossing—When a certain amount of natural crossing occurs the relations above described are somewhat disturbed. The population, of course, tends to reach an equilibrium, and for all practical purposes does reach one very soon, but the mathe- 21 322 GENETICS IN RELATION TO AGRICULTURE matical relations are much more complex than those given above. We may consider a simple case, however, and show the relations in that case. If we start out with a population consisting of equal numbers AA and aa forms, and assume that a given percentage of crossing occurs, then an equilibrium will be reached when the number of homozygotes produced by the heterozygotes in the population is equal to the number of hetero- zygotes produced by spontaneous crossing. Thus, if we assume 10 per cent. of spontaneous crossing in such a population, in the first gen- eration of the 10 per cent. of AA which cross with other plants, half will be fertilized by other AA plants and half by aa. The latter will give heterozygotes, consequently the proportions of different genotypes produced by the AA plants will be 0.954A:0.05Aa. Similarly aa plants produce 0.05Aa: 0.95aa, so that in the first generation the ratio is 0.95AA : 0.10Aa :0.95aa. Now in the next following generation if we assume that random mating occurs among the 10 per cent. of plants which cross with other plants, then one-third of the plants in each genotype will mate with the same genotype, one-third with one of the other two geno- types, and one-third with the remaining genotype. That is, of the 0.95A4A one-tenth or 0.095 cross, as follows: 4AA X AA = 0.3244, 1ZAA X aa = 0.032Aa and 144AA X Aa = 0.016AA :0.016Aa. Simi- larly, of the 0.95aa, 0.095 cross: Maa X aa = 0.032aa, 44aa X AA = 0.032Aa and lgaa X Aa = 0.016Aa:0.016aa. Also of the 0.10Aa, one-tenth or 0.01 cross: 44A4a X AA = 0.0016A A: 0.0016Aa, 14Aa X aa = 0.0016Aa :0.0016aa and WwAa X Aa = 0.0008AA : 0.0016Aa: 0.0008aa. Summating like genotypes we have 0.05AA :0.10Aa : 0.05aa. The 90 per cent. of AA and aa plants which are self-fertilized produce 0.855A4A and 0.855aa respectively, while the 0.09Aa@ plants which are self-fertilized produce 0.02254A : 0.045Aa : 0.0225aa. Combining these with the results of cross-fertilization we have the ratio for the second generation, 0.92844 :0.146Aa:0.928aa. Now the ratio of the proportion of homozygotes to the population in the first generation is of course 0.95 and in the second generation it becomes, 0.928 + 0.928 0.928 + 0.146 + 0.928 ~ 9-927 The composition of the third, fourth and fifth generations and the ratio of the proportion of homozygotes to total population for each are shown in Table XLVII. It is evident that, under the conditions assumed in. this case, the rate of change in the ratio of homozygotes to the total population becomes very gradual after the first three generations, so that for practical purposes the population has reached a state of equilibrium in the fourth generation. In this generation the ratio of heterozygous dominants to the sum of the heterozygous and homozygous THE COMPOSITION OF PLANT POPULATIONS 323 dominants is 0.16 +. In this or later generations, therefore, the chances of selecting at random a heterozygous dominant, assuming dominance to be complete, are about one in six. Table XLVII, shows the composition of the population with ref- erence to a single pair of factors, A anda, in the first five generations when there is 10 per cent. of spontaneous crossing, assuming (1) that be- fore crossing began there were equal numbers of AA and aa plants; (2) that among the 10 per cent. of plants which cross random mating occurs: (3) equal fertility and viability in all individuals. Starting again with a population of AA and aa forms we find that, assuming 20 per cent. of crossing in this instance, other conditions being the same, the ratio of homozygotes to the whole population in the first four generations is as fol- lows: 0.90, 0.86, 0.845 and TasLeE XLVII.—Compositi1on or POPULATION 0.837; while the ratio of ere Ratiowyy* : Generation Aa aa heterozygous dominants to ———! | Leesa: the total dominants in 1 0.95 0.10 0.95 | 0.95 the fourth generation is 2 0.928 | 0.146 | 0.928 | 0.927 0.27. Hence, in this and ; ae Fie peng en ; : .915 dif). |) COSINE) S| (Ooi dile later generations the chance 2 Sune | gman ieucaiaes ueant of selecting a heterozygous dominant is about one in *z = proportion of homozygotes in the popula- four. Again, with 50 per tion; y = value of total population. cent. of crossing the ratio of homozygotes to the whole population in the first four genera- tions is 0.50, 0.625, 0.649, 0.662; and the ratio of heterozygous dominants to the total dominants in the fourth generation is 0.50+, so that the chance of selecting a heterozygous dominant is one in two. In the same way the theoretical expectation for any particular amount of crossing may be calculated. It must be borne in mind, of course, that we have made no allowance for greater relative vigor and productivity in the heterozygous plants. However, the method illustrated may be utilized in working out similar problems in which the genetic relations are disturbed by such conditions as difference in viability or fecundity as well as for various amounts of crossing. This brief consideration merely suggests the possibilities of mathe- matical analysis of the composition of populations under assumed condi- tions. It must be clear, however, that such analysis as applied to a given set of conditions would be of very great value in conducting breeding investigations. But it should be remembered that reliable conclusions regarding any particular case cannot be derived from such analysis unless the more important controlling agencies at least have been so carefully investigated that their combined influence can be duly esti- 324 GENETICS IN RELATION TO AGRICULTURE mated. On the other hand, the general principles derived from the mathematical study of the composition of populations are of universal application. These principles may be summarized as follows: 1. (a) Continued self-fertilization tends to eliminate all heterozygotes from the population. (b) The number of homozygous genotypes to which a self-fertilized population will be reduced depends upon the number of pairs of factors involved. (c) Such a population after a few generations will consist entirely of pure lines. 2. (a) With a given amount of natural crossing in the absence of any disturbing effects there will be an approximation toward a definite pro- portion of heterozygotes in the population. (b) Such a population approaches very nearly a condition of equilib- rium within a few generations. (c) Under the influence of disturbing elements the proportion of heterozygotes may be increased or decreased, but the condition of equilibrium will be rapidly approached if the disturbing elements remain fairly constant. CHAPTER XVIII SELECTION The oldest and most generally used means of plant improvement must continue to be the basic method in systematic plant breeding. Although selection is universally recognized as an effective method of breeding, yet all too long the prevailing ideas among empirical breeders regarding the way in which selection effects improvement and the reasons why selection sometimes fails in securing the end desired have been exceed- ingly vague. The confusion of thought concerning this matter which still exists among both scientists and laymen is largely due to a lack of clear understanding concerning the nature of variation. The variations upon which selection can be used effectively owe their origin either to mutations or to recombinations of genetic factors. On account of the differences in the composition of populations in various species of plants the effects of selection differ greatly in different.crops. In order to employ selection most economically the plant breeder should understand the nature of the population with which he is working and the genetic prin- ciples underlying effective selection. It is our purpose in this chapter to set forth the principles of selection in both allogamous and autogamous species. Selection Methods in Maize Breeding.—The maize plant is highly variable and many different varieties and strains have been produced by selection. In most of the states where corn is grown extensively the experiment stations have published bulletins on corn improvement and the subject is discussed in more or less detail in various works on plant breeding. We shall merely: consider here certain methods of maize selection in order to illustrate the principles involved and to compare them with methods used in other crop plants. Inbreeding in Maize.—Self-fertilization in maize results in marked reduction in vigor and hence in size of plant and production of seed. This was first discovered by Shull, who applied the pure line method in corn breeding, and from his results inferred that a field of maize consists of a collection of genetically distinct biotypes which may be isolated by in- breeding. East soon corroborated Shull’s discovery and later East and Hayes summarized the results of inbreeding a naturally cross-fertilized plant substantially as follows: 1. There is partial loss of power of development, causing reduction in 325 326 GENETICS IN RELATION TO AGRICULTURE the rapidity and amount of cell division. This phenomenon continues only to a certain point and is in no sense an actual degeneration. 2. There is an isolation of biotypes differing in morphological char- acters accompanying the loss of vigor. 3. The hereditary differences between these biotypes is often indi- cated by regression away from instead of toward the mean of the general population. 4. As these biotypes become more constant in their characters the loss of vigor ceases to be noticeable. 5. Normal biotypes with such hereditary characters that they may be called degenerate strains are sometimes, though rarely, isolated. 6. It is possible that pure strains may be isolated that are so lacking in vigor that the mechanism of cell division does not properly perform its function, and abnormalities are thereby produced. Thus we know that any commercial variety of corn is a mixture of different genotypes and that inbreeding tends to isolate pure genotypes, 7.e., inbred strains tend to become homozygous. Thus it is evident that the cross-bred progeny of two different inbred strains will be heterozy- gous for many factors. That cross-bred maize frequently displays greater vigor than either parent was first demonstrated by Beal of Michigan in 1878. But it was not until Shull and East demonstrated the existence of genotypes in maize that the genetic significance of this phenomenon became evident. The actual cause of the increased vigor has been ex- plained in various ways. Both Shull and East held that decrease in vigor in inbred strains is due to reduction in the number of heterozygous factor combinations and that increase in vigor in F, hybrids is the result of increase in the number of such combinations. The general occurrence of decrease in vigor upon inbreeding naturally cross-bred species and of increase in vigor upon crossing closely related forms led them to conclude that heterozygosis is the cause of increased physiological vigor in F hybrids. Other explanations of this phenomenon have been offered, one of which was that of Keeble and Pellew, to the effect that it ‘““may be due to the meeting in the zygote of dominant growth factors of more than one allelomorphic pair, one (or more) provided by the gametes of one parent, the other (or others) by the gametes of the other parent.” East and Hayes reject this hypothesis on the grounds that this increase in vigor “is too universal a phenomenon among crosses to have any such explanation. Furthermore, such interpretation would not fitly explain the fact that all maize varieties lose vigor when inbred.” But there is good evidence that all maize varieties do not lose vigor to the same extent when inbred and that certain genotypes produce much more vigorous F; hybrids when crossed than other genotypes. As was stated in Chapter XII, D. F. Jones has explained this increased vigor SELECTION O27 in F, hybrids in terms of dominance and linkage (p. 231, 2). The fact that different genotypes give diverse results when crossed is of immense practical significance. The Ear-to-row Method.—This has been the method of commercial corn improvement for many years and it is well illustrated by the Illinois corn breeding experiments, which have been going on continuously for over 20 years. The original purpose of the experiments was to produce new strains which would be more valuable as a source of feed for livestock. It was found that there was considerable variation in the relative amounts of protein and carbohydrates in the grains of different ears. Accordingly selection was begun with the object of increasing the protein and reducing the starch content of the grains; also of decreasing protein and increasing starch. As oil was worth three times as much as starch per unit of weight, selection for higher oil content was also begun. A low oil strain was started for comparison and such corn was soon found to be desirable for the production of pork and beef of high quality. The work was begun by Hopkins who picked out 163 ears of a local strain known as Burr’s White, made a chemical analysis of a few grains from each ear, and on that basis sorted them into four classes, viz., high and low protein and high and low oil. The strains were grown in isolated plots from the beginning. After 9 years of selection it was found to be necessary to prevent inbreeding. Accordingly in the tenth and succeed- ing years about 24 ears were selected for each plot and one ‘row was planted from each ear, then the even numbered rows were detasseled. Subsequent selections were made from the detasseled rows, the first consideration always being high yield. Usually 20 ears were taken from each of the six higher yielding rows, or 120 ears for each plot. These were tested by chemical analyses and the most extreme variants in the desired directions were selected for the next planting. The results in general have been more regular in the high and low oil series than in the high and low protein strains. In the latter there seems to have been no very decided effect of selection after the first 10 years. Similarly there has been no continuous advance in the low oil strain since the seventeenth year of selection, but in the high strain the per cent. of oil has continued to increase slightly. The progressive effects of selection in the four series are graphically illustrated in Figs. 133 and 134. That the striking results depicted in these graphs were not caused by environ- mental conditions was proved by planting mixed plots with two grains of “high” and two of “low” corn in each hill so arranged that the resulting plants could be identified. This test, according to L. H. Smith, was made for three successive years, and subsequent analyses showed that: under these conditions the different strains maintained their distinguishing chemical characters. 328 GENETICS IN RELATION TO AGRICULTURE The Illinois Station experiments have included selection for many other characters of the corn plant in more recent years. One of the most striking results was obtained by selecting for height of ear on the plant. Data on which to base selection were secured by measuring several hundred stalks in the oil and protein plots, noting height of ear above the ground, total height of stalk, apparent number of internodes below the Per Cent of Oil mm 9 Ow em No «= © 0 96 797 798 799 ‘00 ’01 *02 °03 04 705 706 ’07 08 '09 "10 "11 "12 713 14 °15 Year of Selection Fig. 133.—Two graphs representing the effects of selection for high and low oil content in the Illinois Station corn experiments. (Data from Castle.) ear and number of internodes above the ear. Fig. 135 shows the result of selecting for high and low ears during five generations. Similar results were obtained from selection in the case of position of ear at maturity and total yield. The striking results of these carefully conducted experiments have been cited by various authors as evidence par excellence for the most Per Cent of Protein 796 97 9899 00 *01 02 °03 °04 05 '06 '07 708 ’09 "10 °11 °12 "13 °14 °15 Year of Selection Fig. 134.—Two graphs representing the effects of selection for high and low protein con- tent in the Illinois Station corn experiments. (Data from Castle.) diverse conceptions of the réle which selection plays in evolution and breeding. Thus the earlier allusions of Hopkins and Smith, the discus- sion in EK. Davenport’s text on breeding, and the recent treatment by Castle all seem to attribute a peculiar creative power to selection which meets with a certain ‘‘response”’ on the part of the plant. This is in line with the Darwinian idea that all fluctuating variations are heritable and that the continuous selection of minor fluctuations in a certain direction is always effective in shifting the type. SELECTIONS 329 The futility of attempting to generalize regarding the effects of selec- tion in plants must be obvious from what we now know about the com- position of plant populations. With the application of Johannsen’s genotype conception in analyzing the composition of a field of maize the problem of explaining the role of selection in the Illinois corn breeding experiments was immediately simplified. This was perceived by Shull who pointed out that the results of these experiments might be readily explained on the ground that some hybrid combinations of genotypes have greater capacity for the production of the desired qualities than other combinations, and that the selection has gradually brought about a RE PO Ay Fig. 135.—Result of selecting corn for high and low ears during 5 generations. The white tape marks the position of the ears on the front row of plants in both plots. the segregation of those genotype combinations which had the highest capacity for the production of the desired quality. Meanwhile Surface had made an illuminating analysis of the data from the first 10 years of selection as reported by Smith. This treatment is so valuable as to warrant its examination in some detail. At the time the selections were made a careful record of the pedigree of each ear was kept. These pedigrees are of course for the maternal side only since self-pollination was not practised. From these data Surface prepared a pedigree chart for each of the four strains. The chart for the high- protein strain is reproduced in Tables XLVIII and XLIX. As stated above 24 ears containing the highest per cent. of protein were selected for the 163 ears analyzed in 1896. These were given registry numbers from 101 to 124 inclusive as shown in column one of the two tables. For convenience we may refer to these ears as the first generation of high- 330 GENETICS IN RELATION TO AGRICULTURE TasLe XLVIII.—Perpicree Cuart or Hicu-prorein Cornn—Parrl. (After Surface.) Generation Number 7 2 3 | 4 | 5 | 6 7 | 8 | Da} h0 | WW 101 102— 215— 320— 410— 502 103— 208— 314.. lia 104— 214 212165 421 105 401— 514 308.. | 299 106.. (513. ! 315— ‘418 3 417 319.. | 414 416 219— 301 LOTes (593 415 108. ; (ate 321... | 406 109 407 — 510 110 420 311.. : 506 — 60 111 411.. {513 312 212.. 313 404. 614 112. 309.. | 402 — 515.. | 606 603 408 ie S05) I eanee a08 |412.- {509 — 613 — 705 — g22 210 113.. {509 394 114— 204— 303 115— 224— 304 116— 202 117 118— 218— 302 Do == 307s — Hospeasll 119.. {591 305 120— 203— 308 Ss... SELECTION 331 TaBLE XLIX.—Pepiaree Cuart or Hicu-ProTeEIN Corn—Part II. (A flier Surface.) Generation Number | | 1 | 2 | 3 | 4.155 6 (arate | 9 | LOW} 913 Eile | | 901 808 { L 70.. | Sis 1 908 609... 801 1007 512. \ 702 ae | 1002 912.. | 1014 Cane 1019 ) pela air Tg SEs OY 925 | 1001 | noe ; 1020. 601 — 710... | S101 G20. ag” 1115 ee | ae ime | 1122 Bonen | 916 207 —323.. | 413. | 902 121 a 719) 811---) 995 { 1009 fF iige vee 914 ‘4 Th 1114 ae cate | 1021 } 1107 | 1119 906 806. | 1017 1113 ines | 924.. 4 1024 | 1101 | 1012. 5 3499 802 | 1005 | 1108 903 507... 4 704 | 820.. / 917 910 508 607.. i am 1010, / 1106 { 1123 716— 807 \ 1118 fou | 818.. | 922.. | 1003 me dai a | 804 (915 re of tle age) Pian Me) CR CIP ae eg 720 923 1124 (708 | 815 709... ( 907 oan 803.. 1913 1011 817 | 904 fan 703 ( 918.. | 101s { 1106 (ezOle)- | 809... 1006. ) 1146 2 123 216 — 318 ets tone ya eaio. 504— 610.. / 718— 816— 909 332 GENETICS IN RELATION TO AGRICULTURE protein corn. The next season 4 sound ears were analyzed from each of the twenty-four rows. From these 96 ears the 24 again having the highest per cent. of protein were selected for planting. The distribution of these selected ears among the 24 original ears is shown in column two of the tables. For example, it is seen that ear No. 124 produced 2 ears, Nos. 216 and 209, which were among the first 24 as regards protein content. Ear No. 123 on the other hand failed to produce any ear (so far as the ears analyzed showed) sufficiently rich in protein to be in- cluded among the first twenty-four. Thus 8 of the original ears fail to be represented in the second generation, while 8 other ears contributed 2 ears each for planting the following year. Exactly the same selection was practised in the second year and the resulting selected ears are shown in the third column of the tables. Of the 16 original ears rep- resented in the second generation only one, No. 116, was dropped out in the third generation but in the next generation there is a significant dropping out of some of the original lines, so that in the fourth genera- tion only 9 of the original 24 ears are represented by progeny. Five of the original lines contribute 80 per cent. of this generation, while two lines, 106 and 112, contribute nearly 60 per cent. Hence at the end of the fourth generation it is clear that certain of the original lines have a much greater tendency to produce ears with a high per cent. of protein. By simply selecting on the basis of the protein content of the individual ear for 4 years 70 per cent. of the original lines have been dropped. Thus the elimination of the original lines gradually proceeds until, in the tenth and eleventh generations all of the high-protein corn is the . offspring of a single ear, viz., No. 121. It will be remembered that in the tenth year the method of detasseling alternate rows and saving seed from these only was put into effect. But this change in method could not have induced the results we have noted because line No. 121 had demon- strated its superiority over all the others as early as the seventh genera- tion. This isolation of a single line was brought about therefore simply by selecting each year those individual ears that showed the highest per cent. of protein. Starting with a protein content of 10.92 per cent., at the end of the third year (fourth generation, 1899) the protein content was only 11.46 per cent. or a gain of 0.54 per cent. But the next year (fifth generation) the protein content jumped to 12.32 or a gain of 0.86 per cent. in 1 year. Referring now to Table XLIX it is seen that it is in 1899 that a great reduction was made in the number of lines represented, for in the fifth generation only six of the original twenty-four lines remain. Furthermore it is Just here that line No. 121 begins to show its superiority since 5 of the 15 ears selected in 1900 or 3314 per cent. come from this line. The course of events in the other three strains was similar but not SELECTION 333 quite so striking. In the low-protein strain only two of the twelve original lines are represented in the eleventh generation; in the high-oil strain three lines out of twenty-four are maintained throughout the 10-year period; and in the low-oil strain only two lines out of twelve are represented in the eleventh generation. These results are exactly what would necessarily accrue in any al- logamous species under continuous selection for a given character, pro- vided the degree of expression of that character is dependent upon a number of genetic factors. That several chemical characters of the corn grain, including protein and oil (fat), are inherited in accordance with Mendelian principles was determined by Pearl and Bartlett in 1911. In a cross between a white sweet corn and a yellow TasieL.—Generic Revations BETWEEN CERTAIN s as x1 YHARACTERS OF THE CorRN GRAIN. starchy corn determina- PHYSIOLOGICAL CHAR Ss H NG tions were made by direct Character | Dominant Recessive analysis of the percentage -————— : content of the grains of Mboisture........... High Low the pure parent races and Nitrogen and protein, Low High the F, and F, progeny in Crudestatsi25 33508 Low (incomplete oes : : dominance) respect to nine chemical 4. 0... vise een constituents. These are (Crude fiber......... Low | High listed in Table L, which Pentosans.......... Low (incomplete | High also indicates the dominant | dominance) ves and recessive conditions of SuCrosetm erie utero | Low (incomplete | High : | dominance) | these characters in the IDextrosean. cece ae | Low | High cross studied. Starch ste een fiw: High Low This evidence, although worked out quite independ- ently, supplements Surface’s analysis of the Illinois data in a remark- able way. Although there are technical obstacles to a clear cut de- termination of the factor relations involved, yet there is no question whatever that these characters of high and low protein and oil are con- ditioned by unit factors. A priori there is no objection to assuming the existence of several factors which affect the percentage of protein, for example, and that the original ear, 121, of the superior line in the high protein strain represented a genotype rich in high protein factors. Similarly in the other strains, continual ear-to-row selection has gradually eliminated all genotypes except the one, two or three as the case may be of highest or lowest factor combinations. Thus we see that selection has created nothing in the course of these justly famous experiments; it has served merely as a means of isolating particular combinations of factors which condition oil and protein pro- duction in the corn plant. Moreover, this sorting process has not been 304 GENETICS IN RELATION TO AGRICULTURE entirely regular or continuous. The saltations or jumps revealed by Surface’s analysis were directly consequent upon lump elimination of a number of mediocre lines. These results, therefore, are in entire har- mony with the known nature of allogamous populations. This conclu- sion is further corroborated by the recent report of Reitz and Smith on the statistical study of indirect effects of selection for high and low pro- tein and oil. These authors state: “Tt is found that four distinct types of corn as regards length, circumfer- ence, weight of ears, and number of rows of kernels on ears are so well estab- lished that we may assign orders of values to the means of these characters that persist with but a few exceptions in such changes of environment as have been experienced in 11 years of planting, from 1905 to 1915. : ‘““While a few slight but progressive changes have been noted, the selections for chemical composition from 1905 to 1915 have not changed decidedly the dif- ferences in mean values of these characters. In fact, we are unable to assert with any high degree of probability that the strains differ more or less with respect to these characters during the second half of the period 1905 to 1915 than during the first half.’’ The italics are ours. It is of especial significance that careful biometrical study has failed to reveal any progressive change asa result of continued selection in these strains of corn. For the results of these experiments have been cited as evidence par excellence by Castle in support of his hypothesis of factor variability. The ear-to-row method has been modified in various ways but it still forms the basis of most systems of commercial corn breeding. : . ‘ igs he” eee an aur 3 . 4 6, Sooke © nf Ms ” Sane : Be reo itetseth ae ann < ee : . - Patiala Fig. 170.—Birdproof cereal breeding garden at the University of California. ce ah ast § MAgey “te Fig. 171.—Pedigree cultures of Bursa in greenhouse of the Department of Botany, Prince- ton University. (Photo from G. H. Shull.) of various sorts may appear—damping off, insect pests, snails, slugs, mice, and in the breeding garden birds, gophers, moles, rabbits, etc. In this connection it may be sufficient to say that eternal vigilance is the price 422 GENETICS IN RELATION TO AGRICULTURE of success. Anyone with ordinary ingenuity will usually be able to provide the necessary protection. The important thing is to realize its need in time to prevent loss or contamination of cultures. If the breeding garden is located in or near cities the English sparrow will work havoe with developing seeds, especially of cereals. This menace can be completely overcome only by enclosing the threatened cultures with something that will keep out the birds and at the same time cut off the minimum amount of light. We have found 1-inch mesh poultry netting satisfactory (see Fig. 170). Many plants used in genetic investigations can be handled most satisfactorily in the greenhouse. Fig. 171 shows a greenhouse filled with pedigree cultures of a single species. A systematic method of recording and preserving data is a sine qua non for the pedigree culture. It is absolutely unsafe to trust to memory if any degree of accuracy is to be attained. For work on a small scale a serial number (using Arabic numerals) for each culture is satisfactory. This number then becomes the permanent designation of the given culture, each plant in the row receiving a subscript number. Thus plant No. 5 of culture No. 3 would be designated as 3P;.° If this plant is selected for further testing with self-pollinated seed, its progeny in the next generation will be labeled 3F, P; Pi, 3F:1P;P2 and so on down the row of plants. However, this method is rather cumbersome and for work upon a large scale the ‘‘annual-note-book-page’’ method first described by Shull is much more satisfactory. In this system each culture of a given year is numbered chronologically receiving the number of the page in the note book for that year on which it happens to be recorded. The label bears this number preceded by the distinctive numerals of that year. Thus the particular culture recorded on page 1 of the 1918 note book will be labeled 181 or 18.1. The use of the decimal point is a convenience especially if one is working at an in- stitution where serial numbers are in use in another department. In addition to the annual note book a set of permanent index cards should be arranged each year, including of course only those actually grown in a given year. By writing the current year number in one corner and the corresponding number for the preceding year in the other corner one has a convenient system for securing the complete pedigree of a given cul- ture. To complete this method some designation is necessary for the individual plants selected in any year. This may be a number in paren- theses, a subscript, a letter, or, where plants are set at equal distances from a given base line and each plant is thus numbered automatically, the letter P with subscript is satisfactory. Whatever the individual designation may be, it becomes the name of the particular plant for the remainder of its existence but its progeny will receive a new number when the seed is sown. PLANT-BREEDING METHODS 423 A system of labeling and recording that will be at once concise and definitely descriptive of the individuals and the nature of the matings has obvious advantages. Pearl has devised a system which is especially useful in crossbreeding experiments and in work with self-sterile plants; it can be adapted for any material. By the use of letters to denote indi- viduals or types of individuals that are brothers and sisters and numbers to denote types of matings a perfectly general set of terms is provided Tyres or MatTING IN F, F» indi- Number F's indi- Number F 2 indi- Number F» indi- Number viduals of viduals of viduals of viduals of mated mating mated mating mated mating mated mating AEX, 10 Bx 2" 46 CN as EXE 19 AVY ia a Sa a 2 CIS Rata NY ORS CEA ime eee ay 45 AXZ 40 Be 37 Dax ES 2D EXG 47 AZ! 42 BXxD 29 DDE TW 24 TeX 30 AXA Is Pee ea BB, el a eetay 1 a il CGI 32 AXB 33 BEE 57 DES Zia: FXZ 60 ALK. 25 BXxXG 59 Dis ae ay ia vs Be oo bison-cattle fetus and to Pete wild. ./.| 101 | eile i Dt7 87.1 eth dodt. Fi, Mewild.-.| 156/53. I" 319 4) fa3-9 BUS UNG OTS NEN. eS F,, %o wild...| 173 | 171 | 344 | 101.2 two reasons for this, the Fe, 164 wild...| 58 64 122) ORG increased size of the hybrid F,, }428 wild..| 16 21 37 76.2 fetus, and the development | | ai Sa of a large hump which Motals: ees | 552 600 1152 92.0 | cannot be accommodated by the normal pelvic con- formation in the cow. Consequently the fact that practically all the animals born of this species cross are females, may simply be due to abortion and death of male fetuses. The amount of trouble is sufficient in this case to give room for a potential equality of sexes in such crosses. Detlefsen reports similar results from a cavy species cross, which gave a disproportionately low ratio of males. The data are given in Table LXXI. The earlier generations show a con- siderable deviation from the normal equality of the sexes. With suc- cessive back-crosses, however, the ratio soon becomes one of practical equality. When races are more closely related a different result is produced. Thus Miss King found in crosses between wild and albino rats a sex-ratio of 119.1 males to 100 females among 425 hybrids representing the first three hybrid generations of such a cross. Among guinea-pigs crossed inter se, Minot found among 410 individuals a sex-ratio almost SEX IN ANIMALS 543 exactly the same as that among Miss King’s rats, 119.2 males to 100 females. Among wide crosses of hybrid birds the percentage of males also appears to be abnormally high, but this can hardly be taken as in conformity with the data of King and Minot, because in birds the mode of sex-inheritance is exactly the reverse of that in mammals. uout9eds pe Beery 9q} jo ALO YSTY jong ‘(qybiy) YONG Usnoy pezimojoueag Ajajatdwuog y *(j/a7) Yong usenoy jeuION Y—' AT ALv Id CHAPTER XXXV FERTILITY IN ANIMALS Among domestic animals fertility is of direct economic importance. Problems associated with it have been investigated from many different angles, even from the standpoint of inheritance. Unfortunately, how- ever, with respect to this latter feature of the question, not many inves- tigations have been carried out with higher animals. It is necessary, therefore, to seek for the principles disclosed by investigations with the lower forms of life, and to determine to what extent they may be applied to higher animals. . Factors Influencing Fertility —The factors which affect fertility are extremely numerous and varied. In considering the problems of in- heritance connected with it, it is, therefore, necessary to make the inevi- table scientific distinctions as to kinds of influences which may affect it and as to the different meanings which the term itself may have. In common parlance the term fertility signifies ability to produce active, living young. In higher animals in general fertility is measured by the reproductive capacity of pairs of individuals. Fecundity is the term used to designate the potential reproductive capacity of indi- viduals. It is measured by the ability of the individual to form mature ova or spermatozoa. Fecundity can be measured accurately and di- rectly only in special cases such as in birds; in mammals only fertility can be determined. Several physiological factors must be considered in a treatment of fertility in animals. Of these only a few can be mentioned here. For a more extended treatment, the student should consult treatises on the physiology of reproduction, of which that of Marshall is especially valuable. Among influences which lead to sterility or decrease in fertility are those of domestication. Here the effect depends largely upon the idio- syneracies of the particular wild species which has been domesticated. It has been suggested that food might in some cases be the determining factor, the supposition being that the animal under captivity may not obtain the variety and character of food necessary to maintain a healthy condition of the reproductive tracts. By no means, however, is the sterility of wild animals in captivity visibly correlated with changes in their mode of life, for often the most surprising variations occur in 551 oo2 tTHNETICS IN RELATION TO AGRICULTURE closely related species. Although it is impossible, therefore, to general- ize as to what particular factor of the environment is responsible for the condition of lowered fertility among wild animals in captivity, there can be no question as to the strikingly adverse effect of confinement in certain cases. Unfavorable conditions of the accessory reproductive organs occasion- ally cause sterility. Thus in cattle barrenness is sometimes the result of an acid condition of the secretions of the vagina. Simply injecting a weakly alkaline solution into the vagina has been found effectively to overcome this difficulty. The practice of artificial insemination has, also, been used in cases where the mucous secretions are unfavorable for conception, and in cases where structural bars to conception exist in the accessory reproductive organs. Among cattle especially conta- gious abortion is a serious cause of barrenness. This disease is bacterial in etiology, transmissible from animal to animal, perhaps usually by the agency of the herd bull, although possibly at times through food, and experimentally by intravenous injection. Not only does the disease cause abortion in animals in which it has not developed until after con- ception, but in animals previously infected it leads to barrenness. The disease is characteristic in its lesions and effects and may be controlled by the adoption of proper antiseptic measures. In general domestic animals are much more prolific than their wild progenitors. Several reasons for this fact may be pointed out. Those species which can adapt themselves to conditions of domestication usually find such surroundings more favorable to development and to the production of offspring. Moreover, there is a natural tendency for selection to favor the survival of those strains or races which reproduce most rapidly, and man has augmented this tendency by choosing the more prolific members of the race for breeding stock. But even the long-continued selective processes of domestication have not sufficed to attain to the maximum of fertility for the species. Few realize how great is the field for improvement in this respect. In England horse breeding, according to Marshall, suffers an enormous loss each year because of the failure of no less than 40 per cent. of mares selected for breeding purposes to produce offspring. Cattle, sheep, and swine appear to suffer somewhat less in this respect but the loss is far from inconsiderable. Heape estimates the average loss among cattle to amount to over 15 per cent. Among sheep the loss from actual sterility alone amounts to nearly 5 per cent. In view of such statistics the in- crease of fertility in domestic animals becomes a problem of prime economic importance. The Darwinian Theory of Fertility —The results of Darwin’s extensive investigations of problems of vigor and fertility in plants and animals FERTILITY IN ANIMALS 553 may be summed up in the trite statement, Nature abhors inbreeding. From his extensive investigations Darwin concluded that all organic beings benefit from an occasional cross and that the inevitable effect of continued inbreeding is loss of size and decreased constitutional vigor and fertility, and at times unusual tendency toward the production of malformations. Since Darwin’s evidence was drawn largely from do- mesticated animals, and since other serious detrimental features of in- breeding are pointed out in addition to loss of fertility, it isimportant that enquiry be made into the reasons why inbreeding should result in decreased fertility. It is, also, important to note that we are attempting to harmonize in this treatment Darwin’s conclusions with a theory of heredity unknown to him. Inbreeding not in Itself Harmful.—Although supposed evidence of harmful effects of inbreeding has been presented by a number of inves- tigators, there is nothing in this evidence which necessarily throws the blame upon inbreeding in itself. A single contrary case is all that is necessary for establishing the negative interpretation, and there are a number of such cases. Thus investigations on the effects of inbreeding in the fruitfly have been carried out on a much more extensive scale than would ever be possible with any of the higher domestic animals. For example, Castle and his associates inbred the fruit fly for fifty-nine generations, mating brother with sister throughout the investigations. They reached the general conclusion that inbreeding unaccompanied by selection generally results in decreased productiveness, but that proper selection for high productiveness results in maintaining the original fertility of the race. They found further that low productiveness is sometimes inherited like a Mendelian recessive, as shown by its appear- ance in alternate generations, and that in crosses between strains of high and low productiveness there was evidence of segregation in F. Castle further comments upon a polydactylous race of guinea-pigs which was descended from a single individual. They have been inbred for over 10 years, yet despite this fact they show no signs of diminished fertility; on the contrary, they are superior in size and in constitutional vigor to most races. Moenkhaus’ results with Drosophila also seem to indicate that a high degree of fertility may be maintained in successive generations of inbreeding if sufficient care be taken to select from the most fertile individuals. Hyde, on the other hand, found a decrease in fertility consequent on continued inbreeding. The experimental results, therefore, show that sometimes inbreeding does not result in diminished fertility. The fact, however, that there are so few cases in which in- breeding has not been followed by measurably harmful results calls for some explanation. In the rest of this chapter some reasons for this fact will be pointed out. . 554 GENETICS IN RELATION TO AGRICULTURE Fertility as Related to Mendelian Factors.—There is a considerable body of evidence to show that some Mendelian factors exhibit residual effects upon the fertility of individuals which bear them. Thisis perhaps most clearly established for certain factors in Drosophila. Thus among sex-linked factors Morgan has shown that those for the rudi- mentary and the fused wing conditions are practically always associated with sterility. In rudimentary flies the males are fully fertile, but the females are usually completely sterile. Examinations of the ovaries of rudimentary females demonstrate that the eggs do not develop normally, but for the most part remain in a low stage of development. Similarly the mutant fused is absolutely sterile in the female sex, but fertile in the male. Stock must, therefore, be maintained by mating heterozygous females to fused males. Here again examination of the ovaries has shown reduction in the number of mature eggs normal for the wild type. Between this relatively complete sterility and the normal fertility of the wild type.there exist all possible gradations. In fact even in wild type flies as Castle and his associates and others have abundantly shown strains possessing different degrees of fertility exist. But mutant strains often exhibit lessened vigor and fertility specifically attributable to the residual effects of the mutant factors themselves. This effect appears to be cumulative, for the presence of several mutant factors often greatly accentuates it. The difficulty has often proven a very great obstacle in carrying out some Drosophila experiments, but it serves to demonstrate that sterility may be a consequence of certain combinations of factors. Specifically a number of definite cases may be given. Muller at- tempted to unite the factors for yellow body color, white eyes, abnormal abdomen, bifid wings, vermilion eyes, miniature wings, sable body color, rudimentary wings, and forked spines in one strain of flies. Here, of course, the factor for rudimentary wings in itself might be expected to have a profound effect upon the fertility of the strain, but aside from this effect it was found that the strain was so deficient in viability and general vigor that it was necessary to propagate it by specially devised breeding methods in the heterozygous condition. The heterozygous flies showed only an insignificant reduction in viability and fertility, whereas their full brothers and sisters which were homozygous for the recessive factors were so weak as to be of no value in the experiments. The same difficulties were met with in dealing with combinations of recessive factors belonging to other groups. It is safe to say that almost any combination of several recessive factors in Drosophila results in diminished vigor and consequent decrease in fertility. The effect is, however, specific, for the degree of diminution depends not only upon the number of recessive factors which are combined, but also upon the specific effects of the factors themselves. The specific residual effects « FERTILITY IN ANIMALS 555 of certain Mendelian factors upon fertility cannot, therefore, well be denied. (See Fig. 216.) The Chromosomes and Fertility——Bridges has demonstrated for Drosophila that males of the chromosome constitution XO, instead of the normal XY, are totally sterile. Here a specific chromosome differ- ence, the absence of the Y-chromosome from the _ hereditary mechanism, leads definitely to complete sterility. Not many other cases are known among animals of sterility dependent upon abnormal chromosome constitution, but Bridges reports several known cases of Fie. 216.—Drosophila mutation which exhibits a high degree of sterility. a, Normal wing; b and c, fused wings. (After Morgan and Bridges.) aberrant hereditary behavior which may be dependent upon irregular chromosome distribution and content. Sterility in Other Animals.—In some other animals there are cases of sterility which suggest strongly the effect of definite Mendelian factors. Thus several writers have commented upon the sterility of tortoise- shell male cats, and apparently orange males are also, sometimes at least, sterile. The reason for this particular case has not yet been established definitely by breeding tests, and there is apparently some pos- sibility that irregular chromosome distribution may account for it. An instance from practical breeding history which appears to belong to this category is that of barrenness in Bates’s famous Duchess family of Shorthorns. This family was noted for superior individual excellence, consequently breeders, naturally desirous of maintaining this excellence, followed a practice of close breeding within the family, an example of 556 GENETICS IN RELATION TO AGRICULTURE which is given in Fig. 217. But the family was tainted from the be- ginning with the curse of barrenness, which such a system of breeding must inevitably preserve. Shortsighted breeders at the time considered it a fortunate circumstance that Duchess cows were so often barren, for it kept down the number of individuals of this favorite strain and resulted in prices correspondingly high. But as a result of barrenness, the strain eventually ran out completely. In Fig. 218, an attempt has been made to show diagrammatically how barrenness was inherited in this family. The diagram is not complete, for it includes only the females in the family. Nevertheless it brings out very forcibly how [recived (1706) Short Tail (2621) | Duchess 32nd 4th Duke of Northumber- land (3649) Belvedere (1706) | Duchess 34th ‘ Duchess 29th Duchess 55th (2nd Hubback (1423) Norfolk (2377) Nonpareil Duchess 38th Belvedere (1708) Duchess 33rd Duchess 19th Fic. 217.—Pedigree of one of the latest Duchess cows, illustrating system of close-breeding followed in maintaining the family. Duchess 55th produced two calves. barrenness occurred very early in the family history, and how it re- appeared in about the same proportion of the total population through- out its history. Far from showing an intensification of the defect as a result of inbreeding, this diagram merely illustrates the heredity of a defective family trait. Sterility of Hybrids.—There is a definite type of sterility which is referable to the effects of species hybridity. We have already had occasion to comment upon this type of sterility in connection with other matters, here we shall however refer to it again with particular emphasis upon certain of its aspects. For the higher animals we do not possess much in the way of definite data respective to hybrid sterility. The mule, a familiar and oft-cited example, appears from all accounts to be very nearly completely sterile. The accounts of fertility in mare mules are for the most part shadowed in doubt, but the possibility of a slight fertility should not be denied. The hinny, the homolog of the mule, exhibits as high a degree of ster- FERTILITY IN ANIMALS 557 ility as the mule. Of other hybrids within the genus Equus the evidence is even less satisfactory. Apparently the cross between the horse and zebra is sterile, like the mule. But the zebra and ass appear to be more closely related, and the possibilities of securing offspring from such da Fig. 218.—Illustrating inheritance of barrenness through the female line of the Duchess family of shorthorns; barren cows represented by solid black circles. hybrids appears to be somewhat greater. There is in fact one reference in the literature to a fertile male hybrid between the zebra and the ass. In the genus Bos, taken in the wider sense to include the subgenera Bison and Bibos, there are various degrees of sterility consequent upon hybridization. The domestic cow, Bos taurus, gives fertile male and 558 GENETICS IN RELATION TO AGRICULTURE female hybrids with the zebu, Bos indicus. With the yak, Bibos grun- iens; the gayal, Bibos frontalis, the gaur, Bibos gaurus, and the bison, Bison americanus, the female hybrids with the domestic cow are fertile, but the males are sterile. The banteng, Bzbos sondaicus, and the zebu behave like this latter series in giving fertile female and sterile male offspring. In this respect they resemble Detlefsen’s and Castle and Wright’s results with species crosses among guinea-pigs, the female hybrids of which were fertile, the males sterile. Among domesticated birds in particular the reproductive powers are strongly disturbed by hybridization. Not only are such hybrids often Fig. 219.—Abnormal reduction divisions in spermatogenesis of the mule. (After W odsedalek.) sterile, but very frequently the sexual organs develop in an abnormal fashion strongly suggestive of intersexualism of the kind exhibited by Goldschmidt’s Lymantria hybrids. Smith and Thomas have examined sterile hybrids between species of pheasants. They found that very often ovarian degeneration or imperfect development occurs in the females, as a consequence of which a marked tendency exists to assume plumage patterns and characters peculiar to the male. Here we are dealing rather definitely with a type of sterility different from that which characterizes different families within a species or breed or different mutant types of Drosophila, the sterility here appears to be more deep-seated and strangely enough, far from being associated with a general diminution in vigor, the vigor and size of the hybrids are often very augmented. We are not surprised, therefore, to find that profound disturbances in the hereditary mechanism occur in such hybrids. Wodsedalek has shown that irregular reduction divisions occur in the mule (Fig. 219). Smith and Thomas have shown specifically that in sterile hybrid pheasants of both sexes the abnormal behavior and de- FERTILITY IN ANIMALS 559 generation of germ cells begins in synapsis. They conclude, therefore, that sterility in pheasant hybrids depends upon the inability of homolo- gous chromosomes derived from different species to conjugate normally. In the mule, which apparently receives a different number of chromo- somes from each parent, a morphological cause for such a difficulty obviously exists, but fundamentally the difficulty must be physiological, for it exists as strikingly in hybrids between species having the same numbers of chromosomes as in the rarer cases where the species have different chromosome numbers. We have already discussed this prob- lem at length. Fertility as Related to Heterozygosis—In another place we have discussed the hypothesis that heterozygosis in and of itself has a favorable effect upon vigor and fertility. This hypothesis is difficult to prove or to disprove. With the facts, however, there can be no question. Cross- breeding definitely does in specific cases lead to an increase in vigor and fertility, a fact which has long been known. But it appears more prob- able, as Jones has shown, that this increase is due to the establishment of a more excellent factor-complex than to any mysterious stimulation effect of the heterozygous condition. At the same time the possibility of an enlarged expression of the heterozygous condition of a given pair of allelomorphs must not be denied, but like other effects of heterozy- gosis, it is probably a condition depending upon the specific nature of the factors concerned. As a generalization, however, it must be taken as not proven; certainly the work with Drosophila, which is based upon more definite knowledge of the Mendelian factors than any other investi- gations to which we can refer, does not provide evidence in support of it. The solution of the problem has in it much of practical importance, for upon the hypothesis of heterozygosis it should be impossible to build up a breed which would reproduce in full the complete set of excellent char- acters of the cross-bred. If, however, a more favorable combination of factors is responsible for the excellence of cross-bred animals, then it should be possible by careful breeding to fix them in a new breed. Fecundity in Fowls.—It is a genuine pleasure in a mass of contradic- tory and illy digested data to meet with something which gives hope for the same definiteness with regard to the problem of the inheritance of fecundity that has been attained in the analysis of the inheritance of other more clearly defined characters. We cannot, therefore, but com- mend the patient investigation and brilliant analysis to which Pearl has subjected the problem of the inheritance of fecundity in the domestic fowl. Many criticisms have been launched against his conclusions, it is true, but it is highly probable that these criticisms involve a funda- mental misconception of the nature and results of scientific knowledge. Pearl’s results deal particularly with winter egg production in the 560 GENETICS IN RELATION TO AGRICULTURE domestic fowl. The conclusions are based upon an analysis of data obtained by trapnesting strains of pure-bred Barred Plymouth Rocks and Cornish Indian Games, and F, individuals and F» individuals ob- tained by mating F; individuals inter se and by mating them back to their parents in all possible combinations. Over a thousand birds were subjected to this definite experimental test. With respect to winter egg production hens naturally appear to fall into three well-defined classes; (a) those birds which lay no eggs during the winter period; (b) those which lay something less than about thirty eggs; and finally (c) those which lay more than thirty eggs. Since egg laying is a character strongly influenced by environmental conditions cor) o 20 _— - ——_— 99 80 70 60 50 40 30 20 10 Q Per Cent Producing Indicated Number of Eggs or More Fic. 220.—Contrasted flock curves of winter egg production of Barred Plymouth Rock (solid line) and Cornish Indian Game (broken line) pullets. (After Pearl.) and somatic fluctuations, these classes are not absolute, nor on the other hand are they by any means purely arbitrary as has been determined by statistical studies of flock production during the winter period. The differences which exist between the two breeds under investigation are shown graphically in Fig. 220. Taking a production of thirty eggs or more as the standard of comparison between the two breeds, it may be seen from this graph that only about 6 per cent. of the total flock of Cornish Indian Games produced as many eggs as this during the winter period, whereas 54 per cent. of Plymouth Rock pullets measured up to this standard of excellence. From a consideration of the data obtained from a wide series of crosses, Pearl proposes the following analysis of the inheritance of fecundity in fowls as measured by winter egg production. For the sake of clearness and conformity to treatment in the remainder of the text, we have used symbols different from those used by Pearl without, however, in any way modifying the essential features of his analysis. FERTILITY IN ANIMALS 561 (a) The Sex Factors.—In the fowl the type of sex-inheritance is that designated WZ in Morgan’s terminology. Females are heterozygous for the sex-factor; 7.e., they are WZ in constitution, whereas males are homozygous, ZZ. W like Y in the XY type is neutral and carries no demonstrable factors. (b) A fecundity factor, L, which determines a winter egg production of something less than thirty eggs. It is dominant over the allelomorph 1, which is present in fowls which produce no eggs TasLE LX XIV.—Generic ConstITUrIONS OF ’ during the winter season, BarreD PiymMoutH Rock MALES FoR but the homozygous con- Fecunpiry Facrors dition LL does not CON- Gags] _ Genetic | Sey Aa ditionachigherwintersege =| PPsaHORB ¥ production than does the i | (ZM)(ZM)LL |(ZM)L heterozygous condition LI. 2 | (ZM)(Zm)LL |(ZM)L, (Zm)L (c) A Seax-linked Fe- 3 | (ZM)(ZM)LI |\(ZM)L, (ZM)I cundity Factor, M.—Like : eee ian (ZM)l, (Zm)L, (Zm)t L it determines alone 4 6 |@MyZmu |(ZM)i, zm) winter egg production of | Ga\Zmskh |\Zm) Le something less than thirty 8 | (Zm)(Zm)LlL |(Zm)L, (Zm)l eggs. With L, however, 9 | (Zm)(Zm)ll |\(Zm)l it gives a winter egg pro- duction of over thirty. Pearl was able to classify the individuals which he used in his ex- periments into classes according to their zygotic constitutions. For the sake of clearness these classes are given in Tables LXXIV, LXXV, Taste LXXV —Genetic ConstituTION OF BARRED PLyMouTH Rock FEMALES FOR Frcunpity Facrors | Class | Genetic constitutions Z gametes | W gametes {Winter egg production il (ZM)WLI (ZM)L, (ZM)I | WL, WI Over 30 eggs 2 (Z2ZM)WLL (ZM)L WL | Over 30 eggs 3 (Zm)WLI (Zm)L, (Zm)b | WL, WI Under 30 eggs 4 (Zm) WLL (Zm)L | WL Under 30 eggs 5 (Zm) WU (Zm)l | Wi No eggs 6 (ZM) Wil (Zm)l Wl Under 30 eggs LXXVI,andLXXVII. These tables represent genetic constitutions which were realized and recognized during the course of the experiments. It will be observed that among the Barred Plymouth Rocks every possible genetic constitution was represented, but among the Cornish Indian Games the factor M was not contained in any individual. 36 562 GENETICS IN RELATION TO AGRICULTURE It is difficult to present briefly all the evidence which has led Pearl to advance the foregoing analysis for the data on fecundity in fowls, reference must be made to the complete published results dealing with these experiments. In Table LX XVIII the data aresummarized and com- pared with expectations, but not in a satisfactory manner, because different types of matings have been TaBLE LX XVI.—GENETIC CONSTITUTIONS OF CornisH INDIAN GAME MALES FOR Frcunpity Factors Class Genetic constitutions | Gametes produced lumped together. It does, however, show as well as : ile os rate oe be shown in so short Zm)(Zm m)L, (Zm 3 (Zm)(Zm) I (Zm)l a summary, how closely | fecundity conforms to the requirements of a Men- delian analysis. There can be little doubt that Pearl has laid the broad foundations for a more definite knowledge of the behavior in heredity of a complex character of great economic importance. The practical implications of this analysis are discussed in Chapter XX XVIII. Taste LX XVII.—Genetic Constitutions or CornIsH INDIAN GAME FEMALES FOR Frcunpity Factors Class | Genetic constitutions | Z gametes | W gametes | Winter egg production | | a satin. 1 (Zm)WLL | (Zm)L WL | Under 30 eggs 2 (Zm) WLI | (Zm)L, (Zm)l | WL, WI , Under 30 eggs 3 | (Zm) Wil | (Zm)l. | Wl No eggs Taste LXXVIII.—Ossrervep anv Exprectep DISTRIBUTIONS OF WINTER Eaa PrRo- DUCTION FOR ALL MaTINGs IN PEARL’S EXPERIMENTS ? Winter production of daughters Mating Class Over 30 Under 30 None Barred Plymouth Rock......... Observed 365.5 259.5 31.0 Expected 381.45 DAD PAD il? 3° Cornish Indian Game..........| Observed 2.0 23.0 15.0 Expected 0.0 25.0 15.0 AUTRES 0S OS TR ee ee Observed 36.0 79.0 8.0 Expected 26.5 88.75 9.75 All F2 and back-crosses........ Observed 57.5 98.5 23.0 Expected 68 .6 95.0 15.4 FERTILITY IN ANIMALS 563 Conclusion.—We may conclude, therefore, fairly, that fertility in animals is a complex character highly modifiable under different con- ditions of environment, but nevertheless possessing a definite, although intricate and obscure, factorial basis. The character bears the same relation to systems of breeding as do any other characters, that is, the system of breeding in itself has no effect upon it, it is merely a mode of achieving certain results. This is clearly established by Pearl’s experi- ments on fecundity in fowls, for they demonstrate that factors for high fecundity exist, and that these factors behave like other Mendelian factors. CHAPTER XXXVI SOME BELIEFS OF PRACTICAL BREEDERS It is proposed in this chapter to discuss not only some matters which belong to the discarded remnants of scientific thought, but also some beliefs of animal breeders which have not yet been subjected to the rigid scrutiny of scientific investigation necessary for analyzing them completely. Telegony.—By telegony is designated the supposed influence which a sire exerts upon the females with which he is mated such that the products of subsequent matings with other sires show some influence of the previous ones. The same phenomenon is known in popular speech as infection of the germ, the influence of previous impregnation, ete. One does not need to go far to find support for the hold which the belief in telegony has upon the popular mind. In certain cases the belief has been so strong as to affect rules of registration of pure-bred animals. Riley has collected from flock book records a few typical rules of regis- tration which are founded upon a belief in telegony. They are given below: Vermont Merino Sheep Breeders’ Association. Rule 24.—The record of registered ewes will be forfeited if bred to rams other than pure descendants of importations direct from Spain, and it shall be the duty of members to report to the Secretary the label marks and numbers within the year in which they are so bred, who shall enter them on the records of the flocks in which they are recorded. Any member who shall fail to report according to this rule or offer lambs from such ewes for record shall be suspended or expelled. New York State American Merino Sheep Breeders’ Association. Rule 17.—That this association exclude from its records all breeding ewes that have been previously bred to coarse wooled rams. American Rambouillet Sheep Breeders’ Association. Rule 4.—No product of a Rambouillet ewe shall be eligible for registry after such ewe shall have been bred to any other ram but a registered Rambouillet. Dorset Horn Sheep Breeders’ Association. Rule 6.—No ewe or ewe lambs shall be eligible for entry that have been served with any ram other than a pure bred Dorset Horn from the date thereof. Michigan Merino Sheep Breeders’ Association. Rule 12.—The product of a registered ewe which shall at any time have been bred to a ram not a registered American Merino, or one eligible to register shall be excluded from registry. 564 SOME BELIEFS OF PRACTICAL BREEDERS 565 It is of course possible that motives other than belief in telegony have had some influence in shaping these rules, but presumably this belief has been the chief reason for adopting them. At the same time it must be acknowledged that the popular belief in telegony is by no means universal. Thus E. Davenport calls attention to the fact that breeders of dogs are generally credited with a strong belief in telegony. Nevertheless a correspondence which he carried on with dog fanciers failed to disclose more than one case among thirty-seven which affirmed belief in telegony, and twenty-eight of these breeders were positively opposed to it. Since some credence is still given to telegony in popular circles, even if not among scientific investigators, a detailed account of the evidence against it will be presented below. Lord Morton’s Quagga Hybrids—We can do no better in beginning a discussion of telegony than to refer to the classic example of it, Lord Morton’s mare, for this case was accepted at its face value by no less an authority than Darwin. The details of this experiment are about as follows. Lord Morton bred a seven-eighths chestnut Arabian mare which had never been bred before to a male quagga. The result of the union was a female hybrid which plainly exhibited both in color and in form distinct evidence of its hybrid origin. The mare subsequently passed into the hands of Sir Gore Ouseley who bred her to a very fine black Arabian stallion. To the service of this stallion she bore first a filly foal and in the next year a colt foal. Lord Morton later examined these two colts and as a result of his inspection he wrote as follows to the president of the Royal Society: The 2-year-old filly and yearling colt have the character of the Arabian breed as decidedly as can be expected where fifteen-sixteenths of the blood are Arabian; they are fine specimens of that breed, but both in the color and in the hair of their manes they have a striking resemblance to the quagga. Their color is very marked, more or less like the quagga in a darker tint. Both are distinguished by the dark line along the ridge of the back, the dark stripes across the forehand, and the dark bars across the back part of the legs. The dark stripes across the forehand of the colt are confined to the withers and to the part of the neck next to them. Those on the filly cover nearly the whole of the neck and the back as far as the flanks. The color of her coat on the neck adjoining to the mane is pale and approaching to dun, rendering the stripes more conspicuous than those on the colt. The same pale tint appears in a less degree on the rump, and in this circumstance of the dun tint also she resembles the quagga. Both their manes are black; that of the filly is short, stiff, and stands upright, and Sir Gore Ouseley’s stud groom alleged that it never was otherwise. That of the colt is long, but so stiff as to arch upward and to hang clear of the sides of the neck, in which circumstance it resembles that of the hybrid. This is the more remarkable, as the manes of the Arabian breed hang lank, and closer to the neck than those of most others. The bars across the legs, both of the hybrid and of the colt and filly, are more strongly defined and darker than those on the legs of the quagga, which are very slightly marked; and though the hybrid has several quagga marks, which the colt and filly have not, yet the most striking—namely, the stripes on the forehand are fewer and less apparent than those on the colt and filly. 566 GENETICS IN RELATION TO AGRICULTURE The strength of the evidence in this case can be understood better by reference to Fig. 221, which shows the male quagga which Lord Morton used in his experiments, the hybrid which was produced by the chestnut Arabian mare when bred to this quagga, and the filly which she produced subsequently to the service of a purebred black Arabian stallion. Fig. 221.—Lord Morton’s male quagga, a hybrid between a Chestnut Arabian mare and this quagga, and a filly produced subsequently by the same mare when mated to a pure-bred black Arabian stallion. (After Ewart.) But although many scientists have granted the weight of this evidence, later scientific thought has questioned strongly even the possibility of such effect of the male upon the female. Accordingly many adverse criticisms have been made against the validity of this case, some of which, as for example that of J. Wilson, go so far as to deny the hybrid origin of SOME BELIEFS OF PRACTICAL BREEDERS 567 the first foal produced by Lord Morton’s chestnut Arabian mare. Those criticisms suggested by Ewart, however, because they are tempered by abundant experimental research, are, perhaps, most just. Accepting the hybrid nature of the first foal, the question arises as to how common striping may be in horses, especially those of Oriental ancestry. On this point there is abundant evidence as Ewart points out. The old yeliow-dun horses of the forest type, which have had much to do in the origin of modern breeds of horses, characteristically possessed a broad dorsal band and zebra-like bars on the legs, and in addition to these markings they often possessed faint stripes on face, neck and withers. In fact evidence points to the belief that a remote ancestor of this forest horse was probably as richly striped as some modern zebras. Even today it is a very common thing among mongrel ponies to meet with individuals which possess distinct markings suggestive of the forest horse. They are not uncommon among Arabian crosses. Consequently we are not sur- prised when this later filly is compared with the quagga to find that its pattern, rather than suggesting residual effect of the previous impreg- nation by the quagga, strongly indicates reversion to some ancestral type. The bars on the legs, for instance, were more marked on the hybrid, on the filly, and on the colt than on the quagga. The scanty mane and tail upon which Lord Morton dwells may simply be regarded as additional evidence of reversion. The Penycuik Experiments.—All the debates which may center around Lord Morton’s mare, however, do not carry a fraction of the weight in assigning telegony to the limbo of discarded doctrines of the experimental work of the last two decades. The Penycuik experiments were designed by Ewart to determine whether such a doctrine as telegony were tenable, and, if so, to what extent it exerted influence in animal breeding. They have been carried out on a considerable scale, and included experiments not only with the Equide, but also with other animals. In every case Ewart was forced to the conclusion that alleged cases of infection may be accounted for most easily and most satisfactorily as instances of rever- sion to ancestral types. To illustrate Ewart bred the Burchell zebra stallion Matopa to a chest- nut polo pony. She produced as a result of this mating twin hybrids. The following year she produced a foal to a light chestnut thoroughbred stallion, after which she was again bred to Matopa, and produced a third hybrid foal. Subsequently she produced another foal to the service of a dark chestnut thoroughbred stallion. The three hybrid foals from this mating were all richly striped, in fact the stripes were more numerous, although less conspicuous, than those of the zebra sire. In spite of this fact, however, the two foals produced by mating Valda to the thorough- bred chestnut stallion in no particular, either in color or in form, resem- 568 GENETICS IN RELATION TO AGRICULTURE bled the hybrid foals. They were chestnut in color without any sug- gestion of striping, and in liveliness of temperament or vigor of develop- ment neither of them resembled in the least the hybrid progeny. € Fig. 222.—Jerry, a male Grevy zebra, Hquus Grevit, used in U. S. government breeding investigations. (After Rommel.) Fic. 223.—A registered Morgan mare, Baby Gates, used in U. S. government breeding investigations. (After Rommel.) A subsequent experiment is of interest because of the closeness with which it agrees with particulars of the Lord Morton case. Ewart bred SOME BELIEFS OF PRACTICAL BREEDERS 569 Mulatto, a black West Highland pony, to Matopa and obtained a colt foal Romulus, a beautiful, distinctly striped hybrid. The mare Mulatto was then bred to a black Arabian stallion. To this service she produced Fig. 224.—Juno, a zebra-mare hybrid produced by mating the Morgan mare, Baby Gates, to the Grevy zebra, Jerry. (After Rommel.) Fie. 225.—Georgia, a registered Morgan filly produced by Baby Gates subsequent to the production of the zebra-mare hybrid Juno, to the service of the Morgan stallion, Pat Murphy. There is no trace of telegony. (After Rommel.) a foal which, when examined immediately after birth, showed numerous indistinct markings, so faint, however, that their exact nature was in some doubt. Subsequently Mulatto produced another foal to the serv- 570 GENETICS IN RELATION TO AGRICULTURE ice of a dark brown West Highland stallion which was also indistinctly marked. In themselves these foals suggested as strongly that telegony might occasionally occur as did those described by Lord Morton. But Ewart tested the matter further by breeding two dark West Highland mares closely related to Mulatto to the same black Arabian stallion which had sired the striped foal. Two foals were produced, one of which possessed the same sort of indistinct markings as those characteristic of the foals of Mulatto, the other was much more distinctly striped. There can be no question, therefore, that the striping of Mulatto’s foals was a consequence of normal hereditary processes having nothing to do with telegony. Further evidence as to the non-tenability of the doctrine of telegony might be cited from the Penycuik experiment. A vast amount of ad- ditional evidence has been obtained from other experiments, some of which have been performed with the distinct object of testing the doctrine, others for different purposes. The experiments of Baron de Parana with zebra hybrids closely paralleled those of Ewart and yielded likewise no evidence in support of the doctrine of infection. The series of photographs shown in Figs. 222 to 225 have been drawn from an article by Rommel describing the work of the U. S. government with hybrids between different species of Equus. Here again there is no evidence that Georgia the Morgan filly which Baby Gates produced subsequently to the production of the zebra hybrid Juno shows any effect of the pre- vious impregnation. The apparent stripes on the body of Georgia, it may be mentioned in passing, are merely her ribs showing through. Similarly there is no evidence that Sweepstakes, the dam of Star Pointer and other pacers, was in any way influenced by the fact that previously she had borne two mule foals. The evidence from mule-breeding establishments, in which thousands of mules have been produced, in every dependable instance is against the doctrine of telegony. Mumford has recorded a large number of concrete cases in support of this position. In a few instances mares had produced as high as ten or eleven mule foals before they were bred to stallions, yet in not a single case was there positive evidence of telegony. The development of the Mendelian theory of heredity has robbed most of the old evidence for telegony of all its value. An instance which Ewart quotes is of considerable interest in this connection. A tan Dachs- hund bitch was bred to a tan dog, and produced a litter of puppies having pure white bodies and tan cheeks and ears. Now this bitch had previously borne by misalliance a litter of puppies to a white Fox terrier with tan cheeks and ears. Presumably both the tan Dachshund bitch and dog had long lines of tan or black and tan ancestors; what more natu- ral than to conclude that this was a strict case of telegony? But the SOME BELIEFS OF PRACTICAL BREEDERS 571 breeder to whom the attention of this instance was called remarked that, although the color of this litter was strongly reminiscent of the white fox terrier, the form and general characteristics were otherwise those of pure Dachshunds. Accordingly he traced the pedigree of the dam and found that in the sixth generation it ran back to the kennel of a lady whose’ hobby was white Dachshunds with tan cheeks and ears. This particular mating had simply given opportunity for the expression of latent factors carried by the tan dogs. It was a perfectly intelligible case of rever- sion, not telegony at all. Harmful Effects of Hybridization.—Although we cannot accept the belief in telegony, we must admit that bearing hybrid offspring may sometimes have detrimental effects upon the dam. Thus Ewart quoting from Baron de Parana calls attention to the practice in Brazil of breeders of mules putting their mares to horses after they have reared two or three mules in order to prevent them from becoming sterile. There is a possibility that a hybrid fetus in consequence of its unusual vigor may tax more strongly the resources of the dam and in a sense impoverish her. This is particularly the case in some hybrids like those between the bison and domestic cattle, the production of which is a tremendous drain upon the dam’s system and often leads to fatal consequences. But this is not telegony, it is merely a consequence of disturbing the physiological balance in the dam, and has nothing whatever to do with the transference of the characters of a previous sire to offspring borne subsequently to the service of another sire. Infection of the Male.—The belief in infection of the male is by no means as strong as that in telegony, but occasionally it is met with. Ewart recites an incident of a breeder who refused to allow his Jersey bull to serve Shetland cows for fear that the bull would subsequently earry over old Shetland traits into his Jersey herd. Since we have, however, discarded telegony as applied to the female, there appears to be no warrant whatever for considering it in the male, where an effective mechanism of operation is even less conceivable. Moreover, a vast amount of evidence which has been obtained in Mendelian experiments leaves no room whatever for this belief. Saturation.—The doctrine of saturation is fundamentally based upon a belief in the cumulative effect of telegony. It holds that succes- sive children of given parents come to resemble the sire more and more in their characters. Although this doctrine has been accorded some im- portance at times, like the doctrine of telegony it finds no support from experimental evidence. Here again we may point to the evidence from Mendelian experiments, collected for another purpose, it is true, but yielding direct evidence in opposition to the belief in saturation. More- over, Pearson has collected statistical evidence in human beings with 572 GENETICS IN REALTION TO AGRICULTURE respect to the stature of successive children in the same families. He finds no evidence whatever of saturation, or as he states it of a “steady telegonic influence.” Maternal Impression.—The belief in maternal impression, or in the effect of the pregnant mother upon her growing fetus, is one of the en- during tenets of popular faith. We need not trouble ourselves here with the long series of influences which are supposed to pass from the mother to the unborn child in human beings. Suffice it to state that they are no more varied nor yet more tenable than those cases which have been described for domestic animals. Many curious cases from that of Jacob’s peeled wands! down to those of far more recent times might be cited from the chronicles of maternal impression; but like the belief in telegony, they all spring from the un- scientific attitude of the popular mind toward isolated instances. We suspect for instance that the famous Biblical herdsman used other methods than that of the peeled wands in order to achieve his remarkable results. An instance may be given as typical of those which are recounted in support of the belief in the effect of maternal impressions, although in reality it is stronger than most cases. A section of a well-known Scottish herd of Aberdeen-Angus cattle which was separated from an Ayrshire herd by only a wire fence persistently produced, for several successive genera- tions, red and black-and-white calves. But this was the formative period of the breed, and we have already had occasion to mention the diversity of color which characterized Aberdeen-Angus foundation stock. The oc- currence of red and black-and-white calves, therefore, is a simple con- sequence of the cropping out of recessive factors, the Mendelian ex- planation is adequate and satisfactory. Moreover it is not entirely improper for us to call attention to the utter confusion which would pre- vail in herds of Aberdeen-Angus cattle, if this phenomenen were of gen- eral occurrence. We venture to state that very few breeders of solid- colored cattle have had such trouble from the proximity of herds of Holstein-Friesian, Shorthorn, Ayrshire, and other breeds of cattle, not to mention other sources of contamination which might occur. But some of the legends of Aberdeen-Angus history are even more curious than this one. It is recorded of the famous breeder MecCombie of Tillyfour that he erected a high black fence around his breeding paddock. But it may be expected that McCombie having as his ideal the black polled Aberdeen-Angus cattle used other means of securing a strain breeding pure for the typical Aberdeen-Angus characters. Like the belief in telegony, the belief in maternal impressions arises from an unscientific attitude of mind toward evidence in general. The 1Cf. Genesis 30 : 31—43. SOME BELIEFS OF PRACTICAL BREEDERS 573 particular, unusual instance, because it is so striking fixes itself in the memory and the countless thousands of cases which do not support the doctrine are overlooked. There is something in it akin to the memory of the card player which retains so tenaciously the recollection of an unusual hand, but here it is usually clearly recognized that chance alone is responsible for the good fortune. So also in animal breeding remembrances of strange coincidences are longest borne in mind, but it seems to be a very common fault not to realize that they are after all nothing but coincidences. Prepotency.—It has been an early observation of animal breeders that some animals possess a superior power of impressing offspring with their characters. This is precisely what is meant by prepotency; a prepotent animal is simply one which has the power to stamp its offspring with its own characteristics. Obviously there is much room here for confusion of thought, but at least the existence of prepotent animals can scarcely be denied. The science of genetics unfortunately has not advanced far enough to be able to state precisely what are the requisites for prepotency, nor has it progressed to such an extent, as some seem to think, that prepotency, like those other popular doctrines which have been considered in this chapter, may be analyzed completely and its untenable features discarded. The fact of prepotency we say must be admitted, and this position is justified by a study of the history of any of the established breeds of domestic animals. Without exception such breeds all show a narrowing of the ancestral lines to a few favored families due to the superior excel- lence and transmitting power of the individuals belonging to the family. For prepotency is obviously a family matter. One of the most notable instances of prepotency is that of the Ham- ‘bletonian family of trotters and pacers. The progenitor of this family was Hambletonian 10, a remarkable stallion who appears to have inherited his excellent characters from those famous imported sires of the early days of speed development, Messenger and Bellfounder. Hambletonian 10 himself was no mean performer, having to his credit a record of 2:48 14 as a 3-year old in 1852, at which time the fastest trotting record was 2:28; but it is as a breeder that he has won enduring fame. E. Davenport has studied with considerable care the relation of prepo- tency to the development of trotting and pacing horses in the United States. He found that up to and including 1901, a total of 26,327 horses had been admitted to the list of performers, 7.e., had records of 2 :30 or better. Of these performers, 14,808 traced back to eighty-five grandsires. In other words over 50 per cent. of performers traced back to slightly more than 1 per cent. of the grandsires of the breed. This fact is 574 GENETICS IN RELATION TO AGRICULTURE truly a remarkable demonstration of the relation of prepotency to the development of speed in the American Standard bred. But even more remarkable is the record of the ten greatest producers of speed up to that time. They are given in Table LXIV. Of the ten great sires here listed, one is by Hambletonian 10, eight have Hambletonian 10 for grandsire, and finally Nutwood, the greatest in the list, is by Belmont, by Adallah, by Hambletonian 10. Every one of the ten premier stallions of the breed, therefore, belongs to the great Hambletonian family. Further evidence as to the existence of prepotency has been given from time to time for many other breeds and for different characters. A typical investigation of this kind has been conducted by Hover for butter-fat production of pure-bred Guernsey cattle. From the advanced registry records for this breed up to December, 1915, Hover found that only thirty-two sires had produced three or more daughters having records equivalent to 600 pounds of butter fat at maturity. Of these thirty- two sires only three had produced more than ten such daughters, and all of these belong to the May Rose family. This same family contains six more of the thirty-two superior sires. Of the rest the Masher family contains seven; the Governor of the Chene family, five; the Glenwood family, five; and the Sheet Anchor family, six. Some of the sires of course belonged to two or more of these families. The results are in no particular different from those which might be obtained with any other dairy breed. A demonstration of the existence of prepotency, however, is far from a scientific treatment of the subject. While many geneticists admit that prepotency is as yet an unsolved problem, they have not failed to point out several ways in which prepotency might operate. These suggestions have pointed to the relations of dominance and recessiveness, to variations in the potency of factors themselves, and to interrelations within the hereditary complex as providing firm bases for the existence and in- terpretation of prepotency. We shall discuss each of these briefly below. The Mendelian Interpretation.—That interpretation of prepotency which refers it solely to the particular characters and the relations of dominance and recessiveness within them may be called the Mendelian interpretation. The simplest expression of this interpretation is found in the relation of homozygous dominants to those which are heterozygous. These two classes are often indistinguishable phenotypically, but the homozygous dominant when mated to recessives impresses its characters on all the offspring, whereas the heterozygous dominant only impresses its characters on half the offspring. The practical bearing of prepotency of this kind may be seen by reference to Pearl’s analysis of the inheritance of fecundity in domestic fowls. Here a Barred Plymouth Rock male of the genetic constitution (Z2M)(ZM)LL will. transmit high laying SOME BELIEFS OF PRACTICAL BREEDERS 575 qualities to all his female offspring regardless of the genotypes of the females to which he is mated, whereas one of the genetic constitution (Zm)(Zm)ll would transmit low egg laying capacity to such an extent that among his daughters even from high producing hens none would fall in the high producing class. For favorable characters the validity of this interpretation depends upon dominance of the determining factors, a condition by no means universally fulfilled. The Relative Factor Potency Interpretation.—There is some evidence that the potency of a given factor sometimes varies with the source from which it is derived. Pearl has suggested for example that the factor L when derived from the Cornish Indian Game has a lower absolute fecun- dity value than that of the same factor in the Barred Plymouth Rock. The suggestion amounts to an application of the hypothesis of multiple allelomorphism, a graded series of multiple allelomorphs of differing potencies, or different relations with respect to dominance, being con- ceived to determine the absolute degree of expression of the factors. We recall here Detlefsen’s work with the agouti factor of the wild Cavia rufescens which was recessive to the agouti pattern of the tame guinea- pig, and less decided in its phenotypic expression. The conclusions of Goldschmidt that races of the gypsy moth exist which have sex factors of various potencies, such that crosses between them give series of intersexual forms, while less definite with respect to the actual factors involved, provides some evidence in support of the belief that some of the phenomena of prepotency are dependent upon actual differences in the factors themselves. The Hereditary Complex Interpretation—The characters for which families are prepotent are evidently often complex, as for example speed in horses, total butter-fat production in dairy cows, beef con- formation in cattle, and so on. They must, therefore, depend upon a favorable genetic constitution with respect to series of factors. This interpretation is based upon the conception that factors form physico-chemical reaction systems and it follows the lines which have been developed in the application of this hypothesis to species hybrids. We have pointed out for instance that varieties of Nicotiana tabacum impress their total set of characters upon the hybrids with N. sylvestris because of the dominance of the tabacum reaction system. Certain characters which are recessive within the tabacwm group are expressed in such species hybrids apparently because of their interrelations with other factors in the tabacum group. This idea is also borne out by certain of the Drosophila experiments. Thus Morgan notes that the factor for truncate wings, usually recessive, is dominant in races which have the black factor. The hypothesis rests upon a belief that sometimes factor interrelations determine whether a particular member of an 576 GENETICS IN RELATION TO AGRICULTURE allelomorphic pair shall be dominant or recessive; and that this influence becomes stronger when large sets of factors determine a particular character. Greater Prepotency of the Male.—There has been a decided tendency to credit the male with greater prepotency than the female. Many investigators have pointed out that extra-biological influences such as the more rigid choice of males and the greater opportunity they have for impressing offspring may account for this belief among animal breeders. Some of the statistical evidence which Pearson has collected on this point seems to indicate no constant behavior in this respect. At the same time it should be noted that phenomena of sex-linkage and crossing-over may play an important role here. The operation of the former we see in Pearl’s investigations of fecundity in fowls. Here the male is obviously the more prepotent with respect to the transmission of fecundity. The operation of the latter we see in Drosophila experi- ments. Here there is no crossing-over in the male, as a consequence of which hybrid males more often transmit the particular set of factors which determine a phenotype like their own than do hybrid females. While the possibility of extending this phenomenon to mammals appears to have been destroyed by Castle’s work with rats, which demonstrated the occurrence of crossing-over in the male, nevertheless as a possible factor in relative prepotency of the sexes it should not be ignored. Conclusions with respect to prepotency. For the present then we must regard prepotency as an established fact, a phenomenon which has not yet been subjected to scientific analysis. From a biological stand- point, however, it is clear that even with our present restricted knowledge there is room for prepotency based upon the existence of different kinds of relations between factors. . CHAPTER XXXVII METHODS OF BREEDING Like modes of research, methods of breeding are the means by which certain results are attained. It is necessary to emphasize this fact, because even yet there is much confusion in the minds of breeders as to the relation which a particular method of breeding bears to results which have been produced by its employment. Not infrequently statements are made to the effect that a certain method of breeding is the cause of the excellence of one race or strain or the inferiority of another. There is a wide difference between the method of producing a given result, and the cause of its attainment. For the sake of clarity of thought we shall endeavor to emphasize this distinction, so far as is possible in the present state of our knowledge, in the discussions which follow. Phenotypic Selection.—The oldest method of breeding was simply that of mating together the most excellent individuals. In popular phraseology this is the method of breeding from the best—its funda- mental postulate is expressed in the old statement, like produces like. We have called it the method of phenotypic selection in order to empha- size the fact that the basis of choice for breeding in this method is the sum total of expressed characters of the individual. It is not necessary to recount here at any great length the sort of improvement which has been effected in modern breeds of domestic animals by the application of this method of breeding. Let it be sufficient to state that much of the excellence of modern breeds is an earnest of the efficiency of phenotypic selection as a mode of breed amelioration. It may, also, be stated justly that all later methods of breeding; out- breeding, line-breeding, inbreeding, and genotypic selection; are simply refined methods of breeding from the best—they are methods of pheno- typic selection plus something else; the something else usually ill-defined, but sometimes, as in genotypic selection, more definitely conceived. The limitations of the cruder form of phenotypic selection depend upon two primary causes, somatic modifiability of characters and geno- typic differences among like phenotypic individuals. Since differences which are due to modifiability tend in the long run to group themselves around a mean in the form of a normal variability curve, it may be stated dogmatically that long-continued phenotypic selection should tend to obliterate them. But it is not enough for the practical breeder to know 37 LY Ai 578 GENETICS IN RELATION TO AGRICULTURE that eventually a given result may be produced, his time is limited and he, therefore, desires, and rightly, to achieve a given result in the shortest possible time. A case in point is that which we have already discussed in some detail, modifiability in relation to selection for high egg produc- tion in the domestic fowl. Here Pearl found that modifiability was so great that simple phenotypic selection of the highest producers for breed- ing stock resulted in no improvement whatever in laying capacity. On the other hand, the application of a method of breeding which fully allowed for this effect of modifiability and which further took into account the germinal constitutions of the fowls selected for breeding purposes immediately resulted in gratifying improvement. With most characters the influence of modifiability is not so great as in fecundity of fowls. Often in fact modifiability may actually be utilized to advantage by the breeder in determining relative excellence. Thus any system of develop- ment which tends to call forth the highest possible expression of the capabilities of individuals tends to widen the differences between superior and inferior individuals. Both good and poor dairy cows tend to give increased milk yield when fed richly, but the increase is often more marked in the good cows. On the other hand, with horses in general it is possible by training to increase speed, but it is a question whether the increase in such a case is more marked in good or poor horses. We may say with confidence, however, that here training by developing the full capa- bilities of the animal tends to bring its speed up to such a standard that when compared with breed records, the superior excellence of the individual is definitely established. Modifiability, therefore, is on the .one hand a factor which tends to decrease the possible effectiveness of the method of breeding from the best; on the other hand, if properly utilized it is a powerful aid in the accurate selection of those individuals which possess the highest inborn capabilities. When we come to consider the influence of germinal diversity in phenotypic selection, we approach more nearly the problem of the real limitations under which the method of phenotypic selection labors. Here we may distinguish different ways in which germinal diversity may hinder phenotypic selection. Phenotygic Selection Does Not Distinguish Between Homozygous and Heterozygous Individuals—To the student of Mendelism this diffi- culty requires no further comment. It may be pointed out, however, that the difficulty increases as the number of factors for which selection is being practised increases. As with modifiability, however, this diffi- culty tends to be obliterated by long-continued selection, for such selection inevitably increases the proportion of homozygous individuals within a given phenotype or standard of selection. Roughly it may be said that the rate of increase of the proportion of homozygous individuals METHODS OF BREEDING 579 is inversely proportional to the number of factors concerned in the selection, for the greater the number of factors the slower is the rate at which the population approaches a uniformly homozygous condition. Theoretically complete attainment of this condition is only reached after an infinite number of generations, but practically the number of genera- tions which is necessary to measure up to within 5 per cent. of the possible limit is much smaller. It is, however, often so large that the animal breeder would prefer to use some other method, if by so doing, he could more quickly reach the desired standard of excellence and stability of type. At this point, however, it should be mentioned that selection is often made for characters which are recessive, or which give intermediates when in the heterozygous condition. In such cases, of course, the relation between phenotype and genotype is simpler and methods of selection gain in effectiveness in consequence thereof. Phenotypic Selection Does Not Make Allowance for the Differences Which May Exist Among the Genotypes of a Given Phenotype.— Simple examples of this proposition may be quoted without number. In fowls for example there are dominant whites like the White Leghorn and recessive whites like the White Plymouth Rock. The diverse progeny which is obtained by mating these two breeds together has been described in detail in a previous section. There is some evidence that a similar condition may obtain in cattle with respect to white coat color. White is, likewise, dominant in the horse, and may therefore conceal a large number of latent factors. In the pig the same differences in behavior with respect to white coat color have been noted. There is reason to believe that the same kind of diversity in genetic constitution obtains for economic characters, as for those not so strictly utilitarian. The breeder who follows a method of phenotypic selection should not, therefore, be surprised if crossing different strains results in a disappointing lack of uniformity in hisherd. It is not difficult to see that in differences of genotype such as have been noted here, the breeder of best to best meets one of his most perplexing problems. Phenotypic Selection Fails to Allow for Heterozygosis——In other portions of this book the assumed effect of heterozygosis on vigor and fertility has already been discussed at considerable length. If a hetero- zygous condition ever can determine a more vigorous development than the homozygous condition, then the breeding practice of the future will sometimes be materially altered in order to take advantage of this fact. But aside from this possible difficulty there is sometimes a very real diffi- culty in the fact that selection has set as its standard a type absolutely conditioned by a heterozygous genotype. The striking and ever-quoted instance of this fact is the Blue Andalusian fowl, which no amount of 580 GENETICS IN RELATION TO AGRICULTURE breeding has ever been able to establish in a pure form. If more than one pair of factors is concerned in such a case, the progeny is correspond- ingly of greater variety—it becomes a case of the Blue Andalusian fowl on a larger scale. It is probable that this condition is not often met with. The only remedy for it is to change the standard of selection. Pedigree Breeding.—If to phenotypic selection be added the concep- tion of family excellence we obtain the foundation upon which pedigree breeding is based. Pedigree breeding, therefore, is merely a refined system of phenotypic selection; in one form or another it is a very old system of breeding. The principle of pedigree breeding is a laudable one, for it judges the individual not only upon its own expressed charac- ters but also upon ‘those which its ancestors have exhibited. It is, therefore, one more step in the direction of strict genotypic selection. From an ideal standpoint the effect of pedigree breeding is to empha- size the value of breeding ability. The existence of strikingly prepotent animals must have been a large factor in the development of this method. By insisting upon breeding ability as a measure of excellence, the tendency has been to eliminate the effects of modifiability and heterozygosis, and to favor the selection of the most excellent homozygous individuals for breeding purposes. By so much it has concentrated blood lines within breeds to a few of those which have proven most excellent, and thereby it has amply justified its adoption as a method of breeding practice. The weakness of the method lies not so much in inherent defects as in the uses to which livestock men have put it. The establishment of herdbooks in which pedigrees are recorded, while undoubtedly an impor- tant step in advance in the history of any breed, has tended to empha- size unduly the value of pedigree, often to the extent that individual excellence has not been rigidly insisted upon and even inherent family defects, like the barrenness of the Bates’ Duchess Shorthorns, have been regarded lightly. It cannot be too strongly insisted upon that the . fundamental basis of pedigree breeding, as well as all other systems of breeding, is individual excellence. No matter how favorable the ances- try, an inferior individual within a family of superior excellence is likely to have lost one or more of the factors upon which that excellence is based. If that is the case, use of such an animal for breeding purposes merely increases the number of animals which lack that portion of the favorable genotype and by so much multiplies inferiority within the family and breed. There are numerous instances in breed history of pedigree fads which have worked to the ultimate disadvantage of excel- lent families because of the undue prominence given to ancestry in selecting breeding animals. Breeding Systems Based on Blood Relationship.—The influence that kinship has had on marriage laws in human society is familiar to all METHODS OF BREEDING 581 educated people. The old Mosaic laws forbade the marriage of closely related individuals, and complex systems of marriage apparently directed against consanguineous marriages are found even among many uncivilized tribes of peoples. Undoubtedly the existence of these systems of mar- riage in human society has had some influence in shaping the methods which have been adopted by animal breeders, but at most the influence has been small. The most potent factor in livestock practice has un- doubtedly been the type of results which has been attained by following one system or another, and the general utility which the given system has in the hands of the average breeder. With respect to the degree of kinship permitted in matings there are three general systems of breeding: out-breeding, in which consanguinity is avoided as much as possible; line-breeding, which is based upon matings of moderate blood relation- ship; and inbreeding, which is based upon matings of animals closely akin to each other. Each of these methods of breeding will be discussed below. Inbreeding.—Specifically inbreeding is a system of breeding in which sire is bred to daughter, dam to son, or brother to sister. It is, therefore, based upon the closest possible types of mating. This system of breeding naturally has had its origin in the desire to intensify the blood of notably superior individuals. The most used form of it, perhaps, is that in which a famous sire is bred to his daughters and even at times to the second generation of daughters which have been produced by inbreeding. The method of inbreeding has been particularly useful in fixing types in the early, formative period of the breed. It was the powerful tool which that great breeder of the eighteenth century, Robert Bakewell, employed in the improvement of horses, cattle, and sheep; and with astonishing success. Evidently at that time the popular prejudice existing against inbreeding was even stronger than it is today, as we may judge from the statements of Culley written in 1794. _ The great obstacle to the improvement of domestic animals seems to have arisen from a common and prevailing idea amongst breeders—that no bull should be used in the same stock more than three years, and no tup more than two; because (say they) if used longer, the breed will be too near akin, and liable to disorders; some have im- bibed the prejudice so far as to think it irreligious; and if they were by chance in posses- sion of the best beast in the island, would by no means put a male and female together that had the same sire, or were out of the same dam. Mr. Bakewell has not had a cross for upward of twenty years; his best stock has been bred by the nearest affinities; yet they have not decreased in size, neither are they less hardy, or more liable to disorder; but, on the contrary, have kept in a progressive state of improvement. Culley might have written in the twentieth century, for even today inbreeding is popularly blamed for a variety of ill effects. Of these we have discussed decrease in fertility and vigor somewhat, and have reached the tentative conclusion that inbreeding of itself does not always result in diminished vigor and fertility, and therefore in all probability does 582 GENETICS IN RELATION TO AGRICULTURE not stand in any causal relation to it. As a method of breeding, however, it gives abundant opportunity for a race which has any defects what- soever to express them, for by simplifying the genotypic constitutions of the animals within a family and making them like each other, it tends to increase the proportion of recessive defectives produced in the family. But if no inherent defects exist in the family, then such an effect cannot be produced, and the practice is on the whole to be commended. The advantages of a system of inbreeding are found in the close ap- proach which this method makes to a strict method of genotypic selec- tion. It overcomes that difficulty of a system of phenotypic selection which arises from the possibility of mating different genotypes which are alike phenotypically. By this method the breeder is assured of genotypic identity in his breeding stock, because they have received their germinal elements from a common ancestor. Accordingly we are not surprised that this method has proven so notably successful in fixing types in the formative period of a breed’s existence, because it is just at this time that both genotypic and phenotypic diversities are most common, and the difficulties arising from their existence most baffling. The increase in prepotency which is universally acknowledged to ac- company inbreeding is in entire harmony with this interpretation—for by simplifying the genotypic constitutions of the individuals the tendency is to secure more and more individuals which are homozygous for all or nearly all the favorable germinal elements, and which possess in con- sequence of this fact superior transmitting capacity. The breeder who would add the practice of inbreeding to his operations must learn to cull with a firm hand, however, whenever defects appear for they indicate inevitably that some necessary constituents of the hereditary material have been lost. If he can do this, he has added a powerful instrument for improvement to his breeding equipment. Line-breeding.—The term line-breeding designates breeding within a given line of descent. By common agreement the term does not in- clude inbreeding; it begins with those degrees of relationship which are just outside the pale of inbreeding. It is, therefore, a system of breeding in which cousins of different degrees are mated with each other. No system of breeding has been so popular or so generally productive of good results as line-breeding. Like inbreeding, it is a method of breed- ing which approaches as nearly as present knowledge will permit to the ideal of genotypic selection. Because the individuals which are mated belong to the same line of descent and exhibit similar sets of characters, it is logically just to conclude that they possess similar sets of germinal elements. In this fact we have the whole explanation of the uniformity of progeny which is so characteristic of continued line-breeding. Line-breeding is popularly credited with all the excellencies of in- METHODS OF BREEDING 583 breeding and a greatly lessened tendency toward the production of de- fectives. There is a measure of truth in this belief for line-breeding, by the mating of animals of slightly wider relationship than those used in inbreeding, permits the introduction and intermingling of hereditary ele- ments from slightly different lines of descent. It is in this that we must seek the explanation for the greater success which line-breeding has had among most practical breeders. That explanation is not far to seek, for if the production of defectives depends upon factors which are distributed in Mendelian fashion, and there is no reason to believe that it does not, then any introduction of diverse hereditary elements is likely to result in the neutralization of the defective elements in both hereditary systems, for only under unusual conditions would such elements be identical in the two systems. Along with this tendency toward decreased pro- duction of defectives, however, there is the ever present possibility of dissipating the elements characteristic of the ideal family type, of ming- ling them with others not so productive or desirable. The tendency is by no means so strong as it is in out-breeding, but it is stronger than in inbreeding. It serves again to emphasize the fact that any system of breeding must be based upon matings of superior individuals. Out-breeding.—Out-breeding is merely a system of breeding best to best, at the same time avoiding relationship in the animals which are mated. While it may tend to avoid completely the disasters which often attended inbreeding, it is subject to all the defects of the old system of phenotypic selection. Chief among these is its tendency toward lack of uniformity in the herd. The harm which it does, however, depends largely upon the breed in which it is practised. Thus among Shorthorns the extraordinary multiplication of individuals of certain families leaves a wide field for the selection of superior individuals distantly related. and of the same type, so that in this breed, a form of out-breeding which is really not out-breeding at all, but a very mild form of line-breeding, may be adopted without much danger. Out-breeding, however, is in a sense a harking-back to methods which have been discarded, and although the new breeder may do well to start his operations by avoiding too close affinities, he should steadily endeavor to master the problem of dealing with consanguineous matings sanely and effectively. Other Systems of Breeding.—Under the chapter on the utilization of hybrids in animal breeding, we have discussed at some length grading and cross-breeding. The former of these methods of breeding provides a simple and practical method for improving livestock on a large scale, and its practice is to be commended. Grading is not to be contrasted with any of the systems of breeding which have been described, but it may be compared on the one hand with pure breeding and on the other hand with aimless scrub breeding. In grading, any of the systems of breeding 584 GENETICS IN RELATION TO AGRICULTURE which have been discussed above may be used, the only requirement is that the sire must always be pure-bred and of the same breed. By rigid selection of females which approach most nearly to the ideal type of the breed from which the sire is selected, grade herds after three or four gene- rations will approach very nearly to the standard of excellence from an economic standpoint at least of pure-breds. Crossbreeding we have also described in a previous chapter. It is an economic procedure entirely, and is based on the uniting of favorable characteristics of two strains in the cross-bred animals. Along with crossing sometimes comes the increased vigor of hybrids, sometimes striking, other times only slight. Although greatly decried by breeders and advocates of pure-bred livestock, crossbreeding is sound in theory and productive of good results in practice. To reap its benefits, however, it must be followed systematically. The breeder must not be tempted to allow the excellence of cross-bred animals to overcome his better judgment to the extent of permitting their retention in the breeding herd. Increased vigor and size are not alone responsible for the adoption of cross- breeding by some livestock men, but the changing standards of market demands have sometimes favored types of livestock not represented in any existing breed. Two alternatives are then open to the breeder, to establish within existing breeds the type demanded or as it were to synthesize such a type by crossbreeding. The former method is pro- ductive of the most permanent good, but it is a slow and expensive pro- ject and one requiring the good judgment of an unusually critical breeder. It has its illustrations, however, in the establishment of the Cruick- shank family of Shorthorn cattle, the American type of Hereford cattle; and as an outgrowth of crossbreeding in the building up of the Corrie- dale sheep of New Zealand. Crossbreeding, however, often achieves the same result immediately with existing materials; and, the advantage of a particular cross having been established, it does not require as much skill in operation as the establishment of a pure breeding type. As agricultural science develops we may expect to see crossbreeding for specific purposes much more fully utilized than it is at the present time. Genotypic Selection.—The method of genotypic selection is a method based on a knowledge of the genotypic constitution of the individuals used in mating. Although but little breeding can be ordered along this | line on account of the dearth of knowledge of the actual factors which are concerned in particular character complexes, nevertheless to all practical purposes intelligent application of the methods of line breeding and in- breeding amounts to the same thing. Thus far our knowledge of factors is only extensive enough to apply this method of breeding to relatively simple problems, such as that of producing polled breeds of cattle by the METHODS OF BREEDING 585 use of polled mutants, mule-footed breeds of hogs, hornless sheep, or particular coat colors in horses, cattle, and swine. Nevertheless it is a very useful conception to add to the stock-breeder’s fund of knowledge. The method of breeding for increased fecundity in poultry devised by Pearl is the best existing illustration of the employment of genotypic selection in attacking a problem of economic importance. We have pointed out how Pearl on the basis of investigations of winter egg pro- duction in fowls established the fact that two dominant factors for high winter egg production existed. One of these factors, L, determines the production of pullets which lay somewhat less than thirty eggs during the winter period; the other factor, M, which is sex-linked, adds to this so ‘that birds possessing both these factors lay over thirty eggs during the winter cycle. The breeder’s problem, therefore, starting with a mixed flock, is to isolate and breed from individuals of the genetic constitutions (WM)(WM)LL for males and (WM)ZLL for females, to the end that the flock will consist entirely of individuals of these genotypes. So valuable are the specific directions which Pearl has given that they are printed in full below. 1. Selection of all breeding birds first on the basis of constitutional vigor and vitality making the judgment of this so far objective as possible. In particular the scales should be called on to furnish evidence. (a) There ought to exist, for all standard breeds of fowls, normal growth curves, from which could be read off the stan- dard weight which should be attained by a sound, vigorous bird, not specially fed for fattening, at each particular age from hatching to the adult condition. These curves we shall sometime have. (b) Let all deaths in shell, and chick mortality, be charged against the dam, and only those females used as breeders a second time which show a high record of performance in respect to the vitality of their chicks, whether in egg or out of it. This constitutes one of the most valuable measures of constitu- tional vigor and vitality which we have. If for no other reason than to measure their breeding performance, a portion of the females each year should be pullets. In this way one can in time build up an elite stock with reference to hatching quality of eggs and viability of chicks. (c) Let no bird be used as a breeder which is known ever to have been ill, to however slight a degree. In order to know something about this, why not put an extra leg-band on every bird, chick, or adult, when it shows the first sign of indisposition? This then becomes a permanent brand, which marks this individual as one which failed, to a greater or less degree, to stand up under its environ- mental measures of constitutional vigor. (d) Many of the bodily stigmata by which the poultryman, during the last few years, has been taught to recognize constitutional vigor, or its absence, have, in my experience, little if any real significance. Longevity is a real and valuable objective test of vigor and vitality, but it is of only limited practical usefulness, because of the increasing difficulty with advancing age of breeding successfully on any large scale from old birds of the American and other heavy types. 2. The use as breeders of such females only as have shown themselves by trap- nest records to be high producers, since it is only from such females that there can be any hope of getting males capable of transmitting high-laying qualities. 586 GENETICS IN RELATION TO AGRICULTURE 3. The use as breeders of such males only as are known to be the sons of high- producing dams, since only from such males can we expect to get high-producing daughters. 4. The use of a pedigree system, whereby it will be possible at least to tell what individual male bird was the sire of any particular female. This amounts, in ordinary parlance, to a pen pedigree system. Such a system is not difficult to operate. In- deed, many poultrymen, especially fanciers, now make use of pen pedigree records. It can be operated by the use of a toe-punch. All the chickens hatched from a par- ticular pen may be given a distinctive mark by punching the web between the toes in a definite way. If one desires to use a more complete individual pedigree system, he ' will find the system described in Bulletin 159 of the Maine Agricultural Experiment Station a very simple and efficient one. It has been in use at this Station for 7 years, with entire satisfaction, on the score of both accuracy and simplicity. 5. The making at first of as many different matings as possible. This means the use of as many different male birds as possible, which will further imply small matings with only comparatively few females to a single male. 6. Continued, though not too narrow inbreeding (or line-breeding) of those lines in which the trapnest records show a preponderant number of daughters to be high producers. One should not discard all but the single best line, but should keep a half dozen at least of the lines which throw the highest proportions of high layers, breeding each line within itself. In the above set of directions two things will challenge the student’s interest most, namely the emphasis which is laid upon constitutional vigor and vitality in the selection of breeding birds, and the fact that a system of line-breeding or inbreeding is used in order to increase fecundity. The relation of the above directions to the genotypic behavior are not difficult to point out. Of females there are two different types (WM)ZLL and (WM)ZLI which are high producers; the remainder are either mediocre or low producers. It is assumed that by trap-nest records, it has been possible to segregate out a certain number of such high-producing hens from a mixed flock of low, high, and medium pro- ducers. When these are mated to males from the same lot, a variety of results will be produced according to the genetic constitution of the males. In Table LX XIX are collected the results which follow when females of the two high-producing genotypes are mated with the nine possible kinds of males. Now if the numbers of females of genotypes (WM)ZLL and (WM )ZLI in each pen are approximately equal—in practice those of geno- type (WM)ZLI would probably be in excess—then it will be practically impossible to distinguish matings of types (1) to (3) and possibly (4) and (5) unless the number of daughters tested from each pen be relatively large. In this connection we recall the fact, as a further difficulty, that modifiability in egg production is relatively very great. If now an equal number of pens from matings (1) to (5) should happen to have 587 METHODS OF BREEDING a ee ee (At) (FEAL) mop ez | nz(mm) | tat) AL) Ol Ss} ZUM) | orm) (AL) | wUIpour % og ITZ (MM) ITM M) (WM) | cANYpaut %OG | 17Z(w AL) eae ana See eee ea TIMM)WM) | unipour %o¢ | qrz(wiy) | tTZ WAL) of ITM) CATAL) | wAnspour % oor | rTZ(wat) | ATC ALS TUAL | TEEPE (208) ITZ M) | orm) | WMYpo %oor| TIZ(uA) | TIZUA) Sole" So[BUl9 SolByL So[BUlO TT SO[BIL So[BU19 iT So[BVUl8 TT (6) 104M) (HM) (8) 17(@UM) 4M) (1) TT(HM) (uM) sale] (HM ) (WM) MOT % 2481 NZ (uM) (uM) CW AL) MOL % GZ NZMM) | 1TMM) (WM) WNIpeu % GZ 1TZ(MM) | I1TCHM) (WM) | WUNIpou % og ITZ (WM ) 1T*HM) (WM) | AMIpeut %eCZ | ITZ(WM) | TT(MM)(WM) | WNIpem %%{ZT | = TIZ(wM) | TTmM)(WA) | wNIpeu % cz TIZMM) | aac M) WWM) M) | wNrpeut %es | 1Z(WA)| WWM) M) | UNIpou % 241 UZ\NM)) 11 WM)(WM) ysty % Gz 17Z( WM) ‘ 1T( WM) (WM) yay %esS | ITZ(WM) | ITM) (WM) Yay % CS 1TZ(WM) | TH WM) (WM) yay % os TIZ(WM) TIUWM) (WM) ysty % ZL! TIZ(WM) | F ITMM)(WAL) | wnpeu %eg} 7177 (mM) 1T(“M) (WAM) | NIpeut %oOsG | 177MM) | TI(MmM) (WM) UINIpeUl % CZ TIZ“M) | TIMM) (WM) | vantpeut % o¢ TIZ(UM) TIZ NM) 1T(WM) WM) yay %0¢S | 1TZ(WM) | IT (WM) (WM) Yysty %Cs 1TZ(WM) | TIWM) (WM) ysty %0¢ TIZ(WM) TITWM)(WM) yay %¢S| TIZ(WM) SOTBIT | so[BUld solBT So] 8 Ula WT SOB se[B Ula 7 Soe ula, (9) 2(@#M) (WAL) () 17T(“M) (WM) (h) TT(M“M) (WA) SlBIN | y I % GB 1Z(WM) 0 NM) WM) | wrpeut %oo | nz Mm) | WAM) | wnipent ? ITAA) | = YA %OS | rTZCHAL) o 1TWM) (WM) ysty % 0g 1TZ(WM) } 1TZ(WM) 1T(WM)\WM) qq %0¢ | 17Z(WM) TUAW M) WM) yay %ez| Tazdym) | TIAL) va) ysty % 0S TIZ\WM) ITM) M)| ~—-YBEY % OOT | 1Ta AM) | LEAL ALS tate ete | ee eA) | orm) Cra) yay %001) §=«17ZUM) | TTZ(WM) sole] so[eula Ty SO BIL So] BU19 JT Sole] soTeute ; So[BMa iT 1 I Ie] [eu9,T () 1(WwM) (WAM) (3) 17(WM) (WM) (1) T7(WM) (WM) sa[BIN SHIVJY JO SGNIY ATAISsog TTY HLIM SNOF{ ONIONGOUd-HDIF{ ONILVJ, JO SLIOSAY AHL GLVULSATIT OL AIAV], IVOILANOTHL— YXTXXT @IAV 588 GENETICS IN RELATION TO AGRICULTURE been chosen, then we will obtain the following distribution of males with respect to their genotypes and relative numerical frequencies: (1) SGP) (Wa EB Peay oe oe 27 (2) (WM)(WM)LI........... 30 (CK GE ONG) A. BY! ee eee ee 7 (4) GV AE Want IG ocak ss 9 (5) (WM) (Won) Bl. . 0. oes ©. 6 CG) UC CW aah tea teas 1 opal. tree cee ns tore a rere 80 In the next generation, therefore, the probabilities are strongly in favor of the selection of males of types (1) and (2) and the trap-nest ‘records should insure the selection of hens which are mostly of the two genotypes given in the table. At any rate matings will be restricted to types (1) to (6), the inferior types (7) to (9) are excluded. Accordingly 9 E232 (69) X 565 — 0 2 G10 (100) 9 F255 (48) X S564 ye 254 (16) 9 G12 (16): 19 (70) 39 (100) 53 (44) 303 (64) X F563 aie 85 (73) 192 (57) 213 (29) 9 D39 (62) X @D5 | 237 (65) X S554 136 (48) © Mean, 62 2G18 (61) 347 (69) X S562 248 (67) 9 G11 (47) 134 (111) 363 (74) X S567 165 (35) 198 (39) Mean, 61 506 (19) Mean, 67.74 Fig. 226.—Pedigree of line D5D39, characterized by high winter egg production. Bold faced figures are band numbers of females; italics of males. Italics in parentheses give the winter egg records. (After Pearl.) METHODS OF BREEDING 589 this second generation should show a very marked improvement in egg production, if breeding be carried on within the line. 2 F308 (78) 7564-0 2 G30 (45) 62 (58) 117 (46) 197 (66) 428 (49) 495 (1) Mean of high line, Q E248 (47) x 7553 62.8. (Mutant?) 354 (55) X 7566 2 G204 (41) 166 (49) X 7 D381 264 (15) { 9G229 (28) 141 (51) X #DS1 \ 458 (11) Mean of D81’s daughters, 23.76. 172 (50) X #578 — Cross Mean of mutant (high) line, 56.5 9 D168 (33) x 7 DE1 9 E231 (25) X 7552 — 9 F233 (32) x 7573 —0 Q G221 (16) 419 (9)X %451— 9F165 (7) X 669 430 (12) ATT (1) 209 (38) X 655 — 0 Mean of main (low) line, 9.67. _813 (26) X P54 ay en) 363 (11) X 7550 — 9 F249 (30) Mean of main 15 (18) (low) line, 22.0. 163 (19) 200 (12) . 141 (0) 116 (28) 161 (11) 24 (23) Mean of main (low) line, 17.8. Fia. 227.—Pedigree of line D61D168, characterized by low winter egg production. Conventions are the same as in Fig. 226. The progeny of the mutant (?) high producer E248 is included in this pedigree. (After Pearl.) This method of increasing egg production is not entirely theoretical, but it has actually worked after 9 years of patient selection of high trap-nest performers failed to show any improvement. The two pedi- 590 GENETICS IN RELATION TO AGRICULTURE grees given herewith show the type of results which Pearl has secured by an application of this method of breeding. In Fig. 226 is given the pedigree of line D5D39, a high-producing line. It will be observed that a mean winter egg production of about sixty eggs is maintained through- out four generations. In every case, of course, males were selected for mating of a genotype corresponding to the females. The last generation in this pedigree shows how large.a range of phenotypic fluctuation may be expected in breeding operations with fowls. For contrast with the above line we may present line D61D168, a low- producing line. The pedigree is given in full in Fig. 227. Here the mean winter egg production of the main low line is about one-third that of line D5D39. This line is interesting on account of the appearance of a high-producing individual, #248, which is probably a product of Mendelian segregation, but may possibly be a mutant. Not only was this individual herself a high producer, but she transmitted her producing abilities to her daughters, so that in the high line of this race, only one individual G495, possibly pathological, failed to exhibit a considerably higher winter egg production than the highest individual in the low por- tion of the line. These two pedigrees illustrate clearly what a systematic plan of breeding may accomplish after mere phenotypic selection has failed completely. We should not fail to point out as a factor to be considered in in- terpreting Pearl’s directions for breeding poultry for high fecundity, that neither Pearl nor any other scientist claims that the two factors, L and M, are the only ones concerned in breeding for fecundity. In any mixed flock of birds there must be a number of other factors, which aithough they may not have as marked an effect as the two primary factors, nevertheless will appreciably affect winter egg production. For this reason the breeder is admonished to use moderate inbreeding or line-breeding, because by too close inbreeding he may inadvertently breed his flock to a homozygous condition for unfavorable modifiers. Line-breeding is necessary, because by this method a like genotypic constitution is assured. Further, too hasty rejection of lines which do not measure up to standard may result inadvertently in the discarding of some line which had greater potentialities than those at first most productive, consequently the warning not to reject all but the best line. Moreover, we suspect that by isolating different high lines and then crossing them and applying the above procedure to the hybrid progeny still better strains might result. The directions which Pearl has given for poultry breeding may be applied with proper modifications to other livestock. They should be carefully studied by every breeder, with the distinct proviso that no rule of thumb, however excellent, can supply the ability for intelligent practical application, an indispensable feature of successful breeding operations. CHAPTER XXXVIII METHODS OF CONDUCTING BREEDING INVESTIGATIONS Any livestock breeder who wishes to carry on breeding operations in an intelligent fashion, particularly if on a large scale, will find it necessary to adopt some definite system of keeping records. Whatever system is adopted it should fulfil at least three requirements: it should be simple; it should be concise, that is it should confine itself to the essential features of the breeding operations; and it should be adapted to the particular conditions of the individual livestock breeder. The last desideratum makes it impossible to outline here any specific plan for keeping written records, consequently certain features of this problem will be discussed so that some definite conception may be gained of the matters with which records should deal. Judging the Individual—oOf first importance in breeding operations is some method of determining individual worth. In certain cases, as for example, in beef cattle, this depends largely upon visible character- istics, and the breeder has only to build up in his mind by constant association with his livestock an ideal to which he desires to direct improvement in his herd. Whenever he can introduce objective tests, the breeder gains by doing so. The practical breeder has often felt the need of such objective standards of judgment, and from time to time he has attempted to introduce them. Sometimes such tests are very easy to apply, as for instance the speed test in race horse breeding. Some- times, however, they are more difficult of utilization, as for example, individual butter-fat production in dairy cattle or individual egg pro- duction in poultry. Nevertheless even such records may be obtained economically if everything be planned so as to expedite the work con- nected with them. Methods of keeping dairy records have been devised which enable the dairymen to obtain and record accurately the daily production of his cows by spending about 2 minutes per day per cow in doing it. In Fig. 228 is shown the equipment for carrying out such work and a convenient mode of arranging it. It will be noted that all the necessary equipment is at the hands of the operator, so that no time whatever is lost in obtaining and recording the data. For testing butter fat, composite samples are used and the actual test is often made by some central creamery or appointed milk tester, rather than by the dairyman himself, although the latter method is perfectly feasible. 591 592 GENETICS IN RELATION TO AGRICULTURE Fig. 228.—Equipment necessary for obtaining individual records of production of dairy cows. (After Lane.) Fig. 229.—Type of trap-nest used in Maine station poultry breeding operations, shown set and sprung. One side removed to show interior. (After Pearl and Surface.) CONDUCTING BREEDING INVESTIGATIONS 593 Since so much has been said about the Maine Station investigations of fecundity in fowls, perhaps it would be of some interest to know how records are obtained there. The type of trap-nest in use is shown in Fig. 229. Details of construction need not be taken up here, except to remark that durability of materials is a prime requisite for continuous service. The absence of any springs or other involved contrivances has made it possible to use this type of trap-nest in extensive breeding in- vestigations involving a flock of about 2000 hens. Ten such nests are used in a pen of fifty birds, and an attendant visits the pens at intervals of one hour or more, depending upon the rate of egg laying. Obviously a method such as this is expensive even when reduced to the simplest terms, and it is, therefore, applicable only to the selection and production of breeding stock. It is difficult, however, to conceive of any other accurate criterion which might be adopted. It should be noted that statistical requirements do not demand that complete records be obtained, for the existence of modifiability and other kinds of individual variability make it impossible in any event to get anything but an approximate record. Accordingly in recording the data of production of dairy cows, for example, it is not necessary to weigh and test the milk every day for the whole period of lactation, but two or three 7-day periods at stated times with respect to the beginning of lactation will give a sufficiently accurate estimate for all practical purposes. Similarly in poultry breeding, Pearl has found that produc- tion during the winter period is a sufficiently accurate and distinctive index of the egg-laying capacity of a hen. Further the danger from unjust comparisons should always be empha- sized. A comparison between egg production of hens in the second laying season and pullets would favor the pullets, for pullets ordinarily lay more eggs during the first season than they.do as hens in the second season. Moreover different parts of a given season are not equivalent. A pullet lays more eggs in a given length of time during the spring cycle beginning about March 1, than she does during the winter cycle. A cow, likewise, produces more milk during the early part of her lactation period than she does later on, and she reaches her maximum capacity at 5 or 6 years of age. With respect to these points we have reproduced in Table LXVI, the comparative indices which Pearl has calculated and which provide a method of comparing the productions of cows of different ages at different stages in the lactation period. As an additional variable in this case we should include the time at which a cow freshens, whether in spring, summer, or fall, as having a definite influence on herd produc- tion of milk and butter fat. We could go on recounting without end such factors which must be considered in making accurate comparisons. The point, however, is sufficiently obvious, namely, that even objective 38 594 Tilly Alcartra 123459 Calved October 2, 1908 Butter 7 days (6 years) E Hees ret erat 30. Vial ees ret neycusi occ ieteucusne Persie stents 632. Butter 7 days (5 years).......... 29 NTI out hciettagh b+ cholovs events aha ree 715 Butter oO Gays)... a.c.2 =. seh nee 122 Enea eth a kta. s ears erent 3,066 Butter 90rdays.\.jgagsvias eect O00) VET a ee eechae La EREY cece oueaes sera 8,793 Butter fO0"dayal es) sic. 4c « 0 396 164 EA ae Rees 8 Re ea PSU REN 9,702 Butter 7 days (8 months after CHINA No trees« seiske serene ee: 19. 11 RE ee Some racer, are ee ae 473 Poet MEV. C RIS. PaaS sa ake hey teae se 1,189. Ms BORDA ARAN? Beats (ot ee 30,451. (World’s yearly milk record) Butter 7 days (3 years)....:...... 23 Mn Ree Zee teoe eh ea ake cheer ea St & 613 Butter 7 days (8 months after Calvino) ney ene ern eevee 7 (CAA Miglin 28 oot Seacl 420 Butter Ui year fos ste wed bois: 841 es 2 5 ee es Rr GRIN ea pi Nice 21,421. Butter 7 days (30 months)....... ff il] qa ae See sl eee | ea ee 490. Butter 7 days (8 months after Calving) eee oe ae eee 14. Bog ca? hee pe totale Mea tote gece ie aches 362 Butter, Z285idaysrn.... er dee ee 556 Ae cae cant SRT are Reet ie 14,837 Fie. 230.—Pedigree of Tilly Alcartra, world’s record milk producer, showing produe GENETICS IN RELATION TO AGRICULTURE Aleartra Polkadot Corrector 30624 22 A. R. O. daughters: Geneseo Belle Polkadot.......... 34.39 > Mille, oh. eee eee 7” Butter lisvears casa rta oer 916.17 | Milk). ek BOR eee 20,816.20 Alcartra, Abbelark <2 Sasnuo kee 27.87 13 others from 20 to 27.1 pounds i 10 A. R. sons He has 871¢ per cent. of the same blood as oie Watson Prima Donna, 31.10. i Brother to the dams of: Filldale, Seis... o. cee ee eee 33.85 K. P. Alcartra (814 years)....... 30. 87. Butter sOrdayssanee caer (World’s 3-year-old records) By a brother to the sires of: Sadie Viale(Conh4theeeee ee one Four others from 30 to 31.8 pounds. tes nag bo — ric p— aa dee Be a —— D> i Tilly Lou 2d 82057 By a brother to the sire of six 30- pound cows. Her sire is by a son of De Kol Burke, whose 73 A. R. O. daugh- ters include: Lint Burke (4 years)2) 597.072. 32.76) River Sadie D. K. Burke......... 32.29 Mille (itdhay St soe et eee 920.80 Mike 30 davis) see eee oor 3,725.60 Milk*2 years). siieeot. ane 54, 805.20. (World’s milk records) . Five others from 30 to 31.7 pounds, 28 others from 20 to 27.2 pounds; and who is grandsire of Sp. Brook Bess Burke, 34.51 pounds. Five others from 30 to 33.5 pounds. eo iF es ae ay ee =. eS a, CONDUCTING BREEDING INVESTIGATIONS 59 Phiebe De Kol Burke 25368 28 A. R. O. daughters: IManidevBurkesiitc acces saci cae « Chief Phiebe Oak Duchess 28176 W- Ri. Jones. 20's: Phieber:s 5.13.25.: 21 A. R. O. daughters: She will do Uneeda............... Ollie Wat. Prima Donna.......... 31.10 Five others from 20 to 25.6 pounds. allie Geuriiia: 2d). hs osels da es 27 .46 17 A. R. sons. Wisconsin Bride Phiebe........... 27.42 ° Lady Oak 2d 39947 LLIN 0 a0 Ns A 25.98 BURCEr tA ae aes Seer Aee ents Per Mouelas. 2dees: .lleg sees aes 23.02 Maas | Siediny as. a cele deine 8: Seven others from 20 to 22.9 Five A. R. O. daughters: pounds, 5 A. R. sons. Ogk Derikol WMOiyears) tect From a sister to the dams of: Lady Oak 20's Homs.D, Ks. 2035) Grace Fayne 2d’s Hom......... 35.55 Two others with 21 and 27.4 pounds. (World’s record) Two A. R. sons. Jessie Fo. 2d’s Maud Hom...... ibe | Pearl of the Dairy’s Joe De Kol 23450 Three others from 30 to 81 75 A. R. O. daughters: pounds. RearlOrmsbye Burke yee emcees tate Aleartra Polkadot 50798 Pearl Neth. Vergeus.c.. se5-42s56- Leshan re hy Gt (cha pane ee er 29.09 32 others from 20 to 26.9 pounds. Rime oh teeth en FET 597.10 | 9A. R. sons. Butters Oday seus «2. seiner iain: 120.16 Alcartra’s 2d’s Rose 44430 MER ir Ar iors cas ae hs 2,605 . 00 Butter 7 daysct..crccs fet cars «ska « Five A. R. O. daughters: = IN Tl ae Bie Pee Reto ole © picks eis tae aN Lyndon. Al. Polkadot............. 32.54 Three A. R. O. daughters: Al: Polkadot-Ornrmsby 32. daes oe 31.25 Alleartra Polktadotics ni. «exe as. mers: Al. Polkadot 2d (344 years)....... 22.97 IBA COGCENScc abaasoobe peed cle Two A. R. sons. All Gantraheachnraanacase us sty. « cralsias Sister to the sire or dams of: Two A. R. Sons. Sp. Brook Bess Burke............ 34.81 | Phiebe De Kol Burke 25368 pom ball Pink 8d 24 454)26). shes ae eee 31.69 His sire is by a brother to the sire of Ollie Watson Prima Donna........ 31.10 Aaggie Cornu. Heilo Butter Boy Burke 29327 j Pauline (414 years), 34.32 pounds; 12 A. R. O. daughters: the first 34-pound cow. Hedo Oak Burke. 5. e-6.o6...s 0. . 23.40 | Four others from 30 to 33.2 pounds. Niamey DUTKe Nk! 4h. 8. ke a: 20.43 Heilo 4th’s Pet 43611 His sire is by a brother to the sires Butters Gays wa serie eee. Sel aeeeas of: NY OU ie aes ee ana, a Sea Dinwmarouewe ten ee SP 2 Oe SD aL Two A. R. O. daughters, including Prins selencerveld DyiKe 5.9.2... 3: 33.62 Heilo Queen De Kol (33 months) Blanche Lyons De Kol............ 33.91 15.60 pounds butter fat, 309.70 Blanche D. K:. Hengerveld........ 33.20 pounds milk. Blooms Hens Hditihesss.5- 5.8.6. e245 One A. R. son. Grown Pontiae Josey... ... .--c2- 32.34 | Iolena Fairmount’s Statesman Brenestactieng: I); sKe! soe. 32.20 13 A. R. O. daughters, including 11 others from 30 to 32 pounds. re ee vat pou : tter fat, 472.80 pounds milk; Tilly Lou 62052 ah My ) Her sire is by a brother of the sire of: Wait-A-Bit, 19.88 pounds butter : . fat, 461.10 pounds milk; Angos- Pauline Alexis (10 years).......... 32.40 (ater IONS. adiaias Duticneinh Milagre vee se. 645.20 ed SDE eran , i 446.50 pounds milk. Butter s0/days..2......:. 128.35 Ochele Ti 46996 PU uty Oa). ric een 2,629.50 ocd aiceay = pecies eer ei eae Butter 60 days TE 0 SE ee cial eh Milk J Pind) aici g Or 10 A daughter of Eunice Clay’s Sir Reel omreisees Azra sects es : By a brother to a grandsire of: Henry and Ophelia Rose 2d. Maplecrest Pon. Girl (4 years)..... 35.15 Maplecrest Pon. Highlawn (4 years) 30.72 HaneeD i Ky Colantha. -5..4... >. 30.54 Burtonseehe ad shares. - ens |. ne 30.14 and to the grandam of: Ee Barna Mechs Kens ce sce kes 30.49 n and breeding performances of her ancestors. (From Kimball's Dairy Farmer.) 30.! 388. 24. 632. 92 90 22.80 596 GENETICS IN RELATION TO AGRICULTURE data must be handled intelligently. In this connection the need of additional comparative tables like Table LX VII for other characters and other classes of livestock should be mentioned. They are not difficult to obtain and undoubtedly they will be available some day. Pedigrees.—The pedigree of an animal is simply a record of its ancestry, and accordingly the ideal system of recording pedigrees is that system which gives proper emphasis to each animal in the pedigree. The one-time fashionable practice of tracing pedigrees back through five or six or even more generations to some illustrious sire or dam cannot, therefore, be too strongly condemned, for it over emphasizes remote ancestors in certain lines and tends to underrate the importance of a possibility of inferiority in nearer ancestors. As a test of purity of blood, the Arabians require that their horses trace through long lines of descent to the five mares of Al Khamseh; there is, however, no justi- fication for this practice in modern breeding operations. The pedigrees of pure-bred breeding stock are recorded in herd books. For such animals it is only necessary to consult the herd books in order to trace out their ancestry. However, it is usually more con- venient, since the pedigree must be traced through several volumes of the herd-book, to record it in extended form in the herd record. This is not a difficult task; it need be done only once for every animal, and the task is still further lightened by the fact that: the individuals of any established herd will have so many common ancestors that they will duplicate one another’s pedigrees to a great extent. It is, however, necessary to say a word regarding the method of recording such pedigrees. The following pedigree of Roan Gauntlet, a famous old Cruickshank Shorthorn sire, taken directly from Volume XXII.of the “American Shorthorn Herd-book,” illustrates a method of recording pedigrees which should not be followed by breeders: ! Roan Gauntlet 45,276 (35,284).—Roan, calved May 19, 1873, bred by A. Cruick- shank, owned by Mr. Rennie, got by Royal Duke of Gloster (29,864), out of Princess Royal by Champion of England (17,526)—Carmine by The Czar (20,947)—Cressida by John Bull (11,618)—Clipper by Billy (3151)—by Dandy AR Tiptop (7633)—bred by Mr. Mason. The reason why this method should not be followed may be seen very easily in Fig. 232, which illustrates a proper way of recording a pedigree. Here the bold-faced type indicates those animals which were included in the pedigree as given in the herd-book. Of the sixty-two ancestors of Roan Gauntlet in five generations only nine are included in the herd book record. Further the record is defective in that it fails to give any evidence of the type of breeding which was employed in pro- ducing Roan Gauntlet. The way this bull traces back to the great CONDUCTING BREEDING INVESTIGATIONS 597 Cruickshank bull Champion of England is the striking feature of his pedigree. The criticism of the above pedigree is not, it should be clearly under- stood, directed at the method of recording pedigrees in the American Shorthorn Herd-book, although it is a fair statement to make that the method that has since been employed of recording simply the name of sire and dam is more economical and just as satisfactory. Even by the old method, however, the pedigrees are so recorded that the entire set of ancestors may be determined. The point, however, is simply this, that such pedigrees should not be used as standards of judgment of ancestry, but rather those of the type shown in Fig. 230. Fie. 231.—Tilly Aleartra. No. 123459, Holstein. Production for one year, 30451.4 lb. milk containing 951.2 lb. butter fat (average test 3.12 per cent.). The addition of other data to the pedigree indicative of the value from a breeding or productive standpoint of the animals therein listed adds greatly to its value, particularly to the new breeder who is not yet fully familiar with the great names of breed history. The pedigree of Tilly Alcartra 123,459, the record-breaking Holstein-Friesian cow por- trayed in Fig. 231, is given in Fig. 230 along with data relative to the performance and breeding value of the animals whose names appear in the pedigree. A pedigree worked out like this one is a much safer guide in judging merit than one which gives data proving that the animal in question traced in the fourteenth generation three times to some famous sire of ancient history. Performance should be insisted upon all along the line, and when three or four generations of some subdivision in a notable line fail to bring forth performing individuals, it is high time 598 GENETICS IN RELATION TO AGRICULTURE for the breeder to suspect that something has been lost in that line of descent, something that a pedigree, however excellent in remote ancestors, cannot supply. The Coefficient of Inbreeding.—We would call attention to Pearl’s coefficient of inbreeding as an instance of another refinement which has been advocated for use in practical breeding operations. Pearl proposes ies Queen’s Roan (7389) x Lancaster a @Champion Comet { of England (11663) (17526) x Virtue J X Plantagenet Grand Duke \ Verdant (11906) of Gloster (Lord Raglan { Crusade 7938 19900 (26288) | Duchess of (13244) \ Brenda | Gloster 9th Duchess of Lord Garlies 20236 Gloster 6th (14819) Duchess of Gloster Royal Duke of Gloster x Lancaster X Queen’s Roan (7389) 20901 © Champion Comet (29864) of England (11663) ; (17526) x Virtue J X Plantagenet (11906) Mimulus \ < Verdant @Lord Raglan { X Crusade (7938) Mistletoe (13244) xX Brenda Maidstone { Matadore (11800) Phantassie Roan Al A2 A5 Gauntlet Will Honeycomb (5660) 45276 The Queen’s (35284) Roan (7389) { Lancaster Comet ke oval Honeycomb (5660) (11663) Lupin Champion of Plantagenet The Duke of Lancaster, England (11906) 10929 (17526) | Virtue Madaline | Verdant { The Exchequer (9721) Prigg Princess Royal @Lord Raglan { X Crusade (7938) (13244) | X Brenda The Czar (20947) Corianda { The Baron (13833) Czarina | Carmine John Bull J Rumous (7456) (11618) | Mayoress | Cressida Clipper { Billy (3151) Fig. 232.—Pedigree of Roan Gauntlet, a famous Cruickshank Shorthorn bull. that inbreeding be used in a generic sense to include all cases in which some of the matings in the pedigree were of related individuals. In order to indicate degree of inbreeding he suggests the use of a coefficient of inbreeding of the form VA Bi, 100 (Pasa aad Qn+1) ‘ Tee 1 which is essentially a mathematical expression of the relation in per cent. between the maximum number of different ancestors which an indi- CONDUCTING BREEDING INVESTIGATIONS 599 vidual might have in a given generation, and the a of repetition which has occurred in its ancestry. In the above formula for Z, the coefficient of inbreeding, P,4., denotes the maximum possible number of different ancestors which an individual might have in matings of the (n+ 1)th generation and Qn+1, the actual number of different ancestors which he has. For an application of this coefficient we take the pedigree of Roan Gauntlet as given in Fig. 232. It gives the following series of values for Z: Zo = a = 0 per cent. Z4,= eo = 0 per cent. Z2 = a = 25 per cent. Z3= Beat! = 37.5 per cent. Z4 = oe = 40.625 per cent. To determine these values we have started with the A; generation in which p; = 2 and of necessity gq: = 2 also. The value of Zo, therefore, must be 0 in all cases. In the Ae generation p, = 4 and q = 4, also, because all these four animals are different and have not previously appeared in the pedigree. The value for Zi, therefore, is0. In genera- tion A3, p3 = 8. Champion of England appears twice in this genera- tion, and since he has appeared already in Ao, the two reappearances in A; are crossed out. Counting the remaining individuals in this genera- tion, we find g; = 6, and consequently Z2 = 25 per cent. Now this Z: = 25 signifies not only that Roan Gauntlet in the third ancestral generation has 25 per cent. less than the maximum possible number of different ancestors, but also that in any generation further removed he must of necessity have at least 25 per cent. less than the maximum possible number of ancestors. In the next following generation, Ay, ps = 16. In determining g,+1 we strike out Lancaster Comet and Virtue, sire and dam respectively, of Champion of England. It is worth while noting here that these two animals are automatically eliminated in this genera- tion because of the reappearance of an animal in a lower generation in this same line of descent. Reappearances at the apex of a line of descent are called primary reappearances and are marked @ in this pedigree, whereas reappearances which are determined by the primary reappearance of an individual in a lower generation are called secondary reappearances and they are marked with the sign X. It is only necessary to determine primary reappearances in calculating the coefficient of inbreeding, for secondary reappearances may be accounted for by simply doubling the total number of reappearances in the next lower generation. Continuing 600 GENETICS IN RELATION TO AGRICULTURE down the A, generation we meet with Lord Raglan as a primary reappear- ance as the sire of Mistletoe and further down as the sire of The Czar, also. The total number of primary and secondary reappearances in Ag is, therefore, 6; and since the expression p,+1 — Qn+118 merely a measure of the total number of reappearances, the value of Z; = 8{¢= 37.5 per cent. In A; we know the total number of secondary reappearances will be 6 X 2 = 12. There is one primary reappearance, that of Will Honeycomb, which must be added to this value, making the total number of reappearances in this generation thirteen. This gives the value Zs, = 40.625 per cent. If we have, therefore, at hand an extended pedigree of an animal it is a simple matter by this method to determine its coefficient of inbreeding for any number of generations. 100 80 or) Oo Coefficients — Oo 20 Z 2'4 6 8 10 12 14 Generations Fia. 233.—Curves of inbreeding: B X S, continued brother X sister matings; P X O, continued parent X offspring matings; C2 X C2, continued double-cousin matings; C! X C}, continued single-cousin matings. Continued matings of uncle X niece give a curve iden- tical with C1 x C1. (After Pearl.) In Fig. 233 are shown a number of curves of inbreeding which show graphically the rate of concentration of blood lines with different types of matings. Continued brother x sister matings give the maximum values for the coefficient of inbreeding. In this connection Pearl calls attention to the similarity of form of the brother-sister and double cousin curves and of the parent-offspring and single cousin curves. The Coefficient of Relationship——Obviously it is necessary for determining the significance of the coefficient of inbreeding to know how — the reappearances occur in the pedigree. Thus if animals appear on both sire’s and dam’s line of descent sire and dam are related in some degree. But it is possible as Pearl points out to have a high coefficient of inbreed- ing without any relationship whatever between sire and dam. In fact, specifically the limiting value of the coefficient of inbreeding where sire CONDUCTING BREEDING INVESTIGATIONS 601 and dam are totally unrelated lags only one generation behind the value for continued brother-sister matings. Pearl, therefore, proposes to de- termine not only the coefficient of inbreeding, but also a coefficient of relationship which shall express mathematically the degree of kinship existing between an individual’s parents. We again take the pedigree of Roan Gauntlet as an illustration of the method of calculation employed. We obtain the following series of values: Z =0 Kk, =0 Z2,=0 K, =0 Zo = 25 K; = 50 ‘Z3 = 37.5 K, = 75 Zs = 40.6 Ks; = 75 The values for K, the coefficient of relationship, were determined in the following fashion. In A; on the sire’s side, Champion of England which has already appeared on the dam’s side reappears twice. The maximum possible number of animals different from those on the dam’s side in this generation is four. Since two of these are identical with an individual which has already appeared on the dam’s side, K; = 24 = 50 per cent. In A, the double primary reappearance of Champion of England in A; automatically determines a total of four secondary reappearances, and to these are added two primary reappearances of Lord Raglan. In Ag, therefore, K = 6g = 75 per cent. In A; there are no additional primary reappearances involving both sides of the pedigree, consequently the value of K remains at 75 per cent. It seems wise for breeders to use these coefficients in order to gain precision in the use of terms, if for no other purpose. Of course the use of inbreeding coefficients does not alter the prob- lem of inbreeding from a biological standpoint. That problem is concerned with the effect of mating closely related animals. It has already been pointed out that the coefficient of inbreeding may be high when there is no relationship between sire and dam as, for example, when a closely inbred Jersey cow is bred to a closely inbred Holstein-Friesian bull. Such matings are of course not a part of the problem of inbreeding as it is understood in practice. For a precise expression of this problem we must look to the coefficient of relationship. A coefficient of relation- ship of 50 per cent. for A; would probably be a fair mathematical require- ment for inbreeding as conceived in practice. A coefficient of relation- ship of this magnitude includes double cousin matings as well as those of brothers with sisters and parents with offspring, but this appears to be a fair inclusion, if reference be made to the curves of inbreeding given in Fig. 233. For further details of the applications of these coefficients reference must be made directly to Pearl’s work. 602 GENETICS IN RELATION TO AGRICULTURE Marking Individuals.—The problem of marking individuals often is difficult where large numbers of individuals are involved. When very small herds are kept in which the animals may be known individually this matter is not very important, because the animals may simply be given a distinctive name, and any notes which it may be necessary to make may be recorded under that name. But when individuals become more numerous, it is usually necessary to have some safe and effective way of distinguishing them. For cattle aluminum ear tags of various kinds are often used, and these may be obtained stamped with any numbers which are desired. These may be used for smaller animals, also, or the ears may be punched in various fashions. A method used by Dr. Fie. of Heredity.) Bell in sheep breeding experiments is illustrated in Fig. 234. By the use of eight holes with place values such as are indicated in the diagram, it is possible to identify 256 sheep according to the following combinations: Total sheep identified by 0 hole................ 1 Total sheep identified by 1 hole................ 8 Total sheep identified by 2 holes............... 28 Total sheep identified by 3 holes............... 56 Total sheep identified by 4 holes............... 70 Total sheep identified by 5 holes...... UG Ser Sone 56 Total sheep identified by 6 holes............... 28 Total sheep identified by 7 holes............... 8 Total sheep identified by 8 holes............... it This is a very simple mode of identification, and by means of a rubber stamp with a sheep’s head outline or description sheets having such a head as that shown in Fig. 234 printed upon them, it is very easy to record accurately the designation which has been given to any particular sheep. The method can of course be used with other animals, and it avoids the difficulty of loss which sometimes is met with in using ear tags. CONDUCTING BREEDING INVESTIGATIONS 603 Poultry may likewise be marked in two different ways either with web punches or with aluminum bands which fit around the shanks. For large numbers the latter method is preferable. Recording Data.—The keynote of any system of recording data should be simplicity. This requirement must be met in scientific work; it is, however, particularly important in practical breeding for herdsmen have but limited time at their command for keeping records. The time necessary for recording data may often be very much cur- tailed, if properly devised, printed forms are used. They are superior PIGEON DESCRIPTION - porsAL FoLLow SHEET No. Sax No Dy WISCONSIN Aaric. Expt. STATION- Dept. ExPER. BREEDING Fic. 235.—Pigeon description sheet in use in experimental breeding at the Wisconsin Agricultural Experiment Station. (Devised by Leon J. Cole.) to other less accurate methods of registering data not only because they make it easier to set down the data, but because by having items indicated on the sheets or cards, it is very easy to see at any time just what data remain to be determined. The methods in all cases should be* those which are best adapted to the particular conditions which obtain in the case in hand. We may summarize in brief the requirements of a good system of record keeping by discussing the several features of it. The Individual Sheet—By the individual sheet is meant a sheet upon which is recorded vital data for a particular individual. This sheet should have places for recording such data as the date of birth of the in- dividual, its date of death or disposal, from whom acquired and to whom disposed of, and other data of a similar character. This sheet may con- 604 GENETICS IN RELATION TO AGRICULTURE veniently have on its back a pedigree blank for recording all the ancestors for at least four generations back. A separate sheet of this kind should be made out for at least each breeding individual; individuals which are not to be kept for breeding purposes may be noted on other specially. devised condensed blanks, which give only the necessary essential data respecting them. The Description Sheet—The purpose of the description sheet is to provide space for notes bearing upon the characteristics of the animal in question, short items which may be jotted down from time to time whenever they occur to the breeder. This sheet should also bear what- ever extended individual descriptions may be necessary. In many cases, the use of a printed outline such as that shown in Fig. 235, which is used in the investigations of pigeon breeding at the Wisconsin Station, aids greatly in making such descriptions definite and detailed without much labor. An outline form for instance will aid materially in recording the extent and position of black and white areas in Holstein-Friesian and other cattle which usually have broken colors. The Progeny Sheet——For recording matings and progeny a special sheet is often useful, although it is often possible to provide space for this data on the individual sheet. This blank will generally be used in the form of a follow sheet to accompany other sheets of each breeding female. Space should be provided for recording dates of service, name of ‘sire used, date of delivery, sex of offspring, and other vital data of this type. There should be a place for recording the disposition of the offspring; if added to the breeding herd, a cross-reference should be made to its individual sheet. The Performance Sheet.—The performance sheet is necessary only when the data obtained under this heading are relatively extensive as is the case jn milking records of dairy cows or egg records of hens. This sheet should be devised in such a fashion as to permit the recording of data quickly and accurately. In Fig. 236 is reproduced a summary egg sheet such as is used in breeding investigations at the Wisconsin Station. It will serve as a type of the kind of sheets which may be used in recording data of performance. Sheets for Special Purposes—Uf the breeder is following out any particular type of operations which require special data it should be an easy matter to devise sheets which will help him in that matter. As an illustration we give in Fig. 237 a reproduction of a sheet used at the Wisconsin Station in an investigation of multiple births in cattle. General Considerations.—Any system which is adopted should be convenient. For that reason a loose leaf system, because it is not bulky and offers the maximum freedom in rearrangement and filing, will prob- ably prove most satisfactory in practical work. Such systems have CONDUCTING BREEDING INVESTIGATIONS 605 > PSE 30! 31 TOTALS | two | tnrent. | Diep €. | Oo L. | Bromen HATCHED =a SSssSesoe 2) ie ee re | vow [eo [won [arn [war [sone] ver] Ave [serr] ocx] | @] @] | | ® ia EEE z ae a arn eh ee % % % Sa aEaieae Pe oe ees ee ees ae = pe : evar SUMMARY EGG SHEET Wisconsin Aaric. Expt. STATION—DEPT. EXPER. BREEDING SusBvect SuBJECT FATHER a Es ed a Ser Ee Pa Pal al wl «| FATHER'S FATHER” MOTHER'S FATHER Fig. 236.—Summary egg sheet in use in experimental breeding investigations at the Wis- consin Agricultural Experiment Station. (Devised by Leon J. Cole.) Report only one pair of twins on each sheet. BREED SS Breep, NAME Sire DaTe OF BIRTH (es NumBeR ) OF TWINS IF REGISTERED) OF Dam AGEICE DAM ATERIME How MANY TIMES HAD ae oAccoae |_OF Prooucina Twine J. Da Cat veo Paeviousty $A OTHER Twins PP Do You KNow OF ANCESTORS OR RELATIVES OF Sine oR DAM THAT HAVE PRODUCED TWINS 7 Record twins separately, one under A and the other under B. A_SeEx NAME : ReGisTRATION No. Were HEALTH AND SIZE NORMAL 2? ——— Was IT Ever Bred? AT WuHaT AGE? Dip iT PropUCE OFFSPRING ? = =e WHEN AND How DisposeD OF ? Wisconsin Aaric. Expt. STATION—DePT. EXPER. BREEDING B Sex NAME ReaisTRATION NO. WERE HEALTH AND SiZE NORMAL ? Was IT EVER BRED ? AT WHat AGE ? Dip 1T PRODUCE OFFSPRING ? WHEN AND How DisposeD OF ? In Case ONE Was A FREE-MARTIN, DESCRIBE APPEARANCE OF GENITAL ORGANS AND UDDER INVESTIGATION OF MULTIPLE BIRTHS IN CATTLE Record any other points of interest (Comparison of horns, color. etc.) on back of this sheet. NAME AND ADDRESS DaTe Fig. 237.—Printed form used in investigation of multiple births in cattle at the Wisconsin Agricultural Experimental Station. (Devised by Leon J. Cole.) 606 GENETICS IN RELATION TO AGRICULTURE been devised for the use of practical breeders. Various aids such as different colored sheets for different purposes help to make these systems still more convenient. Obviously for the sake of convenience sheets should be of the same size so that they may all be filed in the same style of binder. The sheets which have been illustrated in this account are of size 5 by 8 inches and are very convenient for most purposes. Cooperative Breeding.—Most farmers who raise livestock cannot afford themselves to keep a good bull for breeding purposes for the few cows they have, or still less a stallion for the few mares which they may need for their farm labor. Since such a large proportion of stock-raisers are in this class it becomes a grave question as to how these farmers may be provided with the advantages which accrue from the use of pure-bred sires. Any plan which has for its purpose the raising of the general average excellence of livestock must take account of these farmers, for taken all together they own a very large proportion of the livestock in the country, and in the future they will own an increasingly larger pro- portion of it. One of the best ways of meeting this difficulty is by forming codperative associations among the farmers themselves for the purpose of purchasing pure-bred sires. There is no reason why a given section of country should find it necessary to have a different breed of horses or cattle or swine on every farm, consequently the first step in the formation of such ‘a company should be to agree upon the particular breed and type of bull or stallion which should be purchased. Thereafter under no circumstances should this decision be changed, but the farmers should endeavor to grade their herds up to the highest standard of that breed. A definite plan such as this would work an enormous improye- ment within a few years in the character of the livestock in a given rural district. In passing it may be mentioned that it has often been found advisable and feasible to lend government aid to the improvement of livestock. This has been particularly the case in European countries where long decades of breeding have reduced types within a given district to a fair degree of uniformity, so that the government might follow a simple uniform practice in dealing with a given district. The success which such a policy may achieve is testified to by the popularity of the Percheron and French Coach horses, breeds to which the French government has lent considerable official encouragement. These are, however, details to be worked out in every section; the important point in every case is to follow up thoroughly and consistently for a considerable period of time whatever scheme is adopted. CHAPTER XXXIX CONCLUDING REMARKS Although we have discussed a deal of material in this account of genetics in relation to animal breeding, it must be apparent to any student that we are still woefully lacking in detailed and precise knowledge. In fact, as yet we seldom have accurate information with respect to the most simple and easily determined matters, such as growth curves in the various breeds and races of livestock, comparative production curves, and the like, the obtaining of which is largely a matter of routine. Such data are not even genetic data, strictly, but they are so necessary for the application of genetics to animal breeding that genetics proper must almost mark time until they can be obtained. The necessity of having accurate standards of judgment obviously need not be debated before an intelligent audience. Our dearth of detailed knowledge is particularly noticeable, however, in the field of genetics proper. It has been said—and there is much justification for the statement—that our knowledge of heredity is not secure with respect to any character until it has been found possible to analyze it and determine the factors which enter into it. If such a eri- terion then be applied to our knowledge of heredity in horses and cattle, for example, we have little cause to congratulate ourselves upon the extent of our accurate knowledge; for in either of these animals the number of factors accurately known could be counted on the fingers of both hands. It is a far call from such a state of affairs to that obtaining in Drosophila ampelophila in which knowledge has been derived concerning about 150 factors, many of which have been arranged in a systematic, coordinated scheme. Our knowledge is very meagre especially with respect to those factors which affect primarily economic characters in domestic animals, such as milk production in dairy cows, fertility, vigor, and like characters or character-complexes. Here we have a very good beginning in Pearl’s analysis of winter egg production in domestic fowls; but compared with the amount of information yet to be gained we see how long is yet the road to betravelled. But this beginning which Pearl has given us is very encouraging; it leads us to feel confident that our knowledge of accurate details of heredity will be pushed further and further. 607 608 GENETICS IN RELATION TO AGRICULTURE For after all in spite of our present dearth of detailed knowledge of heredity in domestic animals there is no real cause for discouragement. It is not yet two decades since the rediscovery of Mendel’s law of heredity; and the most rapid progress has been made within the last five years. It is not, therefore, at all strange that we have not yet obtained extended data from experimental research; in fact, most of the Mendelian data we now have on the larger domestic animals is of the interpretive kind, that is, the conclusions have been drawn from records already in existence. Ixxperimental research such as has been employed in the study of the inheritance of coat color in rodents, has not yet been carried out to determine the relations of the various coat colors and patterns in horses and cattle; the best that has been found possible thus far is the study of herdbook records and breeders’ notes. The Need of Research.—Students know too well how difficult it is to make due allowances for all the variable factors which may enter into a given body of data. Consequently, however simple the conditions may be, those conclusions which are based on records as crude as those of herd books and breeders’ notes are subject to a great deal of uncertainty. Moreover, it is usually impossible under practical conditions to find matings which have been carried out in such a way as to give crucial tests of a given hypothesis of factor relations. We have emphasized this difficulty in the discussion of Mendelian inheritance in domestic animals, pointing out that very often alternative interpretations could be made of the crude data extant; interpretations which could be very easily sub- jected to a crucial test in the case of accurate scientific research. In the domestic animals, as in Drosophila, the ideal goal of genetic analysis should be ‘that which determines accurately the mode of inheritance and expression of as many Mendelian factors as is possible. The task is difficult, but the increasing knowledge of heredity in lower forms will immensely simplify its execution. The time and expense necessary for carrying out studies of heredity has often deterred investigators from attacking problems in higher animals because the possibility of economic application of the results has seemed to be remote or almost certainly nil. But this is not the point at issue, as may be clearly seen when the interrelations between factors are considered. Accurate determination, for example, of the various factors and factor interactions in the heredity of coat color in cattle would give a secure and definite basis from which to prosecute other investigations more intimately concerned with problems of economic im- portance. It iseven highly justifiable to commend such investigations, because the problem is then first approached in its simplest form. There is grave question as to the advisability of plunging pell mell into difficult problems before the simpler ones have been solved, were it not for the ap AIOE RAR Re a me i i eet i a ee CONCLUDING REMARKS 609 fact that simultaneous attack may be made in such investigations both against the more obvious and the more obscure questions. Since work of this kind requires relatively large funds and consecutive attention during many years, it is the kind of research which is eminently suited to the facilities provided by agricultural experiment stations. In the present state of knowledge in genetic enquiry, investigations in heredity to be of value must be planned and directed by carefully trained men such as should make up the research staff of experiment stations. Undoubtedly as the need for this type of research becomes felt more strongly, as it inevitably will when agricultural methods become more intensive, special facilities will be provided such as are particularly adapted to genetic research. We cannot well apply genetic principles to their full value before we have definite genetic knowledge. The Service of Genetics.—At the present time genetics can without question render an important service to animal breeding, for excellent Fig. 238.—Laboratory devoted to genetic research at the University of Illinois. as may be the art of the skilled practical breeder it remains a regrettable fact that it is neither practised nor known by the great body of practical breeders in this country. The great fundamental conception of genetics that heredity is the primary guiding hand in determining the character- istics of the individual, whether physical or mental, has not become a part of the fund of knowledge of the general public. The firmly grounded belief of the geneticist that the phenomena of heredity have a definite knowable basis are still flauted by the less informed among our practical brethren, not only in speech but also in deed, for nothing is more pitiable than the blind hope manifested among some of them that something good may come out of their hit-or-miss methods of breeding. Superiority does not arise from inferiority in animal breeding; planless breeding operations are not less deplorable than lack of systematic action in any other department of rural activity. It is here indeed more than in any other fashion that genetic instruc- tion finds its justification. For as more and more men become familiar with the laws of heredity and by inference and example broaden the 39 610 GENETICS IN RELATION TO AGRICULTURE circle of those who begin to appreciate the significance of those laws, it must inevitably follow that general breeding practice will thereby be gradually raised. It is not possible for a geneticist, however broad his knowledge, to map out rules of procedure in breeding operations such that success must inevitably follow their application. Such procedure is not to be commended; it is not even scientific, by very nature. For intelligent application of the principles of genetics, which is the ideal of the scientific animal breeder, presupposes a knowledge of such prin- Fic. 239.—Genetics laboratory (for general course) College of Agriculture, University of California. ciples; the service of the geneticist, therefore, should be to determine principles and to indicate insofar as may lie within his power the signifi- cance of these principles. It is in this direction that the study of genetics is not only advisable but needful, for it provides as it were the framework to which the breeder may add the necessary empirical elements for the construction of his finished plan of procedure. And he will find as he becomes more and more familiar with that framework that it is not a mere indifferent edifice to which he may attach things here and there as convenience dictates, but that it is a codrdinated and interrelated structure which provides definite places for different kinds of things, so that when CONCLUDING REMARKS 611 they are fitted in their proper places they tend that much to add to the completeness and unity of the whole structure. It is a fortunate breeder who is able to approach his problems from such a point of view. The Need of Other Knowledge.—Proficiency at any sort of game may be gained only by practising the game. No amount of reading and study of methods of play will suffice to make a good card player or a billiardist; it is required that the player be able to put the principles to effective use if he would achieve any measure of success. It is not far different in the practice of animal breeding. Genetics provides merely the principles of a game, the effective employment of those principles necessitates a thoroughly grounded knowledge of a wide range of matters pertaining to the technique of rearing, training, mating, and what not of the particular type of animal which is being bred. We might say somewhat enigmatically that successful animal breeding requires a knowledge both of principles and principals. He who has studied genetics has only begun the study of the broader subject of animal breeding. Ordinarily it would be a much safer procedure to entrust the future of a carefully built-up herd of pure-bred livestock to the sympathetic care of the herdsman trained in the old school rather than to the most thoroughly trained genetic investigator in the land. For after all success in animal breeding depends very largely upon the ability of the breeder to build up in his mind an ideal type; and there is no more reason or assurance that such a type will arise full-formed in the mind of the breeder than that any other good thing may be obtained without effort. Here indeed is a rare opportunity for good sound judg- ment to work toward a definitely appointed end. For the ideal type of the breeder will in a sense be a composite of many types, in determining which the particular force of any one factor must be weighed with con- summate skill. Thus to take a single illustration, that of the ideal type of beef Shorthorn, we may point out some of the types which must be welded so to speak into one. There is first the market type of beef cattle; broad, deep, built upon the plan of the parallelogram, carrying a maximum percentage of high priced cuts, and a minimum percentage of offal. In the second place we may consider the feeder’s type of beef cattle. He desires an animal which will lay on flesh rapidly and econom- ically. Consequently he looks for a bright and alert, but not overly active disposition, and a high degree of functional excellence in the digestive system and body in general, such that the animal will consume a maximum amount of food and convert it into flesh of the proper quality. Perhaps as a slight compensatory allowance here the feeder permits a slight increase in volume of digestive and other vital organs with a consequent increase in percentage of offal for the sake of more economical gains. In the third place we must consider the breeders’ type of beef 612 GENETICS IN RELATION TO AGRICULTURE cattle. Here questions of the regularity of breeding, of the type of cow best suited for the production of young, of the ability of cows to provide sufficient nourishment for their offspring, of adaptability to the conditions of climate and to the other environmental features of the locality in which they are produced, and many other considerations enter in. Finally we have the ideal breed type to consider: the animal must possess those characters which distinguish Shorthorns as a breed from other beef breeds such as Aberdeen Angus, Galloway, or Hereford cattle; and very likely it will be necessary in order for it to meet with favor that it display those particular characteristics of the Shorthorn breed which mark it as belonging to some favorite family or strain. We have seen how difficult it is to deal with Mendelian experiments involving differences in five or six definite, allelomorphic pairs of factors; how much more difficult must it be to deal with all the variable considera- tions which enter into the discussion of the method of constructing an ideal beef type of Shorthorn cattle. And yet even in the face of all these requirements the results of intelligent, systematic breeding opera- tions are surprising in excellence and uniformity of product. When we consider this fact we can only become more strongly convinced of the definite, knowable operation of the laws of heredity. But these factors which enter into the determination of ideal types are largely considerations outside the pale of genetics proper. These are the matters which must be added to a knowledge of genetics in order to complete the equipment which would be at the command of the animal breeder. To this knowledge, also, must be added information bearing on the technique of managing breeding herds in order to realize the full returns which it should be possible to secure. This information will include a large and varied range of topics such as the methods of feeding breeding stock and of developing young stock, the determination of the proper number and use of service animals, methods of coping with disease of various kinds; a knowledge of methods and appliances by which the greatest possible use may be made of particularly excellent animals, such as by artificial insemination, and a thousand and one items to recount which would only make this discussion more tedious and uninteresting. But these elements are none the less essential to the equipment of the successful animal breeder. So we come tothe end of our account of genetic principles in animal breeding, realizing very keenly the limitations in our knowledge, and the inadequacy of the principles of genetics alone and unsupported to serve as a working equipment for the practical animal breeder. But we take a deal of courage and satisfaction out of the fact that a consideration of those principles has a proper and important part to play in animal breeding, first by the emphasis which it lays upon heredity as a factor CONCLUDING REMARKS 613 in production, and secondly by the firm foundation of coédrdinated principles which it provides asa guide to procedure in breeding operations. It is necessary thus toemphasize the importance of heredity as a determin- ing factor in production, because of the erroneous ideas which are held by the generality regarding the fact of heredity; it is wise to study genetics as a guide in breeding practice, because any knowledge which is reduced to a basis of known principle or is coérdinated with principle is that much clearer of comprehension and more assured of intelligent application. But with all this the study of genetics has failed of its highest purpose, if it has not encouraged in the mind of the student the open attitude toward truth and the healthy skepticism of the true scientist. 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