GENETICS GENETICS AN INTRODUCTION TO THE STUDY OF HEREDITY BY HERBERT EUGENE WALTER ASSOCIATE PROFESSOR OF BIOLOGY BROWN UNIVERSITY WITH 7$ FIGURES AND DIAGRAMS fgork THE MACMILLAN COMPANY 1914 All rights reserved COPYRIGHT, 1918, BY THE MACMILLAN COMPANY. Set up and electrotyped. Published February, 1913. Reprinted June, October, 1913; February, July, 1914. Xortoooti J. 8. Gushing Co. — Berwick A Smith Co. Norwood, Mass., U.S.A. THIS VOLUME IS AFFECTIONATELY DEDICATED TO MY MOTHER PREFACE THE following pages had their origin in a course of lectures upon Heredity, given at Brown Univer- sity during the winter of 1911-1912, which were amplified and repeated in part the following sum- mer at Cold Spring Harbor, Long Island, before the biological summer school of the Brooklyn Institute of Arts and Sciences. An attempt has been made to summarize for the intelligent, but uninitiated, reader some of the more recent phases of the questions of heredity which are at present agitating the biological world. It is hoped that this summary will not only be of interest to the general reader, but that it will also be of serv- ice in college courses dealing with evolution and heredity. The subject of heredity concerns every one, but many of those who wish to become better informed regarding it are either too busily engaged or lack the opportunity to study the matter out for themselves. The recent literature in this field is already very large, with every indication that much more is about to follow, which is a further discouragement to non- technical readers. It may not be a thankless task, therefore, out of the jargon of many tongues to raise a single voice viii PREFACE which shall attempt to tell the tale of heredity. There may be a certain advantage in having as spokesman one who is not at present immersed in the arduous technical investigations that are making the tale worth telling. The difficulties in under- standing this complicated subject may possibly be realized better by one who is himself still struggling with them, than by the seasoned expert who has long since forgotten that such difficulties exist. Among others I am particularly indebted to Dr. C. B. Davenport for many helpful suggestions, to my colleague, Professor A. D. Mead, for reading the manuscript critically, to Dr. S. I. Kornhauser who gave valuable aid in connection with the chapter on the Determination of Sex, and to my wife for assistance in final preparation for the press. I wish to thank Professor H. S. Jennings and Dr. H. H. Goddard, who have given generous permission to copy certain diagrams, as well as The Outlook Company and The Macmillan Company for the use of figures 24 and 66, respectively. The fact that all the suggestions which were at various times offered by my kindly critics have not been incorporated in the text, absolves them from responsibility for whatever remains. H. E. W. PROVIDENCE, R. I., September, 1912. CONTENTS CHAPTER PAGE I. INTRODUCTION. 1. The triangle of life . . . . . . . 1 2. A definition of heredity 4 3. The maintenance of life 5 4. Somatoplasm and germplasm 10 n. THE CARRIERS OF THE HERITAGE. 1. Introduction . . 14 2. The cell theory 14 3. A typical cell 15 >4. Mitosis 18 5. Amitosis 20 6. Sexual reproduction 20 7. Maturation 22 > 8. Fertilization 24 9. Parthenogenesis ....... 26 10. The hereditary bridge 27 11. The determiners of heredity 28 12. The chromosome theory .29 13. The enzyme theory of heredity 33 14. Conclusion . .35 III. VARIATION. 1. The most invariable thing in nature .... 36 2. The universality of variation 37 3. The kinds of variation with respect to their — a. Nature 38 6. Duplication 39 c. Utility 39 d. Direction in evolution ..... 39 e. Source ........ 40 /. Normality 40 g. Degree of continuity 40 h. Character 41 i. Relation to an average standard . . .41 j. Heritability 41 ix x CONTENTS CHAPTER PAGE 4. Methods of studying variation 42 5. Biometry 42 6. Fluctuating variation 43 7. The interpretation of variation curves ... 47 a. Relative variability 47 6. Bimodal curves 47 c. Skew polygons 50 8. Graduated and integral variations . . . .52 9. The causes of variation .52 a. Darwin's attitude 52 6. Lamarck's attitude 53 c. Weismann's attitude ..... 55 d. Bateson's attitude 55 IV. MUTATION. 1. The mutation theory 56 2. Mutation and fluctuation 57 3. Freaks 58 4. Kinds of mutation 59 5. Species and varieties 60 6. Plant mutations found in nature .... 63 7. Lamarck's evening primrose 64 8. Some mutations among animals .... 67 9. Possible explanations of mutation .... 69 10. A summary of the mutation theory .... 72 V. THE INHERITANCE OF ACQUIRED CHARACTERS. 1. Summary of preceding chapters 74 2. The bearing of this chapter upon genetics ... 75 3. The importance of the question 75 4. An historical sketch of opinion 76 5. Confusion in definitions 77 6. Weismann's conception of acquired characters . . 78 7. The distinction between germinal and somatic charac- ters 79 8. What variations reappear ? 80 9. What may cause germplasm to vary or to acquire new characters ? 81 10. Weismann's reasons for doubting the inheritance of acquired characters 84 CONTENTS xi CHAPTER *AGH 11. No known mechanism for impressing germplasm with somatic characters 84 12. Evidence for the inheritance of acquired characters inconclusive ....... 86 a. Mutilations 87 6. Environmental effects 88 c. The effects of use or disuse .... 91 d. Disease transmission 92 13. The germplasm theory sufficient to account for the facts of heredity 94 14. The opposition to Weismann 95 15. Conclusion 96 VI. THE PURE LINE. 1. The unit character method of attack ... 97 2. Galton's law of regression 98 3. The idea of the pure line 102 4. Johannsen's nineteen beans . . . . . 103 5. Cases similar to Johannsen's pure lines . . . 107 6. Tower's potato-beetles 108 7. Jennings' work on Paramecium .... 110 8. Phenotypical and genotypical distinctions . .113 9. The distinction between a population and a pure line 115 10. Pure lines and natural selection .... 118 VII. SEGREGATION AND DOMINANCE. 1. Methods of studying heredity 120 2. The melting-pot of cross-breeding .... 120 a. Blending inheritance 121 6. Alternative inheritance ..... 121 c. Particulate inheritance 121 3. Johann Gregor Mendel 123 4. Mendel's experiments on garden peas . . . 124 5. Some further instances of Mendel's law . . . 128 6. The principle of segregation 130 7. Homozygotes and heterozygotes .... 131 8. The identification of a heterozygote . . . .132 9. The presence and absence hypothesis . . . 132 10. Dihybrids 133 xii CONTENTS CHAPTER PAGE 11. The case of the trihybrid 140 12. Conclusion 143 13. Summary 144 VIII. REVERSION TO OLD TYPES AND THE MAKING OF NEW ONES. 1. The distinction between reversion and atavism . 146 2. False reversion 149 a. Arrested development 149 6. Vestigial structures 149 c. Acquired characters resembling ancestral ones 150 d. Convergent variation 150 e. Regression 151 3. Explanation of reversion 161 4. Some methods of improving old and establishing new types 152 a. The method of Hallet 152 b. The method of Rimpau 153 c. The method of de Vries 154 d. The method of Vilmorin 155 e. The method of Johannsen .... 155 /. The method of Burbank . . . .156 g. The method of Mendel 157 6. The factor hypothesis 159 a. Bateson's sweet peas 160 6. Castle's agouti guinea-pigs .... 163 c. Cuenot's spotted mice 164 d. Miss Durham's intensified mice . . . 165 e. Castle's brown-eyed, yellow guinea-pigs . . 166 6. Rabbit phenotypes 169 7. The kinds of gray rabbits 171 8. Conclusion 173 IX. BLENDING INHERITANCE. 1. The relative value of dominance and segregation . 174 2. Imperfect dominance 175 3. Delayed dominance 177 4. "Reversed" dominance . 178 CONTENTS xiii IHAFTER PAGE 5. Potency 179 a. Total potency 179 6. Partial potency 180 c. Failure of potency ...... 180 6. Blending inheritance 182 7. The case of rabbit ears 183 8. The Nilsson-Ehle discovery 186 9. The application of Nilsson-Ehle's explanation to the case of rabbit ear-length . . . .193 10. Human skin color 196 X. THE DETERMINATION OF SEX. 1. Speculations, ancient and modern .... 197 2. The nutrition theory 198 3. The statistical study of sex 200 4. Monochorial twins 201 5. Selective fertilization 202 6. The neo-Mendelian theory of sex .... 205 a. Microscopical evidence 207 1. The " x " chromosome .... 207 2. Various forms of x chromosomes . . 208 3. Sex chromosomes in parthenogenesis . 210 6. Castration and regeneration experiments . . 210 c. Sex-limited inheritance 213 1. Color-blindness 214 2. The English currant-moth . . . 216 d. Behavior of hermaphrodites in heredity . . 220 7. Conclusion 222 XI. THE APPLICATION TO MAN. 1. The application of genetics to man .... 224 2. Modifying factors in the case of man . . . 225 3. Experiments in human heredity .... 227 a. The Jukes 227 6. The descendants of Jonathan Edwards . . 228 c. The Kallikak family 229 4. Moral and mental characters behave like physical ones 230 5. The character of human traits 231 6. Hereditary defects 232 xiv CONTENTS CHAPTER PAGE 7. The control of defects 235 8. Inbreeding 238 9. Experiments to test the effects of inbreeding . . 240 10. The influence of proximity 241 11. Inbreeding in the light of Mendelism . . . 242 XII. HUMAN CONSERVATION. 1. How mankind may be improved .... 244 2. More facts needed 245 3. More application of what we know necessary . . 247 4. The restriction of undesirable germplasm through — a. The control of immigration .... 248 6. More discriminating marriage laws . . . 250 c. An educated sentiment 251 d. The segregation of defectives .... 252 e. Drastic measures 254 5. The conservation of desirable germplasm . . . 255 a. By subsidizing the fit 256 b. By enlarging individual opportunity . . 258 c. By preventing germinal waste .... 258 1. Preventable death 258 2. Social hindrances 259 6. Who shall sit in judgment ? 260 XIII. BIBLIOGRAPHY 263 XTV. INDEX . 265 GENETICS CHAPTER I INTRODUCTION 1. THE TRIANGLE OF LIFE WITHIN a generation the center of biological inter- est has gradually been swinging from the origin of species to the origin of the individual. The nine- teenth century was Darwin's century. His monu- mental work "On the Origin of Species by Means of Natural Selection," which appeared in 1859, not only dominated the biological sciences but also influenced profoundly many other realms of thought, partic- ularly those of philosophy and theology. Now, at the beginning of the twentieth century, a particular emphasis is being laid upon the study of heredity. The interpretation of investigations along this line of research has been made possible through the cumulative discoveries of many things that were not known in Darwin's day. Trained students have been patiently and persistently bend- ing over improved microscopes, untangling the mysteries of the cell, while an increasing host of in- vestigators, inspired by the Austrian monk Mendel, have been industriously devoting their energies to 2 GENETICS breeding animals and plants with an insight denied to breeders of preceding centuries. The study of the origin of the individual, which has grown out of the more general consideration of the origin of species, forms the subject-matter of heredity, or, to use the more definitive word of Bate- son, of genetics. It is not with the individual as a whole that H E R I T A G E FIG. 1. — The triangle of life. genetics is chiefly concerned, but rather with char- acteristics of the individual. Three factors determine the characteristics of an individual, namely, environment, training, and heri- tage as expressed diagrammatically in Figure 1. It may indeed be said that an individual is the result of the interaction of these three factors since he may be modified by changing any one of them. Although no one factor can possibly be omitted, the student of genetics places the emphasis upon heritage as the factor of greatest importance. Heritage, or INTRODUCTION 3 "blood," expresses the innate equipment of the indi- vidual. It is what he actually is even before birth. It is his nature. It is what determines whether he shall be a beast or a man. Consequently in the diagram (Fig. 1), the triangle of life is represented as resting solidly upon the side marked "heritage" for its foundation. Environment and training, although indispensable, are both factors which are subsequent and secondary. Environment is what the individual has, for example, '» housing, food, friends and enemies, surrounding aids which may help him and obstacles which he must j overcome. It is the particular world into which he conies, the measure of opportunity given to his particular heritage. Training, or education, on the other hand, repre- sents what the individual does with his heritage and environment. Lacking a suitable environment a good heritage may come to naught like good seed sown upon stony ground, but it is nevertheless true that the best environment cannot make up for defective heritage or develop wheat from tares. The absence of sufficient training or exercise even when the environment is suitable and the endowment of inheritance is ample will result in an individual who falls short of his possibilities, while no amount of education can develop a man out of the heritage of a beast. Consequently the biologist holds that, although what an individual has and does is un- questionably of great importance, particularly to the individual himself, what he is, is far more important 4 GENETICS in the long run. Improved environment and educa- tion may better the generation already born. Im- proved blood will better every generation to come. What, then, is this "blood" or heritage? Ex- actly what is meant by heredity ? 2. A DEFINITION OF HEREDITY Professor Castle, in his recent book on " Heredity in Relation to Evolution and Animal Breeding," has defined heredity as " organic resemblance based on descent." The son resembles his father because he is a " chip off the old block." It would be still nearer the truth to say that the son resembles his father because they are both chips from the same block, since the actual characters of parents are never trans- mitted to their offspring in the same way that real estate or personal property is passed on from one generation to another. When the son is said to have his father's hair and his mother's complexion it does not mean that paternal baldness and a vanish- ing maternal complexion are the inevitable conse- quences. Biological inheritance is more comparable to the handing down from father to son of some valuable patent right or manufacturing plant by means of which the son, in due course of time, may develop an independent fortune of his own, resembling iri charac- ter and extent the parental fortune similarly derived although not identical with it. So it comes about that "organic resemblance" INTRODUCTION 5 between father and son, as well as that which often appears between nephew and uncle or even more remote relatives, is due not to a direct entail of the characteristics in question, but to the fact that the characteristics are "based on descent" from a common source. In other words, an "hereditary character" of any kind is not an entity or unit which is handed down from generation to generation, but is rather a method of reaction of the organism to the constellation of external environmental factors under which the organism lives. To unravel the golden threads of inheritance which have bound us all together in the past, as well as to learn how to weave upon the loom of the future, not only those old patterns in plants and animals and men which have already proven worth while, but also to create new organic designs of an excellence hitherto impossible or undreamed of, is the inspiring task before the geneticist to-day. 3. THE MAINTENANCE OF LIFE So far as we know, every living thing on the earth to-day has arisen from some precedinSform of life. How the first spark of life began will probably always be a matter of pure speculation. Whether the beginnings of what is called life came through space from other worlds on meteoric wings, as Lord Kelvin has suggested ; whether it was spontaneously generated on the spot out of lifeless components ; or whether life itself was the original condition of 6 GENETICS matter, and the one thing that must be explained is not the origin of life, but of the non-living, no one can say. Leaving aside the first speculation as un- tenable and the third as irrational, since it jars so sadly with what astronomers tell us of the probable evolution of worlds, the theory of spontaneous gener- ation seems to be the last resort to which to turn. In prescientific days this idea of spontaneous generation presented no great difficulties to our imaginative and credulous ancestors. John Milton, with the assurance of an eye-witness, thus described the inorganic origin of a lion : — " The grassy clods now calved ; now half appears The tawny lion, pawing to get free His hinder parts — then springs as broke from bonds, And rampant shakes his brindled mane." (" Paradise Lost," Book VII, line 543.) Ovid also in his "Metamorphoses," not to mention a more familiar instance, easily succeeded in creating mankind from the humble stones tossed by the juggling hands of Deucalion and Pyrrha. Although under former conditions on the earth it might have been possible for life to have originated spontaneously, and although it may yet be possible to produce life from inorganic materials in the labora- tory or elsewhere, the exhaustive work of Pasteur, Tyndall and others effectually demonstrated a genera- tion ago that to-day living matter always arises from preceding living matter and this conclusion is gener- allyliccepted as an axiom in genetics. INTRODUCTION 7 There are various methods of producing more life, given a nest-egg of living substance with which to start. Any organism, whether plant or animal, is continually transforming inorganic and dead material into living tissue. Through the process of repair, for example, an injury to a form as highly developed even as man is frequently made good, if it is not too extensive, as in the case of a skin wound. When the intake of non-living material is in excess of the outgo, growth results, with the consequence that more living substance is built up than existed before. Thus a fragment of a living sponge or a piece of a begonia leaf are each sufficient to restore a duplicate of the original organism. A process similar to the repair of the begonia leaf is that employed so effectively in the great groups of the one-celled animals and plants, the Protozoa and Protophyta, by means of which their numbers are maintained. These one-celled organisms multiply by fission, that is, by equal division into halves, and each half then grows to the size of the parent organism from which it sprang. When two daughter pro- tozoans are thus formed, they are essentially orphans because they have no parents, alive or dead. The parental substance in such a process, along with the regulatingjDower necessary to reorganization, goes over bodily into the next generation in the forma- tion of the daughter-cells, leaving usually no re- mains whatever behind. In primitive forms of this description, continuous life is the natural order, and death, when it does occur, is, as Weismann has 8 GENETICS pointed out, accidental and quite outside the plan of nature. In these cases it is easy to see the reason for "or- ganic resemblance" between successive generations. Parent and offspring are successive manifestations of the same thing, just as the begonia plant, restored from a fragment of a begonia leaf, is simply an ex- tension of the original plant. Many modifications of the process of multiplica- tion by fission occur, all of them, however, agreeing in the fundamental principle that the progeny re- semble the parents because they are pieces of the parents. Thus the greening apple maintains its individuality although coming from thousands of different trees, because all of these trees through the asexual process of grafting are continuations of the one original Rhode Island greening tree grown by Dr. Solomon Drowne in the town of Foster, nearly a century ago. Again, certain fresh-water sponges and bryozoans, quite unlike any of their marine relatives, keep a foothold from year to year within their particular shallow fresh-water habitats by isolating well pro- tected fragments of themselves in the form of gemmules and statoblasts. These structures may drop to the muddy bottom and live in a dormant condition throughout the icy winter when it would not be possible for the entire organism to survive near the surface. In order to meet the conditions imposed by winter, however, these fragments have become so modified INTRODUCTION 9 as temporarily to lose their likeness to the parent generation, although readily regaining that likeness when springtime brings the opportunity. The unity of two succeeding generations, although interrupted by the temporary interposition of something ap- parently different in the form of gemmules or stato- blasts, is thus essentially maintained. The bryozoan colonies of two successive seasons in a fresh-water pond may be regarded as parts of the same identical colony, since they present an "organic resemblance based on descent," although the sole representatives of the parent colony during midwinter may be the sparks of life locked up within the statoblasts buried in the mud. Similarly, the asexual spores of many plants, such as molds, mosses and ferns, may be regarded as gemmules reduced to the lowest terms, namely, to single cells. As in the preceding cases so in this instance the resemblance of the offspring which may arise from these spores, to the parents which pro- duced them, is due to the essential material identity of two generations. These illustrations of heredity in its simplest mani- festations give the key to "organic resemblance" higher up in the scale. Sexual reproduction is no less plainly the direct continuation of life though in this instance two sporelike fragments out of one generation contribute to form the new individual of the next generation instead of one fragment. In all cases there is a material continuity between succeeding generations. Offspring become thus an extension of 10 GENETICS a single parent or of two parents, while heredity is simply "organic resemblance based on descent." 4. SOMATOPLASM AND GERMPLASM In forms that reproduce sexually there theoretically occurs a differentiation of the body substance into what Weismann terms somatoplasm and germplasm. The somatoplasm includes the body tissues, that is, the bulk of the individual, which is fated in the course of events to complete a life-cycle and die. The germplasm, on the contrary, is the immortal fragment freighted with the power to duplicate the whole organism and which, barring accident, is des- tined to live on and give rise to new individuals. The germplasm thus carries potencies for develop- ing both germplasm and somatoplasm, while the somatoplasm, according to this conception, has only the power to reproduce more of its own kind. More^ over, the germplasm is not formed afresh in each gen- eration, neither does it arise anew when the individual reaches sexual maturity, but it is a continuous sub- stance present from the beginning. Although this theory of the continuity of the germplasm has been actually demonstrated in comparatively few instances, all the facts we know concerning the behavior of the germinal substance are consistent with it. In many of the Protozoa the entire organism is possibly comparable to germplasm, but in all forms of life that are compounded of several cells the germ- plasm is probably set aside early in the development INTRODUCTION 11 of the individual, and this remains undifferentiated, or in reserve, like a savings-bank account put by for a rainy day, while the somato- \ •• plasm is expended in the immediate demands of the tissues that make up the individual. In one instance at least, that of the nematode worm Ascaris, according to Boveri, this splitting off or isolation of the germplasm occurs as early in the cleavage of the fertilized egg as the sixteen - cell stage, when fifteen of the cells go to form the somato- plasm and the sixteenth is set aside as germ- plasm. Thus there re- sults a continuous stream of germplasm, receiving contributions from other germplasmal streams at the GERMPllASM OPLASM FIG. 2. — Scheme to illustrate the continuity of the germplasm. Each triangle represents an individual made up of germplasm (dotted) and somatoplasm (undotted) . The beginning of the life cycle of each individual is represented at the apex of the triangle where germplasm and somatoplasm are both present. As the indi- vidual develops each of these component parts increases. In sexual reproduction the germ- plasms of two individuals unite into a common stream to which the somatoplasm makes no contribution. The continuity of the germ- plasm is shown by the heavy broken line into which run collateral contributions from suc- cessive sexual reproductions. 12 GENETICS time of sexual reproduction, as shown diagrammati- cally in Figure 2, in which individuals are represented by triangles. From this continuous stream of germ- plasm there split off at successive intervals complexes of somatoplasm, or "individuals," which go so far on the road of specialization into tissues that the power to be " born again " is lost, and so after a time they die, while the germplasm, held in reserve, lives on. This is what is meant by saying that a father and son owe their mutual resemblance to the fact that they are chips off the same block rather than by saying that the son is a chip off the paternal block. Both somatoplasms are developments at different inter- vals from the same continuous stream of germplasm instead of one somatoplasm being derived from a preceding one. As a matter of fact the germplasm from which the son arises is modified by the addition of a maternal contribution, so that father and son in reality hold the same relation to each other that half- brothers do. From the point of view of genetics, then, the real mission of the somatoplasm, which is so marvelously differentiated into all the various forms that we call animals and plants, is simply to serve as a temporary domicile for the immortal germplasm. Thus the parent becomes as it were the "trustee of the germ- plasm, " but not the producer of the offspring. In the light of these preliminary explanations it is plain that the hopeful point of attack in the science of genetics must inevitably be the germplasm which INTRODUCTION 13 is the source, or point of departure, in the formation of each new individual, rather than the somatoplasm, which represents the end stages of the hereditary processes. This has not been the method of the past. The resemblances of the visible father and son have usually been traced instead of the character of their unseen germplasms. By following this old method, investigators have often been misled because the visible or apparent is not always the true index of what lies behind it. A gray and a white rabbit, for example, may produce some offspring that are entirely black just as two white-flowering sweet peas when crossed may sometimes produce purple blos- soms. Consequently it is a great fallacy to affirm that in heredity "like produces like," since the op- posite is quite often the case. The new heredity, embodied in the science of genetics, attempts to go deeper than the surface appearance of the somatoplasm. It aims to get at the source or origin of organisms, that is, the germ- plasm which is the only connecting thread between succeeding generations of living forms. It is con- cerned not so much with somatoplasm, which repre- sents what the germplasm has done in the past, as with the germplasm and what it can do in the future. CHAPTER H THE CARRIERS OF THE HERITAGE 1. INTRODUCTION HEREDITY, as has been shown in the introductory chapter, is essentially a matter of continuity between succeeding generations of living organisms. This continuity may be direct, as when a mother protozoan divides into two daughters, or it may be indirect, as illustrated by the relationship of a father and son, an uncle and nephew, or any other relatives of varying degrees of kinship which, taken singly or collectively, are somatoplasms derived from a common stream of germplasm. It is the purpose of the present chapter to consider this material continuity between succeeding genera- tions and to discover, if possible, just what are the carriers of the heritage from one generation to another. To this end it will be necessary in the first place to take up what is meant by the "cell theory." 2. THE CELL THEORY In 1838-1839 the "cell theory" of Schleiden and Schwann, which affirms that all organisms, both plant and animal, are made up of cellular units, had its birth. 14 THE CARRIERS OF THE HERITAGE 15 Robert Hooke, as early as 1665, had described "little boxes or cells distinguished from one another" which he saw in thin slices of cork, and to him is due the rather unfortunate use of the term "cell" which has survived in biological writings to this day. The reason this term is unfortunate is because walls, which are ordinarily the characteristic feature of any cell, such as a prison cell, are usually the least im- portant part of the structure of a living cell, often indeed being entirely absent. 3. A TYPICAL CELL A typical undifferentiated cell is represented diagrammatically in Figure 3. Near the center of the cell the nucleus is shown surrounded by a •Cell wall Cvjto plasm Cent rose me Nuclear membrane Nucleus -Chromattn network FIG. 3. — Diagram of a typical cell. nuclear membrane. The nucleus, in common with the enveloping cytoplasm, is made up of living substance called protoplasm (Hugo von Mohl, 1846), and around the whole there is usually formed a 16 GENETICS wall or membrane which serves to separate one cell from another. Within the protoplasm there may be a considerable amount of non-living substance in the form of salts, pigments, oil-drops, water, and other inclusions of various kinds. The nucleus is to be regarded as the headquarters of the whole cell, since changes which the cell under- goes seem to be initiated in it, while cells deprived of their nuclei cannot long survive. A single instance will serve to show the vital part which the nucleus plays in the life-history of the cell. In 1883, Gruber found that after rocking a thin cover-glass back and forth in a drop of water containing a collection of the protozoan Stentor, which has a long chain-like nucleus, these tiny animals could thus be cut into fragments, which would in some instances recover from the operation and regenerate into complete individuals. Only those pieces, however, which contained a frag- ment of the nucleus regenerated into new Stentors, while pieces of relatively large size which lacked a frag- ment of nuclear substance very soon disintegrated. The nucleus, it should be said, is made up of more than one substance, a fact that is easily demonstrated by processes of staining, in which certain dyes, through chemical union, stain a part but not the whole of the nuclear substance. The part most easily stained is called chromatin, that is "colored material," and during certain phases of cell life the chromatin masses together within the nucleus into visibly definite structures or bodies termed chromo- somes. THE CARRIERS OF THE HERITAGE 17 Throughout all the various cells that make up the individuals of any one species these chromosomes appear to be practically constant in number with some exceptions to be mentioned later in connection with sex. This law of the constant chromosome number for any species was first stated by Boveri in 1900. The chromosomes of different organisms vary in number from two in the worm Ascaris up to perhaps 1600, according to Haecker ('09), in certain radiolaria. Species which apparently are closely related may differ widely with respect to the number of their chromosomes, while species of unquestionably re- mote relationship may have an identical number of chromosomes in each of their cells. The number of chromosomes characteristic for a species, therefore, is in no way an index to the complexity or degree of differentiation of the species. Besides the nucleus there may often be identified in the cytoplasm of the animal cell a tiny body known as the centrosome. At certain times in the life-cycle of a cell the centrosome becomes the focal point of peculiar radiating lines, which play an important part in the behavior of the cell, particularly during the period of division. Every cell passes through a cycle of life which may be compared with that common to individuals. It is born from another cell ; passes through a vigorous youth characterized by growth and transformation ; attains maturity when the metamorphoses of its earlier life give place to a considerable degree of stability; and finally, after a more or less extended 18 GENETICS period of normal activity old age ensues, and death completes the cycle. In most instances, however, before this final phase is reached, the cell gives place to daughter-cells through fission, after the manner of most protozoans, and a new cell cycle is begun. Sometimes the road of differentiation has been traveled so far that it is apparently impossible, as in the case of the complicated brain-cells, to retrace these steps of differentiation and begin again. In such instances the outfit of cells provided in the em- bryo determines the numerical limit of the cells available throughout life. When this supply is ex- hausted no more cells appear to replace those which have been worn out. 4. MITOSIS The ordinary process by which two cells are made out of one is termed mitosis. It occurs constantly, and particularly during growth, in all cellular organ- isms. A series of diagrams, modified from Boveri, illustrating the typical phases of mitosis is given in Figures 4 to 13. The resting cell (Fig. 4) is characterized by the presence of a nuclear membrane, a single centrosome, and by a chromatin network within the nucleus. In the beginning of the prophase (Fig. 5) the centrosome has divided into two parts, while in the early prophase (Fig. 6) the two centrosomes have moved farther apart and definite separate chromosomes have formed out of the chromatin network. The prophase proper (Fig. 7) is marked by the vanishing of the nuclear THE CARRIERS OF THE HERITAGE 19 membrane and the more compact form of the chromo- somes. At the end of the prophase (Fig. 8) the chro- mosomes have come to lie at the equator of the cell, fig.4. The nesting cell Rg.5. Beginning Prophase F%6. Earlq Prophase Fig. 7 Prophase F7g. 8. End of Prophase F7g.9. Metaphase Fig.12 Beginning Telophase Flg.13. End of Telophase FIGS. 4-13. — Diagrams illustrating mitosis. After Boveri. being connected by the mantle fibers with the cen- trosomes, each of which has now come to occupy a polar position. In the metaphase (Fig. 9) the chromo- somes split lengthwise, and at the beginning of the 20 GENETICS anaphase (Fig. 10) these half chromosomes commence to separate from each other and to move toward the poles, while the mantle fibers shorten. During the anaphase (Fig. 11) the cell body lengthens and begins to divide, while the migration of the half chromosomes toward the poles is completed. In the beginning of the telophase (Fig. 12) the half chromosomes grow until they attain full size and the division of the cell body into two parts becomes complete. The mantle fibers have disappeared and the nuclear membrane begins to re-form around the chromosomes. Finally, at the end of the telophase (Fig. 13) the nuclear membrane becomes complete, the chromosomes break up into a chromatin network, and two resting cells take the place of the single one with which the process began (Fig. 4). 5. AMITOSIS Amitosis, or the formation of two cells from one without the machinery of mitosis, is comparatively rare. It occurs in certain rather isolated instances among animals and plants, particularly in old cells late in their life-cycle or in cells that are on the road to degeneration. When amitosis takes the place of the more elaborate process of mitosis it is fre- quently, though not always, a signal of the death- warrant for that particular cell. 6. SEXUAL REPRODUCTION The mechanism by means of which two cells unite to make one in sexual reproduction is quite as com- THE CARRIERS OF THE HERITAGE 21 plicated as that of mitosis by which one cell is trans- formed into two. In sexual reproduction there are two kinds of germ- cells, the egg and the spermatozoan respectively, which take part in producing a new organism. These cells are structurally unlike each other in nearly every particular, but each is a true cell, which von Kolliker made clear as early as 1841, and each has typically the same number of chromosomes in its nucleus, a fact more recently determined by van Beneden in 1883. The egg-cell is often supplied with one or more envelopes of protective or nutritive function, and it is usually distended with stored up yolk, in consequence of which it is comparatively large and stationary. The result is that whatever locomotion is necessary to bring the two cells together for union devolves upon the sperm-cell. Consequently the sperm-cells are practically nuclei with locomotor tails of cyto- plasm, and frequently, in addition, with some struc- tural modification for boring a way into the egg-cell. They are, moreover, much more numerous than the egg-cells, so that although many go astray, never fulfilling their mission, the chances are nevertheless good that some one of them will reach the egg and effect fertilization. Ordinarily only one sperm enters the egf , but when several succeed in penetrating into the cyto- plasm only one proceeds to combine with the egg nucleus, that is, only one sperm nucleus is normally concerned in the essential process of fertilization. 22 GENETICS It was formerly thought by the school of "ovists" that in fertilization the essential process is a stimu- lation of the all important egg by the sperm. The opposing school of "spermists," on the other hand, regarded the egg simply as a nutritive cell the func- tion of which is to harbor the all important sperm. It is now known that both the egg- and the sperm-cell are equally concerned in fertilization, which consists in the union of their respective nuclei within the cytoplasm of the egg. 7. MATURATION Certain preliminary changes of a preparatory nature, termed rqaturation, regularly precede the union of the nuclei of the two sex-cells in fertiliza- tion. These maturing changes result in reducing the outfit of chromosomes in each sex-cell to one half the original number, a process which is necessary in order to maintain the chromosome count which is characteristic for any particular species and which is known to exist unbroken from generation to genera- tion. If there were no such reduction, then the fertilized egg, formed by the union of egg and sperm nuclei, would contain double the characteristic number of chromosomes, and during the formation of a new individual, the number in all the cells arising by mitosis from such a fertilized egg would like- wise be double. When the germ-cells of such indi- viduals unite in fertilization, the original number of chromosomes would be quadrupled, and so on in THE CARRIERS OF THE HERITAGE 23 geometric progression throughout subsequent genera- tions. In 1883, too late for Darwin to learn of it, van Beneden discovered the important fact that the «— Manq similar cell divisions-* MATURATION "Spermatocyle 1- OoqU- FIG. 14. — Scheme to illustrate maturation of germ-cella. mature germ-cells, as expected, actually contain only half the normal number of chromosomes. The mature egg- or sperm-cell, with half its normal number of chromosomes, is termed a gamete (marry- 24 GENETICS ing cell), while the fertilized egg which is formed by the union of two gametes (mature egg- and sperm- cell), and which consequently has the characteristic number of chromosomes, is called a zygote (yoked cell). A diagrammatic representation of the process of maturation is shown in Figure 14. The number of chromosomes (not shown in the diagram) remains constant in each germ-cell respectively until the divi- sion of second spermatocytes into spermatids which are subsequently transformed into spermatozoa, and of the second oocytes into mature eggs and second polar cells, when it is reduced to one half the normal number. As spermatozoan and mature egg unite in fertilization, the original number of chromosomes is restored in the fertilized egg (zygote). 8. FERTILIZATION The stages concerned in a typical case of fertiliza- tion, according to Boveri, are illustrated in Figures 15 to 23. In Figure 15 the "head" and the "middle piece" of the sperm-cell have penetrated into the egg cyto- plasm, while in Figure 16 the tail of the sperm-cell has become lost and the middle piece, which furnished thg_centrosome, has rotated 180° so that it lies between the nucleus, or head, of the sperm-cell and that of the egg-cell. Figure 17 shows an increase in the size of the sperm nucleus and a division of the centrosome into two parts which begin to migrate towards the poles. This process of polar migration of the centrosomes is carried further in Figure 18 as THE CARRIERS OF THE HERITAGE 25 Rg.15. Entry of Sperm Rg,J6. Loss of SpermT&ll Rg.17. Division of Cenlrosojne fi£.16. Approach of Spero f^.ia Increase of Sperm fig.20. Formation of Nucleus Nucleus Chromosomes F7&2.1. Splitting of Chromosomes H£. 22. Anaphase . Two-celled Sta^e FIGS. 15-23. — Diagrams illustrating fertilization. After Boveri. 26 GENETICS well as the increase in the size of the sperm nucleus, until in Figure 19 the process is complete so that the centrosomes have assumed a polar position and the sperm nucleus is equal in size to the egg nucleus and lies in contact with it. In Figure 20 the chro- matin network of the two nuclei has formed into an equal number of chromosomes which in each case is half the number characteristic for the species. Figure 21 shows the complete disappearance of the nuclear membrane, a process that had already begun in the preceding figure, and also the arrangement of the chromosomes, connected with mantle fibers, in the equatorial plane where the former split longitudinally. In Figure 22, when the half chromosomes thus formed pull apart and migrate toward the poles, the segmenta- tion of the fertilized egg has begun, and there finally occurs, as shown in Figure 23, the two-celled stage following fertilization in which each cell contains the normal number of chromosomes, half of which came from the egg and half from the sperm. 9. PARTHENOGENESIS Fertilization is by no means an essential process in the formation of a new individual, even in those ani- mals which produce both eggs and sperms. Many animals and plants reproduce parthenogenetically, that is, the egg-cell may develop without first uniting with a sperm-cell. In these instances the chromo- somes of the egg are not halved during maturation, and the offspring, therefore, have the same number THE CARRIERS OF THE HERITAGE 27 of chromosomes as the parent, since they are simply fragments of the parent. Professor Loeb, by the use of certain chemicals, has succeeded in doing artificially what apparently is never accomplished in nature, namely, making an egg that normally requires fertilization develop par- thenogenetically. 10. THE HEREDITARY BRIDGE Whatever may ultimately prove to be deter- miners of the hereditary characters which appear in successive generations, it is obvious that, in any event, such determiners must be located in the zygote, that is, in the fertilized egg. This single cell is the actual bridge of continuity between any parental and filial generation. Moreover, it is the only bridge. In the majority of animals the egg develops en- tirely outside of and independent of the mother, thus limiting to the egg-cell itself all possible mater- nal contributions to the offspring. Although there is abundant evidence that half of the filial char- acteristics come from the male parent, the janly actual fragment of the paternal organism given .over to the new individual is the single sperm-cell, which unites with the egg in fertilization, and the whole of this even is not usually concerned in the process of fertilization. The entire factor of heritage is packed into the two germ-cells derived from the re- spective parents and, in all probability, into the nuclei of these germ-cells, since the nuclei are ap- 28 GENETICS parently the only portions of these cells that in- variably take part in fertilization. To the new individual developing by mitosis from the fertilized egg into an independent organism, the factors of environment and training referred to in Figure 1 are subsequently added. When it is remembered that the human egg-cell is only about 1^3 th of an inch in diameter, a gigantic size as compared with that of the human sperm-cell, and, furthermore, when one passes in rapid review the marvelous array of characteristics which make up the sum total of what is obviously inherited in man, the wonder grows that so small a bridge can stand such an enormous traffic. A sharp-eyed patrol of this bridge as the strategic focus of heredity is proving to be one of the most effective points of attack in the entire campaign of genetics. It is not desirable at this time to discuss possible ways in which the determiners of the heritage, what- ever they may be, are originally packed into the germ-cells, for this question can be more conven- iently considered in a later connection. It is im- portant at present, however, to emphasize the ob- vious conclusion that determiners of heredity must inevitably be present in the germ-cells in order to account for the fact of "organic resemblance based on descent" between parents and their progeny. 11. THE DETERMINERS OF HEREDITY What are the determiners of hereditary qualities ? Do they actually exist in the germ-cells as visible THE CARRIERS OF THE HERITAGE 29 entities, and is there such a thing as a mechanical Basis— for heredity as the German embryologist Wilhelm His suggested years ago when he wrote: "It is a piece of unscientific mysticism to suppose that heredity will build up an organism without mechanical means" ? Can we find these determiners by the aid of microscopes and differential stains, or are they some sort of intangible entities, such as enzymes or hormones or the 'like; which only the chemist can detect ? Whatever the answer to these questions, it may at least be affirmed that the determiner represents the adult, structure without resembling it. It is something which controls the unfolding of the developing or- ganism with respect to both quantity and quality, and which also governs the time and rate of appear- ance of its various characteristics so that certain combinations rather than others shall come about in definite sequence. To use the words of Conklin : "The mechanism of heredity is the mechanism of differentiation." 12. THE CHROMOSOME THEORY Certain investigators, who seek a morphological basis for heredity, regajd-tlie chromosomes as the car- riers, of the heritage ; in other words, as the source of the determiners of ontogeny or the effective factors in the process of differentiation. A few of the grounds for this theory are briefly indicated below. First: In spite of the great relative difference in 30 GENETICS size between the egg-cell and the sperm-cell, in hered- ity the two are practically equivalent, as has been repeatedly shown by making reciprocal crosses be- ween the two sexes. The only features that are apparently alike in both the germ-cells are the hromosomes. The inference is, therefore, that they contain the determiners which are the causal factors for the equivalence of adult characters in heredity. The existence of an extra chromosome in probable connection with the matter of sex is, as will be pointed out later, an exception to the exact chromosome equivalence of the two sexes, which only goes to strengthen the supposition that the chromosomes are the carriers of hereditary qualities since extra chromo- somes are always associated with the character of sex. Second: The process of maturation, which always results in halving the chromosome material of the germ-cells as a preliminary step to fertilization, is a series of complicated manceuvers not practised by I other cells. During this process no other part of | the cells appears to play so consistent and important i a role as the chromosomes. Provided they act as ? hereditary carriers, their peculiar behavior during maturation is just what is needed to bring together an entire complement of hereditary determiners out of partial contributions from two parental sources. Third : Sometimes abnormal fertilization occurs, as in the case when two or more sperm-cells, instead of one, enter the egg cytoplasm and unite with the egg nucleus. This unusual performance has been artifi- cially induced by chemical means in the case of sea- THE CARRIERS OF THE HERITAGE 31 urchins' eggs. The fertilized egg, or zygote, thus formed with an excess of male chromosomes, re- sults in the development of abnormal larvse. It is thought that a causal connection may exist, there- fore, between the additional male chromosomes in the fertilized ovum and the abnormalities of the progeny. Fourth: The fact that chromosomes may retain their individuality throughout the complicated phases of mitosis, as has been proven in some instances, agrees with the corresponding fact that certain characteristics of the somatoplasm maintain their individuality from generation to generation. Moreover, certain chromosomes in the fertilized egg have been identified with particular features in the adult developing from that egg. Tennent sum- marizes his recent work on Echinoderms (1912) by the statement that from a knowledge of the chromosomes in the parental germ-cells, particular characters in the adult hybrids may be predicted, and, conversely, that from the appearance of sexually mature hybrids the character of certain chromosomes in their germ-cells may be predicted. Again, the correlation of a particular chromosome in the germ-cells with a definite adult character, namely sex, has been repeatedly demonstrated in connection with the so-called "extra chromosome" to which reference has already been made. Fifth: Finally, excellent evidence of a definite causal connection between certain chromosomes of the germ-cells and particular somatic characters has 32 GENETICS been furnished by certain critical experiments upon the eggs of sea-urchins. Boveri found that he was able in some instances to shake out the nuclei bodily, chromosomes and all, from the mature eggs of the sea-urchin, Sphcer echinus, and when there was added in sea water to such enucleated eggs the sperm-cells of an entirely different genus of sea-urchin, namely, Echinus, the Echinus sperm-cells entered the Sphcer- echinus eggs, which had been robbed of their nuclei, and from this peculiar combination larvae developed which exhibited only Echinus characters! Such cumulative circumstantial evidence as the (foregoing has convinced many that in the chromo- somes we have visibly before us the carriers of heredity. Several biologists, however, raise an objecting voice to this theory, protesting against the mo- nopoly of the heritage by the chromosomes. They point out that there always exists an intimate physiological relationship between the nucleus and the cytoplasm, and that it is unreasonable to expect the isolation of one from the other, since the two must always act together as parts of an organic cell unit. In sexual reproduction, moreover, some small amount at least of spermatic cytoplasm in the form of the so-called "middle piece," which is situated between the head and the tail of the sperm-cell (Fig. 15), may enter the egg about to be fertilized along with the sperm " head " or nucleus, containing the chromosomes. In this way the cytoplasm of the THE CARRIERS OF THE HERITAGE 33 male sperm-cell may not necessarily be entirely excluded from taking part in the formation of the zygote. As a matter of fact, this extra-nuclear part of the sperm-cell sometimes apparently forms the centrosome of the fertilized egg and in consequence may have a hand, as well as the nucleus with the chromosomes, in determining what follows. 13. THE ENZYME THEORY OF HEREDITY It is not unlikely that the key to this whole prob- lem will be furnished by the biochemists and that the final analysis of the matter of the heritage-carriers will be seen to be chemical rather than morpho- logical in nature. It has been found that the blood of greyhounds and dachshunds is chemically different, although from a morphological point of view it is apparently identical. The idea of "individual albumen" or "protein specificity" for each animal of a species, to say nothing of the animals of different species, has been advanced as not improbable. Miescher has shown that an albumen compound having only forty carbon atoms, a number by no means unusual, would make possible a million com- binations of atoms or isomers. The possibilities in this direction seem to be un- limited if we take into consideration those invisible actuators of chemical processes, the enzymes, which the chemist brings forward with the prodigality of an astronomer dealing in star-dust, to explain dif- ferent chemical reactions. 34 GENETICS Montgomery has suggested that the chromosomes themselves may be masses of enzymes although, ac- cording to the chemist, enzymes are not morpho- logical entities, since they seem to be able to flourish and- maintain their identity while bringing about chemical reactions in their neighborhood without being visibly demonstrable. As said before, it is quite likely that in the final analysis heredity will be reduced to a series of chemi- cal reactions dependent upon the manner in "which various enzymes initiate, retard, or accelerate suc- cessive chemical combinations occurring in the pro- toplasm. When the same enzymes act upon the same chemical combinations in successive genera- tions, they bring about that "organic resemblance" known as heredity. E. B. Wilson, whose brilliant work in the entire field of cell activity makes it possible for him to speak with authority, has recently said : "The es- sential conclusion that is indicated by cytological study of the nuclear substance is, that it is an ag- gregate of many different chemical components which do not constitute a mere mechanical mixture, but a complex organic system and which undergo perfectly ordered processes of segregation and dis- tribution in the cycle of cell life. That these sub- stances play some definite role in determination is not mere assumption, but a conclusion based upon direct cytological experiment and one that finds support in the results of modern chemical research.5* THE CARRIERS OF THE HERITAGE 35 14. CONCLUSION The supposition that -the chromosomes, with cer- tain chemical reservations, are the morphological carriers of the heritage forms an excellent working hypothesis, and this chapter may suitably be closed with a second quotation from Professor Wilson. " In my view studies in this field are at the present time most likely to be advanced by adopting the comparatively simple hypothesis that the nuclear substances are actual factors of reaction by virtue of their specific chemical properties; and I think that it has already helped us to gain a clearer view of some of the most puzzling problems of genetics." CHAPTER III VARIATION 1. THE MOST INVARIABLE THING IN NATURE IN the introductory chapter it was shown that "organic resemblance based on descent," by which is meant heredity, is due principally to the fact that offspring are material continuations of their parents and consequently may be expected to be like them. The fact that this is the case in the great majority of instances has given rise to the popular formula, " Ifke, produces like," as a rule of heredity. But this formula by no means always fits the facts. Like often produces something apparently unlike. For instance, two brown-eyed parents may produce a blue-eyed child, although brown-eyed children are more usual from such a parentage. It is a common experience, indeed, for breeders of plants and animals to nieet with continual difficulties in getting or- ganisms to "breed true." On the other hand, it is exactly these variations which so constantly interfere with breeding true that furnish the sole foothold for improvement. If all organisms did breed strictly true, one generation could not stand on the shoulders of the preceding generation, and there would be no evolutionary advance. 36 VARIATION 37 The most invariable thing in nature is variation. ThisTacTis at once the hope and the despair of the breeder who seeks to hold fast to whatever he has found that is good and at the same time tries to find something better. When the similarities and dissimilarities between succeeding generations are clear, then heredity can be explained. The entire subject of variation is intimately and inevitably bound up with any consideration of genetics. 2. THE UNIVERSALITY OF VARIATION Much of the variation in nature is patent to the most casual observer, but it requires a trained eye to see the universal extent of many minor differences. A flock of sheep may all look alike to a passing stran- ger, but not to the man who tends them. A dozen blue violet plants from different localities might easily be identified by the amateur botanist as be- longing to the same species when, to a specialist on the genus Viola, unmistakable differences would doubtless be clearly apparent. The fact that every attempt at an intimate ac- quaintance with any group of organisms whatsoever invariably reveals previously unrecognized varia- tions, indicates that variability is much more wide- spread in nature than is commonly believed. The key to Japanese art, as pointed out by Dr. Nitobe, consists in being natural and in faithfully copying nature. It is for this reason that the Jap- anese artist makes each object that he produces 38 GENETICS unique, because nature herself, whom he strives to follow, never duplicates anything. The Bertillon system of personal identification is based upon the constancy of minor variations found in each individual. Its importance is shown in Figure 24. The faces of the criminals there pictured would be easily confused by the ordinary observer, but an examination of their thumb prints shows unmis- takable differences between these three individuals. 3. KINDS OF VARIATION A brief enumeration of some of the kinds of varia- tion will reveal their diverse character. a. With respect to their nature variations may be morphological, physiological, or psychological. Under morphological variations are included differences in shape, size, or pattern as well as differences in number and relation of constituent parts. Differences in activity are of a physiological nature. Many animals in captivity are less fertile than when free, while different individuals are well known to vary widely with respect to their susceptibility to disease. Nageli, for example, reports the presence of tubercles in 97 percent of the cases in five hundred autopsies, although a majority of the deaths in ques- tion was not due to tuberculosis at all, — a fact which indicates a great diversity in the resistance of differ- ent individuals to the tubercle bacillus. Psychological variations in man, such as those which determine the disposition or mental traits of individuals, are apparent to every one. VARIATION 39 b. With respect to their duplication variations may be single or multiple. A legless lamb 1 is an ex- ample of a single variation or "sport." Four-leaved clovers, on the contrary, are multiple for the reason that this variation, although not common, neverthe- less occurs frequently. c. With respect to their utility variations may be useful, indifferent, or harmful to the organism possess- ing them. Useful variations are of the kind empha- sized by Darwin as being effectively made use of in natural selection. Indifferent variations, on the other hand, are those which apparently do not play an important part in the welfare of their possessor, as, for example, the color of the eyes or of the hair. Finally, the degree of degeneration in certain organs may be cited as an illustration of harmful variations. The amount of closure of the opening from the in- testine into the vermiform appendix in man is an ex- ample of a harmful variation, since the larger the opening, the greater is the liability to appendicitis. d. With respect to their direction in evolution varia- tions may be either definite (orthogenetic) or indefinite - (fortuitous) . Paleontology furnishes numerous instances of the former category, such as the series of variations from a pentadactyl ancestor, all apparently tending in one direction, which have culminated in the one- toed horse. The fact that the paleontologist deals historically with a completed phylogenetic series in which the side lines lack prominence, while the suc- 1 "A Peculiar Legless Lamb." Stockard. Biol. Bull, xiii, D. 288. 40 GENETICS cessful line stands out with distinctness, makes it easy for him to view successive variations as orthogenetic, that is, as definitely directed in one course either through intrinsic (Nageli) or extrinsic (Eimer) causes. Fortuitous or chance variations in all possible directions furnish the repertory of opportunity, according to Darwin, from which natural selection picks out those best adapted to survive in the strug- gle for existence. e. With respect to their source, vaript^nfi TYIQY ^fi somatic or germinalCl^Qwa^c, or body variations, arise as modifications due to environmental factors. They are individual differences which may be quite transitory in nature, while germinal variations may arise without regard to the environment, are deep- seated, and of racial rather than of individual sig- nificance. /. With respect to their normality variations may fall within expected extremes and thus be considered normal, or they may be outside of reasonable expec- tations and consequently be reckoned as abnormal, as in the case of a two-headed calf. g. With respect to the degree of their continuity varia- tions may form a continuous series, grading into each other by intermediate steps, or they may be discon- tinuous in character. An example of continuous variation is the height of any hundred men one might chance to meet, which would probably represent all intermediate grades from the highest among the hundred to the lowest. The number of segments in the abdomen of a VARIATION 41 shrimp, on the other hand, which may, for instance, be either eight or nine but cannot be halfway between, illustrates what is meant by discontinuous variation. The widespread occurrence of this later category of variations has been pointed out by Bateson in his encyclopedic volume "On Materials for the Study of Variation." h. With respect to their character variations may be quantitative or qualitative. A six-rayed starfish represents a quantitative variation from the normal number of five rays, whereas a red variety of ,a flower may differ chemically from a blue variety, or a bitter fruit may differ from a sweet fruit in a qualitative way dependent upon the chemical constitution of the fruit in question. i. With respect to their relation to an average stand- ard variations may have a fluctuating distribution around an arithmetical mean, as when some of the offspring have more and some less of the parental character, or the variations in the progeny may all center about a new average quite distinct from the parental standard and consequently come under the head of mutations. j. Finally, and most important in the present connection, with respect to heritability, variations may possess the power to reappear in subsequent genera- tions, or they may lack that power. It is this aspect of variability which bears most directly upon genetics. Other possible categories might be mentioned, but a sufficient number have been cited to show the great diversity of variations in general. 42 GENETICS 4. METHODS OF STUDYING VARIATIONS Roughly stated, there are three ways of studying variations : first, Darwin's method of observation and the description of more or less isolated cases ; V second, Gal ton's biometric method of statistical inquiry ; and third, Mendel's experimental method. The second of these methods will be considered in this chapter. 5. BIOMETRY The new science of biometry, that is, the applica- tion of statistical methods to biological facts, has been developed within recent years. Sir Francis Galton, Darwin's distinguished cousin, may be re- garded as the pioneer in this field of research, while Karl Pearson and his disciples constitute the modern school of biometricians. Although mathematical analysis of biological data when not sufficiently ballasted by biological analysis of the same facts may sometimes lead the investigator astray, yet often the only way to for- mulate certain truths or to analyze data of some kinds is by resort to statistical methods. Biome- tricians are quite right in insisting that it is frequently necessary to go further than the fact of variation, which may be apparent from the inspection of an individual case, and to deal with cumulative evidence as presented through statistical analysis. In matters of heredity, however, facts as they occur in single cases and definite pedigrees seem to VARIATION 43 offer a more hopeful line of approach than statistical generalizations. It is better to become acquainted with the real parent than to evolve a hypothetical " mid-parent " mathematically. In this connection it is well always to bear in mind the warning of Johannsen, himself a past master in biometry, when he writes : "Hit Mathematik nichr als Mathematik treiben wir unsere Studien." 6. FLUCTUATING VARIATION With respect to any measurable character there are bound to be deviations from an average con- dition. According to the mathematical laws of chance these deviations sometimes are plus and sometimes minus, and consequently they may be termed fluctuating variations. Pearson gives as a simple illustration of fluctuating variation the number of ribs present in two sets of beech-leaves, as shown below. These sets were taken from two different trees, and each contains twenty- six leaves. NUMBER OF RIBS 13 14 15 16 17 18 19 0 TOTAL First tree . . . 1 4 7 9 4 1 26 Second tree 3 4 9 8 2 26 Total .... 3 4 10 12 9 9 4 1 It will at once be seen that, while certain leaves might well belong to either tree, as, for example, those with sixteen ribs, the entire group of leaves from 44 GENETICS either tree is unlike that of the other tree. In the first instance the number of ribs fluctuates around of individuals 55 /5 10 LIST or CONSTANTS Arithmetical Mean(A.M)=49 Mode CM) -5 Average Deviation (A.D.)=.5£ Standard Deviation (cr) = j£4 Coefficient of Variability (C.V> \.-ri FORMULAE A.D. = 2(x.f) x- deviation of the class from A.M. f -number in the class n -total number Number of Rays £ 3 4 5 6 7 Fia. 25. — The fluctuating variability of starfish rays. From data by Goldschmidt. eighteen as the commonest kind ; in the second case, around fifteen. Such a difference could not easily VARIATION 45 be detected or expressed by any other method than the statistical one. Again, in the case of forty-seven starfishes all of which were collected from one locality the variation in the number of rays proved to be, according to Goldschmidt, an amount indicated graphically in Figure 25, where the data are arranged in the form of a so-called frequency polygon or curve. From such a polygon certain constants may be computed which conveniently express in a single number, for purposes of abstract comparison, dis- tinctions that otherwise could be handled only in the most indefinite way. Thus in this instance the arithmetical mean, ex- pressed by the hypothetical number 4.915, a number which of course does not actually occur in nature, is simply the average number of rays in forty-seven starfishes selected at random. The mode which represents the group containing the largest number of individuals of a kind, namely, thirty out of forty-seven, is five in this particular polygon. The average deviation, which is an index of the amount of variation going on among the starfishes in question, is .52. In other words, .52 is the average amount that each individual starfish deviates from the arithmetical mean of 4.915. Although the one seven-rayed starfish which happens to be in the lot varies from the standard of 4.915 to the extent of 2.085 (7 - 4.915) rays, there are thirty five-rayed starfishes which vary only .085 (5 — 4.915) of a ray, 46 GENETICS and consequently the average of the entire forty- seven amounts to .52 of a ray. In another collec- tion of starfishes where either more seven-rayed or two-rayed specimens might be present, the average deviation would probably be greater. By computing the average deviation, therefore, and using it as the criterion of variation, a compar- ison of the variability of organisms that have been taken from different localities or subjected to differ- ent conditions can be definitely expressed. A measure of variability more commonly in use by biometricians, since for mathematical reasons it is more accurate, is the standard deviation. This is the square root of the sum of all the deviations squared, according to the formula / V («••/) in which x represents the deviation of each class from the arithmetical mean ; /, the number of individuals in each separate class ; 2, the sum of the classes ; and n, the total number of individuals.1 In the present instance the standard deviation is .724, an arbitrary number that has valuable sig- nificance only when brought into comparison with standard deviations similarly derived from other groups of starfishes. Such a variation polygon as the above expresses the law that the farther any single group is from the 1 For directions explaining the use of such formulae see Davenport's "Statistical Methods." VARIATION 47 mean of all the groups making up the polygon, the fewer will be the individuals that represent it. 7. THE INTERPRETATION OF VARIATION POLYGONS a. Relative Variability The statistical determination of the relative vari- ability of two lots of organisms with respect to a certain character may be illustrated by the case of the oyster-borer snail, Urosalpinx cinereus, as seen in the accompanying table. ATLANTIC AND PACIFIC SHELLS COMPARED LOCALITY NUMBER OF SHELLS A.M. er PROB- ABLE ERROR West Shore 1,001 58.928 2.339 ±.0352 Penzance Point .... 1,002 61.718 2.737 ±.0412 Nobska Point .... 1,002 61.737 2.152 ±.0324 Woods Nobska Point .... 1,001 61.944 2.234 ±.0337 Hole Nobska Point .... 496 66.944 2.366 ±.0507 Barnacle Beach .... 998 63.932 2.604 ±.0393 BigWepecket . . . . Mid-Wepecket .... 1,006 500 57.426 57.606 2.052 2.098 ±.0308 ±.0447 Average for Mass. . . 61.066 2.335 ±.0386 Cali- fornia ' Belmont Beds .... L San Francisco Bay . . . 1,008 520 59.051 60.892 3.023 3.361 ±.0454 ±.0703 Average for Cal. . . . 59.664 3.138 ±.0538 Difference ... .803 The obvious conclusion to be drawn from this table is that the snails which were unintentionally carried from the Atlantic coast to California in the 48 GENETICS transplantation of oysters show more variation in their new habitat than they did in the old one with respect to the particular character measured, namely, the relative size of the mouth aperture compared with the height of the entire shell.1 b. Bimodal Curves Sometimes two conspicuous modes make their ap- pearance in a frequency polygon, as Jennings found, *£» Individuals 25 - 20 10 20 £4- 2.8 32 36 40 44 5£ 56 60 64 FIG. 26. — The body width of a population of the protozoan Paramecium\ showing a polygon with two modes. A, Paramecium aurelia. B, Paramecium caudatum. After Jennings. for example, in measuring the body width of a popu- lation of the protozoan Paramecium (Fig. 26). 1 ' ' Variation in Urosalpinx. ' ' Walter, pp. 577-594. Amer. Nat. 1910, Vol. XLIV. VARIATION 49 It was subsequently found that the two modes in this polygon were due to the fact that the material in question was a mixture of two closely related species, Paramecium aurelia and Paramecium cauda- tum, the individuals of which arranged themselves around their own mean in each instance. FIG. 27. — The ribs' of leaves from two beech trees. When put together they form a polygon which does not reveal its double origin. From data by Pearson. Although such an explanation does not always turn out to be the right one, the biometrician is led to suspect when a two or more moded polygon appears that he is dealing with a mixture of more than one kind of material, each of which fluctuates around its own average. Heterogeneous material, it should be noted, does not always give a bimodal curve. For example, if Pearson's two lots of beech leaves mentioned 50 GENETICS above are mixed together, they form a regular series from the inspection of which no one could infer their double origin. (See the heavy line in Figure 27.) c. Skew Polygons The direction in which variations are tending may sometimes be determined by the statistical method. As an illustration of this may be cited the number of ray florets on 1000 white daisies (Chrys- anthemum leucanthemum) , 500 of which were col- lected at random by the writer from a small patch in a swampy meadow in northern Vermont, while the other 500 were selected in the same random manner upon the same day from a dry hillside pas- ture hardly more than a stone's throw distant. Among these two lots of daisies the number of ray florets varies from twelve to thirty-eight and their frequency polygons, as shown in Figure 28, form what are termed "skew polygons," because the mode in each case lies considerably to one side of the arith- metical mean. It will be seen that lot A from the swampy meadow, which in spite of the greater fertility of the soil and the unquestionably greater luxuriance of the plants themselves, produced heads with fewer florets, fluctuates around the number 21, while the dry pasture population B, characterized by blossoms which were in general noticeably smaller, fluctuates around the number 34. VARIATION 51 The habitats of the two lots were so near together, however, that there was probably a considerable intermixture of the two types, as shown by the tendency of each polygon to produce a second mode. 12 is H is 16 17 ia 19 to ^1 ^^ 23 35 34 »S 36 57 58 39 FIG , 28. — Variation in the ray florets of the white daisy (Chrysanthe- mum leucanthemum) . A, from a swampy meadow. B, from a hillside pasture near by. Both the polygons are " skew " because in each case there is an admixture of the other type. The distinction between the two types is due to heredity rather than to environment. Thus the A polygon shows that there is an increasing tendency or variability in the twenty-one floret type toward the thirty-four floret type, due probably in this particular instance to invasion resulting from the proximity of the B colony. 52 GENETICS 8. GRADUATED AND INTEGRAL VARIATIONS It is comparatively simple to treat statistically integral variations, illustrations of which have been given in the case of beech-leaf ribs, starfish rays, and daisy florets, all of which are characters that can be readily counted. In the same way any measurable character, such as the size of snail shells, may fall into easily limited groups, as, for example, 10 to 11 mm., 11 to 12 mm., 12 to 13 mm., etc. It is somewhat more difficult to classify variations when color or pattern is the character in question, since it then becomes necessary to define certain arbitrary limits for each class of the series within which to group the individual variants. Tower, in his famous researches on potato-beetles, encountered variations in the pigmentation of the pronotum all the way from entire absence of color to complete pigmentation. By cutting up this continuous series of variations into arbitrary groups of equal extent, however, it was quite possible to arrange the data so that they could be statistically treated just as conveniently as the integral variations mentioned above. Groups or classes of this kind are termed graduated variations. 9. THE CAUSES OF VARIATION With respect to the causes of variation authori- tative biologists have taken different points of view. a. Darwin considered variations as axiomatic. An axiom is self-evident, requiring no explanation. VARIATION 53 The absence of variations in organisms rather than the occurrence of variations is, from this point of view, the phenomenon requiring an explanation. Although Darwin himself spent some time in point- ing out the universal occurrence of variability, he accepted it as a primary fact and proceeded from it as a starting point without attempting to seek its causes. b. Lamarck and his followers have regarded the causes of variation either as extrinsic, that is, refer- able to external factors making up the environment of the organism, or as intrinsic or physiological, that is, based upon the efforts which an organism puts forth to fit into its particular environment success- fully. The causes of variation are to be sought ac- 50 35 40 45 SO 55 60 65 70 75 80 85 90 95 100 /o < Ratio of height of head to length of shell > FIG. 29. — Schematic curve of the head height of Hyalodaphnia under various conditions of nourishment. Adapted from Woltereck. cording to the Lamarckian school, in the "environ- ment" and "training" sides of the triangle of life rather than in the "heritage" side (Fig. 1). For example, Woltereck, by controlling the single 54 GENETICS extrinsic factor of food supply, was able to modify the height of the "head" of the microscopic fresh- water crustacean, Hyalodaphnia, in the remarkable manner indicated in Figure 29. When poor food Number of Fl FIG. 30. — Variations in the number of stamens in the flowers of the " live- for-ever" (Sedum spectabile) under various controlled conditions. For detailed description, see text. After Klebs. was supplied, the percentage of the head height to that of the body averaged hardly forty, while with rich food it was increased to over ninety. Similarly Klebs succeeded in changing at will the number of stamens in the common " live-for-ever," Sedum spectabile, by manipulating the environment in which the plants were kept. Some of his results are shown in Figure 30. Polygon A combines the data for 4260 flowers which were raised in well-fer- tilized dry soil under bright light ; polygon B repre- sents 4000 flowers grown in a moist greenhouse under red light ; and polygon C includes 4390 flowers VARIATION 55 from well-fertilized soil in moist hotbed conditions under a weak light. c. Weismann, on the contrary, believes that the causes of variation, at least of heritable variations, are ipirj^sic_or jnborn in the germplasm. His con- ception of sexual reproduction is that it is a device for doubling the possible variations in the offspring Jpy the mingling of two strains of germplasm (am- phimixis). By far the greater number of observa- tions recorded go to substantiate this theory. Tower found among his potato-beetles, for exam- ple, that two strains reared in the same environment showed striking differences in variation, — a fact necessarily due to intrinsic rather than to extrinsic factors. Similar cases may be recalled by any one. d. Lastly, Bateson, whose work "On Materials for the Study of Variation" already cited is a classic, takes the agnostic attitude that it is rather futile to guess at the causes of variation before the facts are well in hand. He consequently discourages such attempts by saying : "Inquiry into the causes of variation is, in my judgment, premature." In conclusion, the words of Darwin written half a century ago— "Our ignorance of the laws of variation is profound" -may still be appropriately quoted, notwithstanding the fact that in biometry we have at least an excellent analytical method by means of which considerable insight into variation is being gained. CHAPTER IV MUTATION 1. THE MUTATION THEORY AMONG the possible kinds of variation already hinted at are so-called mutations which are clearly defined from the fluctuating variations to which reference has just been made. Darwin was fully aware of the existence of muta- tions or "sports" and incidentally gave time to their consideration, but the great task which he accomplished in such a masterly manner was to overthrow the widespread and deep-seated belief of his day in a sudden special creation of distinct species. To this end he marshaled evidence in support of the gradual transition of one species into another, emphasizing fluctuations rather than muta- tions which seemed to him to play a minor role in the origin of species. It remained for the Dutch botanist Hugo de Vries to analyze the character of mutations. There is something distinctly suggestive of Darwin's method in the fact that de Vries worked in silence for twenty years before he gave to the world the " Mutations- theorie" with which his name will forever be con- nected. 56 MUTATION 57 2. MUTATION AND FLUCTUATION A mutation is something qualitatively new that {•appears abruptly without transitions and which breeds •true \from the very first! To use the musician's If phraseology, it is not a variation elaborated upon an old theme, which would correspond to a fluctuating variation, but it is an entirely new theme. The difference between mutations and fluctuating varia- tions is generally not one of quantity or magnitude, although it sometimes may be so, — since muta- tions are often much smaller than fluctuations. Mutations are discontinuous in the same sense that chemical combinations, such as carbon monoxide (CO) and carbon dioxide (C02), are discontinuous, but the leap from one to the other may be so small that frequently it is difficult to ascertain by inspec- tion alone whether the difference is due to a mutation for a fluctuation. The test comes in breeding, for the rogeny of a fluctuation will vary around the old average of the parental generation, while the progeny f a mutation will vary around a new average, set y the mutation itself. When a series of mutations is treated statistically, it does not arrange in frequency polygons as readily as a series of fluctuations do. The latter mass around the average standard according to the laws of chance much in the same way that a hundred shots by a good marksman may center around a bull's- eye. Mutations never act in this way. They find no correspondence even with wild shots at the bull's- 58 GENETICS eye. They are shots directed at a different target altogether. To the student of heredity there are two distinc- tions of prime importance with respect to mutations. ; First, that they usually appear full-fledged without f preparatory stages, and second, that they breed true ' from the start. Fluctuations, on the contrary, ordin- arily "revert" to the parental type in subsequent gen- erations. The great practical importance to the breeder of a knowledge of these heritable mutations is at once apparent. 3. FREAKS A further distinction should be made between mutations and so-called freaks or monstrosities, namely, that the former breed true, while the latter do not. A human physical deformity, such as club-foot, for example, or a humped back, is not mutation, because it does not reappear as a heritabh character. Variations of this kind are not predeter- mined in the germplasm, but are usually instances oi something that went wrong during the development of the individual somatoplasm. Thus, among normally "right-handed" snails "left- handed" individuals have occasionally been dis- covered which, when bred, were found to produce all normal "right-handed" progeny. They therefore not mutations at all, but freaks or mon- strosities due probably to some unusual occurrenc early in the cleavage stages of the embryo. MUTATION 59 4. KINDS OF MUTATION De Vries has classified mutations according to their component units into three categories: pro- gressive, regressive, and degressive. Progressive mutations are signalized by the addi- ion of a new character to the sum of complex char- acters making up the individual. If rumor may be Sieved, Anne Boleyn, the second in the interesting series of wives of Henry VIII, was a progressive mu- tant with respect to at least one character, for she is said to have possessed an extra finger on each hand, as well as the abnormalities of supernumerary mammae and extra teeth. Evidences that each of these three characters occur as heritable mutations is presented in Davenport's " Heredity in Relation to Eugenics." Regressive mutations are characterized by the •dropping out of something. Thus albinism is caused •by the absence of pigment or color. Albinic mutants * which breed true are well known, particularly among mammals, such as rats, mice, rabbits, cats, guinea- V pigs, and even man himself. Degressive mutations include cases of the return lof a character which was formerly present in the ipast history of the race, but which has for generations peen absent or latent. Castle's four-toed race of guinea-pigs furnishes an example of this class of mutations. In 1906 Professor Castle discovered a newly born guinea-pig in one of his pens with four toes on each hind foot, from which he has successfully established a four-toed race. The hypothetical an- 60 GENETICS cestor of the rodents probably had five toes on each foot, but the normal number in modern guinea-pigs is four on each of the front feet and three on the hind feet. The individual from which Castle has bred a four-toed race exhibited a degressive muta- tion, tending toward the ancestral type. 5. SPECIES AND VARIETIES The doctors have always disagreed regarding a definition of species. What determines the exclusive boundaries that shall isolate from their fellows any particular group of animals or plants has long been a mooted question, and still remains so. The Linnsean concept of a species was that of an exclusive caste of individuals, inflexibly demarked, over whose high barriers no nondescript tramps would dare attempt to climb. When an entomolo- gist of the old Linnsean school encountered an ins< which did not conform to the morphological tre tions of its fellows, the frequent fate of such a noi conformist was to perish under the boot-heel rath< than to find sanctuary in the cabinet of the pi served. Since it was an exception, and a violate of the divine law of the fixity of species, it deserv< to be annihilated! Those were hard days both f< heretics and for mutations. The method of the older school of systematisl may be described as one which emphasized differt and put up barriers that should keep the unlil apart, at the same time allowing only "birds of feather to flock together." It was a brave and su( MUTATION 61 cessful attempt to bring order out of chaos by classi- fying the living world, and it served its purpose well until Darwin's idea of half a century ago, that the origin of all species is from preceding species, put an entirely new face upon the whole matter. Organ- isms of different species were found to be related to one another, and even man could no longer escape acknowledging his poor animal relations. As a consequence, likenesses rather than differences there- after claimed the most attention. During the reconstruction of phylogenetic trees, which seized the imagination and became the prin- cipal business of biologists as soon as the " Origin of Species " was made common property, the crotched sticks in the woodpile of organisms, that had hitherto been largely discarded, were most eagerly sought after. It was just these scraggly sticks, that were neither trunk nor limb-wood but combinations of both, which told the story of continuity and were indis- pensable in building up a reunited whole. As the analysis of the living world gradually came to shift from species to individuals, it was shown that individuals may be regarded simply as ag- gregates of unit characters which may combine so variously that it becomes more and more diffi- cult to maintain constant barriers of any kind be- tween the groups of individuals arbitrarily called "species." The old species of the systematist, upon analysis into their respective unit characters, dissolve into numerous "elementary species" and "varieties" dif- GENETICS fering from species perhaps only by the addition or subtraction of a single character, and thus the SPECIES f\ FIG. 31. — Diagram to illustrate various ideas about "species." Under Species A are represented two groups of individuals which are near enough alike to be placed within a single species, but which are suffi- ciently unlike each other to constitute the " sub-species " or "varie- ties " of Darwin. Under Species B are various groups of individuals distinguished from each other by the addition or loss of one or more characters. These groups represent the "elementary species" and "varieties" of de Vries. "The "barrier of Linnseus" attempted to separate species absolutely from each other. Darwin sought to find loopholes in this barrier. To-day attention is directed rather to the relation between individuals than to the boundaries between species. possibilities of analytical classification have become almost limitless. An elementary species, according to de Vries, is a progressive mutation differing from the type species MUTATION 63 by the addition of at least a single character, while varieties are regressive mutations distinguished from the parent type by the loss of at least one character. Both breed true to their respective modifications. These different concepts of what constitutes a species, illustrated diagrammatically in Figure 31, pave the way for a better understanding of muta- tions in connection with heredity. 6. PLANT MUTATIONS FOUND IN NATURE The oldest known authenticated case of a plant mutation is the often cited instance of the "fringed jcelandine," Chelidonium laciniatum, which made its appearance in the garden of the Heidelberg apothe- cary Sprengel in 1590 among plants of the "greater celandine," Chelidonium ma jus. The fringed cel- andine bred true at once and is now a widespread and well-known species. The purple beech has appeared historically as a mutant~among ordinary beeches upon at least three occasions in widely separated localities, and it has always given rise to a constant progeny. The "Shirley poppy," notable for its remarkable range of color, originated from a single plant of the small red poppy, Papaver rhoeas, which is commonly found in English cornfields. Instances are known of double flowers among roses, azaleas, stocks, carnations, primroses, petunias, etc., arising from single flowering plants, the seeds of which in turn produce double flowers. 64 GENETICS 7. LAMARCK'S EVENING PRIMROSE The most widely known plant mutations are the progeny of Lamarck's evening primrose, (Enothera lamarckiana, because it was these plants that led de Vries to formulate his mutation theory. It is believed by botanists in general that this plant is a native of the southern United States, al- though it is now, so far as is known, extinct as a wild species in America, and native specimens are included in but few American herbaria. It was exported to London as a garden plant about 1860, and from thence it spread to the continent, where, escaping from gardens, it became wild in at least one locality near Hilversum, a few miles from Amsterdam. Here, in an abandoned potato field, it fell under the seeing eye of Hugo de Vries in 1885, and now both botanist and primrose are famous. De Vries found among these escaped plants not only 0. lamarckiana, but also two other kinds or mutants, 0. brevistylis, characterized by short- styled flowers, and 0. lavifolia, which has smooth leaves. These two were entirely new species hitherto unknown at the great botanical clearing-houses Paris, Ley den, and the Kew Gardens. Since the seeds of the (Enothera are produced by self-fertilized flowers, de Vries felt safe in regard- ing these plants as mutants rather than hybrids, and he continued to study them with especial care. Transplanting the mutants along with representa- tives of 0. lamarckiana to his private gardens cv^ - MUTATION 65 Amsterdam, where it was possible to maintain them in normal healthy condition, de Vries was able to follow their individual histories with certainty. He found that, out of 54,343 plants of the species 0. lamarckiana grown during eight years, there ap- peared 837 mutants comprising seven different ele- mentary species, all of which, with the exception of 0. scintillans, bred true. See table. MUTANTS OF (ENOTHERA LAMARCKIANA 2 g | E 4 | 3 o,. THE PURE LINE 107 factual so far as inheritance is concerned, and it makes no difference whether the largest or the small- >t beans within a pure line are selected from which to breed, the result will be the same, in that there is a complete return to mediocrity or type with no "in- heritance" of the parental modification. As a matter of fact in 1903, 1906 and 1907 the lighter parents gave a heavier progeny than the heavier parents. It will be seen at once that here is a discovery of far-reaching importance which may require us to reconstruct certain cherished ideas about the part played in the evolution of species, as well as in heredity, by natural selection. 5. CASES SIMILAR TO JOHANNSEN'S PURE LINES Although according to Johannsen pure lines are "the progeny of a single self -fertilized individual," it is plain that in at least three other possible cases something quite similar to "pure lines" may be obtained. First, when two similar organisms identical in their germinal determiners with regard to a particular character inbreed, their progeny will form a pure line so far as this particular character is concerned just as truly as two parents that are united in a single in- dividual produce a pure line progeny as the result of self-fertilization. Second, in cases of parthenogenesis, the progeny arising from a single female individual without the customary qualitative reduction of chromosomes that 108 GENETICS accompanies sexual reproduction, constitute a pure line or an unmixed strain. Third, in cases of asexual reproduction where the progeny are simply the result of continued fission of the original individual, a pure line may be said to continue from generation to generation. In the second and third categories it should be pointed out that the "pure line" is assured only so long as asexual reproduction continues. It is quite possible for an organism, heterozygotic in composi- tion, to continue to breed true or to produce an ap- parently pure line so long as asexual methods are employed. As soon as such an organism, however, changes to the sexual method of reproduction, seg- regation of characters may occur and different combinations result. 6. TOWER'S POTATO-BEETLES As an illustration of the effect of selection within pure lines of the first category may be mentioned a case given by Tower in his exhaustive experiments on the Colorado potato-beetle Lepiinotarsa decem- lineata. Among the numerous cultures of this ^beetle which were under control, a considerable variation in color made its appearance. For con- venience in classification these variations were graded into arbitrary classes or graduated variants (see p. 52) ranging from dark to light. When a male and a female from the extreme class at the dark end of the series were allowed to breed together, their progeny were not dark, but fluctuated XT X K OT 1ZH A FIG. 36. — Diagram showing the ineffectiveness of selection through twelve generations within a homozygous strain in the case of the Colorado potato-beetle (Leptinotarsa) . In each generation extreme dark speci- mens were selected as the parents of the succeeding generation but the progeny always swung back to the type. After Tower. 110 GENETICS in color around the original average of the entire series. This process of selecting each time an ex- treme pair of dark parents was continued for twelve generations, as shown in Figure 36, without in any way increasing the percentage of brunette potato beetles in the progeny. Thus in a pure line formed by the breeding of two individuals alike with respect to color, the selection of an extreme variant was quite without effect in modifying the color of the progeny. 7. JENNINGS' WORK ON PARAMECIUM An instance of the third category of pure lines is furnished by Jennings' remarkable work on the protozoan Paramecium, which was published in 1909. Jennings carried on his experiments quite independ- 206 200 194 FIG. 37. — Eight pure races of Paramecium. The actual mean length of each race is given in micra below the corresponding outline. Magni- fied about 230 diameters. After Jennings. ently of Johannsen, but he nevertheless arrived at the same general conclusion, namely, that selection within a pure line is without effect. Jennings found that Paramecia differ from each THE PURE LINE 111 other in size, structure, physical character, and rate of multiplication as well as in the environmental conditions required for their existence and, further- more, that these differences, in an hereditary sense, are "as rigid as iron." With respect to the character of mean length he was able to isolate eight races, or pure lines, whose average size, drawn to scale, is shown in Figure 37. 256 « 80 FIG. 38. — Diagram of a single race (D) showing the variation in the size of the individuals. Magnified about 230 diameters. After Jennings. Each of these pure lines produced a progeny which exhibited a considerable range of fluctuating variation. The offspring of pure line D, for example, varied from 256 to 80 micra l in length with an aver- age of 176 micra, as shown in Figure 38, where samples of the different classes of variants in pure line D are arranged in a series. A single representative of each of the different classes of variants out of all of the eight pure lines bred by Jennings is shown in Figure 39. Each horizontal row represents a single race or 1 A micron is j^^th of a millimeter. 112 GENETICS pure line, the average size of which is indicated by the sign -f . The mean length of the entire lot, as shown I 5 FIG. 39. — Diagram of the species Paramecium as made up of the eight different races shown in Figure 37. Each horizontal row represents a single race. The individual showing the mean size in each race is in- dicated by a cross placed above it. The mean for the entire lot is at the horizontal b'ne. The magnification is about 24 diameters. After Jennings. by the vertical line, is 155 micra. The total number of individuals belonging to each size is not indicated, but in every horizontal line their number is more THE PURE LINE US numerous near the average for that line and less numerous at the extremes, thus forming the typical normal frequency polygons of fluctuating variability. The significant fact about these series is this, that extreme individuals selected from any pure line do not reproduce extreme sizes like themselves, but instead, a progeny varying according to the laws of chance around the average standard of the particular line from which it came. 8. PHENOTYPICAL AND GENOTYPICAL DISTINCTIONS From the foregoing it will be seen that the be- havior of an organism in heredity cannot always be determined by an inspection of its somatic char- acters alone. For example, six Paramecia, each 155 micra in length and apparently identical, could be selected from the six upper pure lines in Jennings' table given in Figure 39 which would produce six progenies definitely unlike, whereas in the case of pure line D, twenty-four Paramecia, all measurably different from each other in size, would be found to produce twenty-four progenies practically identical. Organisms that appear to be alike, regardless of their germinal constitution, are said by Johanssen to be identical phenotypically ', or to belong to the same phenotype. On the other hand, organisms having identical germinal determiners such as those of the varying members of pure line Z), are said to be genotypically alike or to belong to the same genotype. 114 GENETICS Organisms belong to the same phenotype with respect to any character when their somatoplasms are alike. They belong to the same genotype when their germplasms are alike. The word "genotype" was suggested by Johannsen in honor of Darwin and his theory of p&ugenesis, although there are certain objections to its use in this connection for the reason that systematists have already appropriated it in a different sense. Natural history and common usage deal prin- cipally with phenotypes, that is, with organisms as they appear. The older theories of heredity were likewise concerned with phenotypes, but we are now coming to see more clearly than before that heredity must always be a case of similarity in origin, that is, in germinal composition, and that similarity in ap- pearance by no means always indicates similarity in origin or true relationship. The assumption that similarity in appearance does indicate relationship has been made the foundation of many conclusions in comparative anatomy and phylogeny, but to the modern student of genetics who places his faith in things as they are, rather than in things as they seem to be, conclusions based upon phenotypical distinctions alone have in them a large source of error which must be taken into account. In a museum of heredity, should such a collection ever be assembled, the specimens would not be ar- ranged phenotypically as they are in an ordinary museum where things that look alike are placed together as if in bonds of relationship, but they THE PURE LINE 115 would be arranged historically from a genetic point of view to show their true origin one from another. 9. THE DISTINCTION BETWEEN A POPULATION AND A PURE LINE A mixture of pure lines has been called a 'popula- tion. It is not possible to distinguish a pure line from a population by inspection, since both may be pheno- typically alike. Fluctuations about the average occur in both cases with no appreciable difference in character, although such fluctuations, when they occur within a pure line, are simply somatic differ- ences caused in general probably by modifications in nutrition or some other external factor of environ- ment, while fluctuations in a population include not only modifications of this transient nature, but also permanent hereditary differences due to germinal differences in the various pure lines of which the population is composed. Johannsen has made the distinction between pure lines and populations clear by the following figure (Fig. 40), in which five pure lines of beans are com- bined artificially to form a population. The beans which make up the pure lines noted in this figure are represented inclosed within inverted test tubes. The beans in any single tube are all of one size. Tubes vertically superimposed upon each other also contain only beans of one size. Thus it is seen that what may be a rare size of bean in one line, for instance that in the left-hand 116 GENETICS tube of pure line 5, may be identical with the com- monest size in another line, as pure line 2. The PURE LINE five pure lines represented in Figure 40 are combined in a population at the bottom of the figure, making a phenotype that marks the five phenotypes above, which are also five geno- types. In the population, how- ever, the five genotypes are hidden within one phenotype. Hence, while selection within a pure line has POPULAT.ON | ]| lUiiiiiiil II II II no hereditary in- fluence, it is evi- dent that selec- tion within a population may shift or move over the type of the progeny FIG. 40. — Diagrams showing five pure lines and a population formed by their union. The beans of each pure line are represented as as- sorted into inverted test tubes making a curve of fluctuating variability. Test tubes contain- ing beans of the same weight are placed in the same vertical row. After Johannsen. THE PURE LINE 117 obtained, in the direction of the selection simply by isolating out a pure line of one type. Thus beans chosen from the extreme left-hand test tube in the population cited would belong only to pure line 8, while those taken from the extreme right- hand test tube could belong only to pure line 3. Galton's "law of regression," namely, that minus parents give minus offspring and plus parents plus offspring, with a tendency to reversion from genera- tion to generation, depends simply upon a partial but not complete isolation of pure lines out of a population. From this distinction between pure lines and popu- lations it is clear why breeders in selecting for a particular character out of their stock need to keep on selecting continually in order to maintain a cer- tain standard. As soon as they cease this vigilance, there is a "reversion to type" or, as they say, "the strain runs out," which means that the pure lines become lost in the mixed population which inevi- tably results as soon as selective isolation of the pure line ceases. Such reversion must always be the case in dealing with a population made up of a mixture of pure lines, for only by the isolation of pure lines can the constancy of a character be maintained. When, however, a pure line is once isolated, then all the mem- bers of it, large as well as small, are equally efficient in maintaining the pure line in question, regardless of their phenotypical constitutions. 118 GENETICS 10. PURE LINES AND NATURAL SELECTION From the foregoing statements it appears that by means of selection \within a population, such as occurs normally in nature, it is not possible to get anything out that was not already there to begin with. If this is so, the origin of species cannot have come about, as Darwin thought, through natural selection by a gradual accumulation of slight favorable varia- tions. The best that selection can do is to isolate pure lines. Within pure lines it is quite powerless to change fhe genotypical characters. In other words, natural selection can only maintain and strengthen the frontier posts that are already es- tablished. It cannot break into the wilderness and create new centers. Since the extreme members of a pure line, having the same genotypical constitution, always tend to backslide to mediocrity within the limits of the line in question, the crucial question is : How can the critical step from one genotype to another, a step indispensable in the evolutionary derivation of species, ever occur ? That it has repeatedly oc- curred in the course of time is amply proven by the fact that somehow or other we have gone from Ameba to man. ^L present the only loophole of escape seems to lie either in the unlikely inheritance of acquired char- acters, or in mutations which make the leap from one character to another, and so eventually from one type to another, without the aid of selection. THE PURE LINE 119 It is interesting to note that Johannsen himself, who has been so prominently concerned in erecting this barrier in the way of the evolutionary derivation of species by natural selection, has recently reported mutations arising within his pure lines of beans. It must be admitted that to the skeptical there is a vicious circle here, for when a variation fails to re- appear in a subsequent generation, it may be ex- plained as the failure of natural selection to act within a pure line, but when a variation does reap- pear it is hailed as a mutation ! In any event the way of experiment lies open, and the evidence of investigators in this critical field will be awaited with keen interest. CHAPTER VII SEGREGATION AND DOMINANCE 1. METHODS OF STUDYING HEREDITY MODERN studies in heredity have been pursued principally in three directions : first, by microscopical examination of the germ-cells ; second, by statistical consideration of data bearing upon heredity; and third, by experimental breeding of animals and plants. The first two of these methods of approach have already been touched upon as well as experimental breeding with reference to "pure lines." In the present chapter attention will be directed to a con- sideration of experimental breeding with reference to hybridization, that is, breeding from unlike par- ents, a process which Jennings characterizes by the expressive phrase, "the melting-pot of cross-breed- ing." 2. THE MELTING-POT OF CROSS-BREEDING Hybridization, or cross-breeding, as formulated by Gal ton (1888), results in one of three kinds of inheritance, namely, blending, alternative, or par- ticulate. 120 SEGREGATION AND DOMINANCE 121 Of these, blending inheritance may be called the typical "melting-pot" in which contributions from the two parents fuse into something intermediate and different from that which was present in either parent. Galton illustrated this process by the inheritance of human stature in which a tall and a short parent produce offspring intermediate in height. A more thorough consideration of this type of inheritance will be presented in Chapter IX. By the method of alternative inheritance the pa- rental contributions do not melt upon union, but retain their individuality, reappearing intact in the offspring. In inheritance of human eye-color, for ex- ample, the offspring usually have eyes colored like those of one of the parents when the parental eye- color is unlike in the two cases, rather than eyes intermediate in color between those of both parents. According to Galton particulate inheritance results when the offspring present a mosaic of the parental characters, that is, when parts of both the maternal and paternal characters reappear in the offspring without losing their identities by blending or without excluding one another. Piebald races of mice arising from parents with solid but different colors have been cited as illustrations of this sort of inheritance, although it will be seen later in connection with the "factor hypothesis" that another interpretation of this phenomenon is not only possible but probable. The distinctions between these three categories of inheritance are diagrammatically represented in Figure 41. 122 GENETICS In blending inheritance the offspring are seen to be unlike either parent, because the parental deter- miners fuse into a new thing. In alternative in- heritance, on the contrary, the offspring may be like either parent, since the characters in question do not lose their individuality upon union, as shown in the diagram. Only one or the other of the two BLENDING ALTERNATIVE: P/\RTicuLflTE Characteristics of parental oermptasm as shown in the somatoplasm PoMibU kinds xT^ ^^^ S — N. o FIG. 41. — Three kinds of inheritance described by Galton. mutually exclusive characters ..thus becomes effective in determining the nature of each offspring. Finally, in particulate inheritance the double germplasm which determines a new individual may be imagined to undergo a diagonal rather than a vertical cleavage upon maturation, I hereby causing unblended fragments of both parental characters tobecm^e effective at once, in this manner producing a mosaic pffspring. SEGREGATION AND DOMINANCE 123 3. JOHANN GREGOR MENDEL Our understanding of the working of inheritance in hybridization we owe largely to the unpretentious studies of an Austrian monk, Johann Gregor Mendel, who, although a contemporary of Darwin, was prob- ably unknown to him. For eight years Mendel carried on original experiments by breeding peas in the privacy of his cloister garden at Briinn and then sent the results of his work to a former teacher, the celebrated Karl Nageli, of the University of Vienna. At the time Nageli's head was full of other matters, so that he failed to see the significance of his old pupil's efforts. However, in 1866 Mendel's results appeared in the Transactions of the Natural History Society of Briinn,1 an obscure publication that reached hardly more than a local public. Here Mendel's investigations were buried, so to speak, because the time was not ripe for a general apprecia- tion or evaluation of his work. At that time neither the chromosome theory nor the germplasm theory had been formulated. More- over, much of our present knowledge of cell structure and behavior was not even in existence. Weismann had not yet led out the biological children of Israel through the wilderness upon that notable pilgrimage of fruitful controversy which occupied the last two decades of the nineteenth century, and the attention of the entire thinking world was being monopolized 1 Verhandlungen naturf . Verein in Briinn. Abhandl. IV, 1865 (which appeared in 1866). 124 GENETICS by the newly published epoch-making work of Charles Darwin. Mendel died in 1884, and his work slumbered on until it was independently discovered almost simul- taneously by three botanists whose researches had been leading up to conclusions very much like his own. These three men were de Vries of Holland, von Tschermak of Austria, and Correns of Germany. Their papers were published only a few months apart in 1900 and were closely followed by important papers from Bateson in England and Davenport and Castle in America, with a rapidly increasing number from other biologists the world over. To- day the literature upon this subject has grown to be very large, and the end is by no means yet in sight. Concerning Mendel, Castle has well said: "Mendel had an analytical mind of the first order which en- abled him to plan and carry through successfully the most original and instructive series of studies in heredity ever executed." 4. MENDEL'S EXPERIMENTS ON GARDEN PEAS What Mendel did was to hybridize certain varie- ties of garden peas and keep an exact record of all the progeny, in itself a simple process but one that had never been faithfully carried out by any one. Before examining Mendel's results it may be well to state the difference between normal and artificial self-fertilization. Self-fertilization occurs when from the pollen and ovule of the same flower are derived SEGREGATION AND DOMINANCE 125 the two gametes which uniting produce a zygote that develops into the seed and subsequently into the adult plant of the next generation. In artifi- cially crossing normally self-fertilized flowers it is necessary to carefully remove the stamens from one flower while its pollen is still immature, and later, at the proper time, to transfer to it ripe pollen from another flower. . Mendel's cross-breeding experiments on peas showed certain numerical relations which gave rise to what has come to be rather indefinitely known as "Mendel's law." This law may be temporarily formulated as follows : — When parents that are unlike with respect to any character are crossed, the progeny of the first gen- eration will apparently be like one of the parents with respect to the character in question. The parent which impresses its character upon the off- spring in this manner is called the dominant. When, however, the hybrid offspring of this first generation are in turn crossed with each other, they will produce a mixed progeny, 25 per cent of which will be like the dominant grandparent, 25 per cent like the other grandparent, and 50 per cent like the parents resem- bling the dominant grandparent. An illustration will serve to make plain the man- ner in which this law works out. Mendel found that when peas of a tall variety were artificially crossed with those of a dwarf variety, all the resulting offspring were tall like the first parent. It made no difference which parent was 126 GENETICS selected as the tall one. The result was the same in either case, showing that the character of tallness is independent of the character for sex. When these tall cross-bred offspring were subse- quently crossed with each other, or allowed to pro- duce offspring by self-fertilization which amounts to the same thing, 787 plants of the tall variety and 277 of the dwarf kind were obtained, making approx- imately the proportion of 3 to 1. On further breeding the dwarf peas thus derived proved to be pure, producing only dwarf peas, while the tall ones were of two kinds, one third of them "pure," breeding true like their tall grandparent, and two thirds of them "hybrid," giving in turn the proportion of three tall to one dwarf like their parents. These crosses may be expressed as follows : — Tall, T, X dwarf, t, = tall, T(t). That is, tallness crossed with dwarfness equals tallness with the dwarf character present but latent. Mendel termed the character, which became ap- parent in such a hybrid, in this case tallness, the dominant, and the latent character which receded from view, in this instance dwarfness, the recessive. When now the hybrids, T(t)9 were crossed to- gether, the result algebraically expressed was as follows : — T + t (all possible egg characters) T + 1 (all possible sperm characters) TT+ Tt Tt +tt TT+*T(t)+tt SEGREGATION AND DOMINANCE 127 MALE GAMETES T i T t That is, one out of four possible cases was dwarf, t, in character and the other three were apparently tall, although only one out of the three was pure tall, T, while the remaining two were tall with the dwarf Character latent, T (t). The same thing may be expressed more graphically by the checkerboard plan, which Punnett suggested (Fig. 42). Each square of the checkerboard rep- resents a zygote which, having received a gamete from each of the two par- ents, may develop into a possible offspring. The character of the gametes of the parents is shown outside of these squares, while the arrows repre- sent the parental source from which the offspring have received their heredi- tary composition. The essential feature of Mendel's law is briefly this: hereditary characters are usually independent units which segregate out upon crossing, regardless of temporary dominance. Mendel carried on further experiments with garden peas, using other characters. He obtained practically the same result as in the instance already given, for the actual progeny in the second generation of the cross-bred offspring figured up, as seen in the table — * >TT — I Tt >iT 1 t FIG. 42. — Diagram to illustrate theoretically the formation of the four possible zygotes in the second filial generation of a monohybrid. 128 GENETICS below, very nearly to the expected theoretical ratio of 3 to 1. CHARACTER NUMBER OF DOMINANTS NUMBER OF RECESSIVES RATIO Form of seed .... 5474 smooth 1850 wrinkled 2.96 to 1 Color of seed coat . . Color of flowers . . . 6022 yellow 705 colored 2001 green 224 white 3.01 to 1 3.15 to 1 Form of pods .... Color of unripe pods . . Position of flowers . . 882 inflated 428 green 651 axial 299 constricted 152 yellow 207 terminal 2.95 to 1 2.82 to 1 3.14 to 1 Length of vine Total 787 tall 277 dwarf 2.84 to 1 2 98 to 1 Darbishire repeated the yellow-green cross with garden peas, obtaining in the second generation the large total of 139,837 individuals of which 105,045 were yellow and 34,792 green, which is very close to 3tol. 5. SOME FURTHER INSTANCES OF "MENDEL'S LAW" Since the rediscovery of Mendel's law the ratio of 3 to 1 in the second generation has been found by a number of different investigators to be constant in a large array of characters observed both in animals and plants of diverse kinds when these are cross-bred with reference to the characters in question. Botanists have the advantage perhaps in this matter, as they deal with forms which usually produce a large number of offspring from a single cross,— a very desirable thing in estimating ratios. On the SEGREGATION AND DOMINANCE 129 other hand, they are handicapped by being unable usually to obtain more than one generation in a year, while zoologists may secure from many animals like rabbits and mice several generations in a year, al- though ordinarily the number of progeny is much OBGANISM AUTHOR DOMINANT RECESSIVE 1 Nettles Correns Serrated leaves Smooth-margined leaves '03 Sunflower Shull Branched habit Unbranched habit '08 Cotton Balls Colored lint White lint '07 Snapdragon Baur Red flowers Non-red flowers '10 Wheat Biffen Susceptibility to Immunity to rust '05 rust Tomato Price and Two-celled fruit Many-celled fruit '08 Drinkard Maize de Vries Round, starchy Wrinkled, sugary kernel '00 kernel Silkworm Toyama Yellow cocoon White cocoon '06 Cattle Spillman Hornlessness Horns "'06 Pomace fly Morgan Red eyes White eyea '10 Horses Bateson Trotting habit Pacing habit '07 Land snail Lang Unbanded shell Banded shell '09 Mice Darbishire Normal habit Waltzing habit '02 Guinea-pig Castle Short hair Angora hair '03 Canaries Bateson and Crest Plain head '02 Saunders Poultry Davenport Rumplessness Long tail '06 Man Farrabee Brachydactyly Normal joints '05 Barley von Tschermak Beardlessness Beardedness '01 Salamander (Amblystoma) Haecker Dark color Light color '08 smaller and the ratios obtained have a larger chance of error than is the case with the more numerous plant offspring. Semi-microscopic animals, as, for example, the pomace fly, Drosophila, which produces a large progeny every two weeks or so, may combine the general advantages mentioned for the two groups of organisms 130 GENETICS indicated above, but they have the disadvantage of being so small that the detection of their distinctive phenotypic characters is attended with considerable technical difficulty. What the modern experimenter in genetics desires is an organism, first, that possesses conspicuous distinc- tive somatic characters, and, second, which will come to sexual maturity early and breed either in captivity or under cultivation both numerously and frequently. The preceding table, compiled chiefly from Bateson 1 and Baur,2 might easily be much extended. It shows from what diverse sources confirmatory evidence of the truth of Mendel's law has been derived within the past few years. 6. THE PRINCIPLE OF SEGREGATION The essential thing which Mendel demonstrated was the fact that, in certain cases at least, the deter- miners for heredity derived from diverse parental sources may unite in a common stream of germplasm from which, in subsequent generations, they may segregate out apparently unmodified by having been intimately associated with each other. This "law of segregation'', depends upon the conception that the individual is made up of a bundle of unit characters. It may be illustrated by the separate flowers picked from a garden which, after being made into a nose- gay, may be taken apart and rearranged without in 1 "Mendel's Principles of Heredity," 1909. * " Einf Uhrung in die experimentelle Vererbungslehre," 1911. SEGREGATION AND DOMINANCE 131 any way disturbing the identity of the separate blossoms. The general formula of segregation that covers all cases of organisms cross-bred with respect to a single character, that is, monohybrids, is given in Figure 43. DD 2 D(R) RR 2 D(R) DD DD DD *D(R) RR RR RR FIG. 43. — General Mendelian formula for a monohybrid. 7. HOMOZYGOTES AND HETEROZYGOTES A character which is present in the offspring iij double quantity because it was present in both parents is said by Bateson to be homozygous, while an or- ganism which is homozygous with respect to any character is called a homozygote so far as that particu- lar character is concerned. In contrast to the homozygous condition, an organ- ism is said to be heterozygous when it derives the determiner of a character from one parent only. Such an organism is described as a heterozygote with respect to the character in question. A homozygous and a heterozygous dominant may appear alike, 132 GENETICS although not necessarily so, that is, they may have the same phenotypical constitution, but their geno- typical composition is always different. 8. THE IDENTIFICATION OF A HETEROZYGOTE "Homozygote" and " heterozygote " are terms then descriptive solely of the genotypical constitution of organisms, and, as has been said, it is not always possible to distinguish one from the other by inspec- tion, although it may frequently be done, as will be pointed out later. The onj^suwway to ^idgnttfy a heterozygote is by breeding to a recessive and observing the kind of offspring produced. Peas of the formulae TT and T(t), for example, both look alike, since a single determiner for the tall character, T9 is sufficient to produce complete tallness. When, howpwr1 f.hMft two kinds of tall peas_ are each bred to a recessive dwarf pea, of the formula ft, the progeny will differ distinctly in the two cases vas follows : — r Case I. T + T X t + t = 100 per cent T(t). Case II. T + t X t + 't = 50 per cent T(f) + 50 per cent it. That is, if the dominant to be tested is homozygous (Case I), the entire progeny will exhibit the dominant .character, but if the dominant to be tested is heterozy- gous (Case II), then only one half of the progeny will show the character in question. 9. THE PRESENCE AND ABSENCE HYPOTHESIS Mendel's conception that every dominant character is paired with a recessive alternative is now being SEGREGATION AND DOMINANCE 133 largely replaced by the presence and absence hypothesis which was first suggested by Correns\ but later logi- cally worked out by others, particularly by Hurst, Bateson, and Shull. According to this latter inter- pretation, a determiner for any character either is, or is not, present. When it is present in two parents, then the offspring receive a double, or duplex* "jiose," to use Bateson's word, of the determiner. When it is present in one parent only, then the offspring have a single, or simplex, dose of the character. When it is present in neither parent, it follows that it will not appear in the offspring. In this case the offspring are said to be nulliplex with respect to the char-, acter in question. Take the case of tall and dwarf peas, the determiner for tallness when present pro- duces tall peas, even if it comes from one parent only, but if this determiner for tallness is absent from both parents, the offspring are nulliplex, that is, the absence of tallness results and only dwarf peas are produced. ^ The difference between the presence and absence theory and the dominant and recessive theory is that in the former case the "recessive" character has no existence at all, while in the latter instance it is present, but in a latent condition. 10. DIHYBRIDS So far reference has been made exclusively to mono- hybrids, any two of which are supposed to be similar except with respect to a single unit character. Mono- hybrids are comparatively simple, but when two 134 GENETICS organisms are crossed which differ from each other with respect to two different unit characters, the situa- tion becomes more complicated. Mendel solved the problem of dihybrids by crossing wrinkled-green peas with smooth-yellow peas. He found that smoothness S is dominant over wrinkled- ness W and that yellow color Y is dominant over green 0, or, as it would be stated according to the presence and absence theory, smoothness is a positive character which fills out the seed-coat to plumpness while its absence leaves a wrinkled coat, and yellow- ness is a positive character due to a fading of the green which causes the yellow to be apparent. In the absence of this green fading factor or determiner the green, of course, appears. If smooth-yellow SY and wrinkled-green WG are crossed, all the offspring are smooth-yellow, but they carry concealed the recessive determiners for wrinkledness and greenness according to the formula S(W)Y(G). When the determiners of these cross- breds segregate out during the maturation of the germ-cells, they may recombine so as to form four possible double gametes, namely, smooth-yellow *SF and wrinkled-green WG, which are exactly like the grandparental determiners from which they arose, and in addition, two entirely new combinations, smooth-green SG and wrinkled-yellow WY. Since the male and the female cross-breds are each furnished with these four possible gametic combina- tions, the possible number of zygotes formed by their union will be sixteen (4x4 = 16). That is, the SEGREGATION AND DOMINANCE 135 monohybrid proportion of 3 to 1 in dihybrid com- binations is squared, (3 + I)2 = 16. It of course does not follow that the offspring in dihybrid crosses will always be sixteen in number, or that they will always conform strictly to the theoreti- cal expectation of (3 + I)2. The offspring obtained undoubtedly obey the laws of chance, but the greater the number of offspring, the nearer they come to fall- ing into the expected grouping. The sixteen possible zygotes resulting from a dihybrid cross will give rise to sixteen possible kinds of individuals which in turn, as will be demonstrated directly, present four kinds of phenotypic and nine kinds of genotypic constitutions. A dihybrid mating, using the same symbols em- ployed in the case just described, would be expressed algebraically as follows : — SG+ WY+ SY+ WG = all the possible egg gametes SG+ WY+ SY+ WG = all the possible sperm gametes SGSG+ SGWY+ SGSY+ SGWG SGWY + WYWY+ WYSY+ WYWG SGSY + WYSY +SYSY+ SYWG SGWG + WYWG + SYWO+WGWG SGSG+2 SGWY+2 SGSY+2 SGWG+WYWY+2 WYSY+ 2 W YWG+SYSY+ 2 SY WG+ WGWG The second and the ninth items in this result are alike ; by combining them the revised result reads : — SGSG+4SGWY+2 SGSY+2 SGWG+WYWY+2 WYSY+2 WYWG+SYSY+WGWG •« There are then these nine different combinations of germinal characters or nine different genotypes in any dihybrid cross. By placing the recessive char- acters in parentheses, whenever the corresponding dominant is present to indicate that the dominant 136 GENETICS causes the former to recede from view, these nine genotypes may be combined into four phenotypes as follows : — Phenotypes . . 9SY 3SG 3WY IWG Genotypes . . 4S(G)(JF)F 2S(G)SF *SY(W)Y SYSY SGSG 2SG(W)G WYWY 2 WYW(G) WGWG From this analysis it may be said that the Mendelian ratio for a typical dihybrid is phenotypically 9:3:3:1, while that for a monohybrid, as we have already seen, SG SG SG SG © © © ® SG WY SY WG WY-> WY WY WY WY ® © © ® SG WY SY WG SY-* SY SY SY SY ® @ © © SG WY SY WG WG- WG WG WG WG @ © © © Fio. 44. — Diagram to illustrate the possible combinations arising in the second filial generation (Fa) following a cross between yellow -smooth YS and green-wrinkled GW peas. is 3:1. This expected ratio corresponds essentially with the actual results Mendel obtained in crossing smooth-yellow and wrinkled-green peas. SEGREGATION AND DOMINANCE 137 Figure 44 presents a graphic representation of the different combinations resulting from a dihybrid cross following the checkerboard plan used in Figure 42 to illustrate monohybrids. The nine genotypes and four phenotypes which result from a dihybrid cross are shown in the following squares. Number in Each Class GENOTYPE Number of Squares in Fig. 44 PHENOTYPE Number in Each Class 1 SYSY 11 SY 9 2 (W)YSY 7-10 2 S(G)SY 3-9 4 S(G)(W)Y 2- 5 -12- 15 1 SGSG 1 SG 3 2 SG(W)G 13-4 1 WYWY 6 WY 3 2 WYW(G) 8-14 1 WGWG 16 WG 1 16 16 Another illustration of dihybridism is shown in Figures 45 and 46 which is based upon data fur- nished by the Davenports.1 In the matings given here, dark or pigmented hair, represented by the solid black circles, is dominant over light-colored, that is, unpigmented or slightly pigmented hair, symbolized by the open circles, while curly hair is dominant 1 "Heredity of Eye-color in Man," Science, N. S. 26, p. 589, 1907; " Heredity of Hair Form in Man," Amer. Nat. 42, p. 341, 1908. Daven- port, C. B. and G. C. 138 GENETICS over straight, represented by crooked and straight lines respectively in the diagram. In other words, the presence of pigment is dominant over the ab- sence of pigment, while the factor that causes curli- ness is dominant over the absence of this factor, with respect to human hair. KEY TO SYMBOLS • «= Dark O = Light » = Curl4 - = Straight FIG. 45. — The heredity of human hair according to data by C. B. and G. C. Davenport. The arcs represent the somatoplasms of four indi- viduals. Within the arcs are the gametes formed by these individuals. The dominant character is placed on the outside of the arc where it will be visible. SEGREGATION AND DOMINANCE 139 Dark cvr/y When a homozygous individual with dark curly hair crosses with a homozygous individual with light straight hair, all the offspring have dark curly hair. The dark curly-haired individuals of this second generation, however, are heterozygous with respect to each of these two hair characters. When any two individuals having this particular genotypic compo- sition mate, therefore, they may produce any N£«£ |GENOTYPE PHENOTYPE one of four possible phenotypes — dark curly, dark straight, light curly or light straight haired individ- uals. These four phe- notypes in turn will present nine different genotypic combina- tions out of sixteen pos- sible cases, as shown in Figure 46. Figure 45 further- more serves to make clear, first, the distinc- tion between somato- plasm and germplasm; second, the maturation of germ-cells ; third, the segregation of gametes; and fourth, the formation of zygotes in sexual reproduction. The cells of the somatoplasm are represented as Dark straight Light curly 16 FIG. 46. — Diagrams showing the pos- sible genotypic and phenotypic com- binations resulting when two hetero- zygous individuals, with dark curly hair, mate. Symbols are the same as in Figure 45. 140 GENETICS making up the arcs within which are inclosed the germ-cells after their reduction through maturation, which results in giving to each germ-cell half the number of determiners that are present in the soma- tic cells. ; • : It will be remembered that when two gametes, or mature germ-cells, unite, they form a zygote having the proper number of determiners normal to the species in question instead of double that number. Symbols for dominant characters in the diagram are placed on the outside of the somatic arcs, because these are the characters that are visible or pheno- typic, while the non-apparent recessives are placed on the inside out of sight. 11. THE CASE OF THE TRIHYBRID Mendel went even further and computed the possibilities which would result when two parents were crossed differing from each other with respect to three unit characters. He found that the results actually obtained by breeding closely approximated the theoretical expectation. This expectation in the case of a trihybrid cross is that the cross-breds resulting will all exhibit the three dominant characters, while their genotypic constitution will include six factors, namely, these three dominant characters plus their corresponding recessives or "absences." Cross-breds of the first generation will, therefore, have eight possible kinds of triple gametes and when interbred may form a possible range of sixty-four SEGREGATION AND DOMINANCE 141 (8 X 8) different zygotes, which corresponds to a monohybrid raised to the third power (3 -f I)3. These sixty-four zygotes group together in eight RSP RsP RSp Rsp rSP rsP r SP r«p 1 1 1 I I I 1 I RSP RSp R*p rSP RSP RSP RsP RSP RSp RSP Rsp RSP rSP RSP rsP RSP rSp RSP rsp RSP RSP RsP RsP RsP RSP RsP RsP RsP rSP RsP rsP RsP rSp RsP rsp RsP RSP RSP RsP RSP RSP RSp Rsp RSp rSP RSp rsP RSp rSp RSp rsp RSp RSP Rep RsP Rap RSP Rsp Rsp Rep rSP Rsp rsP Rsp rSp Rsp rsp Rap RSP rSP RsP rSP RSp rSP Rsp rSP rSP rSP rsP rSP rSp rSP rap rSP RSP rsP RsP rsP RSp rsP Rsp rs.P rSP rsP rsP rsP rSp rsP rsp rsP RSP rSp RsP rSp RSp rSp Rsp rSp rSF rSp rsP rSp rSp rSp rsp rSp RSP rsp RsP rsp RSp rsp Rsp rsp rSP rap rsP rsp rSp rsp rep rsp FIG. 47. — Diagram showing the possible combinations in a guinea-pig trihybrid of the F2 generation. R, resetted coat ; r, non-rosetted coat (absence of R) ; S, short hair ; s, angora hair (absence of