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MONOGRAPHS ON EXPERIMENTAL BIOLOGY
EDITED BY
JACQUES LOEB, Rockefeller Institute
T. H. MORGAN, Columbia University
W. J. V. OSTERHOUT, Harvard University
THE PHYSICAL BASIS OF HEREDITY.
BY
THOMAS HUNT MORGAN
MONOGRAPHS ON EXPERIMENTAL
BIOLOGY
PUBLISHED
VOLUME I
FORCED MOVEMENTS, TROPISMS, AND
ANIMAL CONDUCT
By JACQUES LOEB, Rockefeller Institute
IN PREPARATION
THE CHROMOSOME THEORY OF
HEREDITY
By T. H. MORGAN, Columbia University
INBREEDING AND OUTBREEDING: THEIR
GENETIC AND SOCIOLOGICAL
SIGNIFICANCE
By E. M. EAST and D. F. JONES, Bussey Institution,
Harvard University
PURE LINE INHERITANCE
By H. S. JENNINGS, Johns Hopkins University
THE EXPERIMENTAL MODIFICATION OF
THE PROCESS OF INHERITANCE
By R. PEARL, Johns Hopkins University
LOCALIZATION OF MORPHOGENETIC
SUBSTANCES IN THE EGG
By E. G. CONKLIN, Princeton University
TISSUE CULTURE
By R. G. HARRISON, Yale University
PERMEABILITY AND ELECTRICAL
CONDUCTIVITY OF LIVING TISSUE
By W. J. V. OSTERHOUT, Harvard University
THE EQUILIBRIUM BETWEEN ACIDS AND
BASES IN ORGANISM AND
ENVIRONMENT
By L. J. HENDERSON, Harvard University
CHEMICAL BASIS OF GROWTH
By T. B. ROBERTSON, University of Toronto
PRIMITIVE NERVOUS SYSTEM
By G. H. PARKER, Harvard University
COORDINATION IN LOCOMOTION
By A. R. MOORE, Rutgers College
OTHERS WILL FOLLOW :
MONOGRAPHS ON EXPERIMENTAL BIOLOGY
THE PHYSICAL BASIS
OF HEREDITY
BY
THOMAS HUNT MORGAN
PROFESSOR OF EXPERIMENTAL ZOOLOGY IN COLUMBIA UNIVERSITY
117 ILLUSTRATIONS
PHILADELPHIA AND LONDON
J. B. LIPPINCOTT COMPANY
COPYRIGHT, 1919, BY J. B. LIPPINCOTT COMPANY
Electrotyped and Printed by J. B. Lippincott Company
The Washington Square Press, Philadelphia, U.S.A.
EDITORS’ ANNOUNCEMENT
THE rapid increase of specialization makes it im-
possible for one author to cover satisfactorily the whole
field of modern Biology. This situation, which exists in
all the sciences, has induced English authors to issue
series of monographs in Biochemistry, Physiology and
Physics. A number of American biologists have decided
to provide the same opportunity for the study of
Experimental Biology.
Biology, which not long ago was purely descriptive
and speculative, has begun to adopt the methods of the
exact sciences recognizing that for permanent progress
not only experiments are required but quantitative experi-
ments. It will be the purpose of this series of monographs
_to emphasize and further as much as possible this develop-
ment of Biology.
Experimental Biology and General Physiology are one
and the same science, in method as well as content, since
both aim at explaining life from the physico-chemical
constitution of living matter. The series of monographs
on Experimental Biology will therefore include the field
of traditional General Physiology.
Jacques Logs,
T. H. Morean,
W. J. V. OsterHovt.
5
CONTENTS
CHAPTER PAGE
L. INTRODUCTION: dus e2egiielc: weeds pad eae Cee set eaten as 15
II. Menvet’s First Law—SEGREGATION OF THE GENES......... 19
TIT. Tez Mucuanism oF SEGREGATION..........0.00.0 0c eee ee eee 39
IV. Menvew’s Seconp Law—Tue INDEPENDENT ASSORTMENT OF
THE: GUNES: pic Goi siietein ae swe gaskis de ghia ae guided keg am ee eA 59
V. Tue MrcHaANIsM OF ASSORTMENT...........0. 000000 ce ce eeee 73
VI EAN BAGH 2's ors Say dee baad ee Ree wage Hi whe ba smuacde notin 80
VIT. Crossing Over.) oc soh5 os sak nc baile ca ed aw whe Rh hee eed eae 87
VIII. Crossinc OVER AND CHROMOSOMES........... 000.00 ce ee eeee 96
IX. Top ORDER OF THE GENES...... 6.0.0.0 e ee 118
XX; INTERFERENCE 5 o yyad ds Gh ac eee ee ERE He Yeu T Tsetse s 126
XI. Limitation or THE LINKAGE GROUPS............. cece eee ee 133
XXII. VARIATION IN LINKAGE.......... 0... eect cnet eaes 139
XIII. Variation In THE NUMBER OF THE CHROMOSOMES AND ITS RE-
LATION TO THE TOTALITY OF THE GENES.................- 147
XIV. Szex-CHrRoMosoMES AND SEX-LINKED INHERITANCE............ 165
XV. PARTHENOGENESIS AND Pure LINES...............00 00 ee eeee 204
XVI. Tae EmpryYoLocicaL AND CyToLocicaL EVIDENCE THAT THE
CHROMOSOMES ARE THE BEARERS OF THE HEREDITARY UNITs.. 212
XVII. Cyropuasmic INHERITANCE........... 000 ee cece eee eens 219
XVIII. MaTeRNaL INHERITANCE. ......... 0000 c cece eee eens 227
XIX. Tae Particutate THEory or HEREDITY AND THE NATURE OF
PHB GENE i for Fa wees wd eat ae Hoe Dae Ze aes 234
FEX) MUTATION as os din iie die Wing aden lad Si nae See Ea ees 247
ILLUSTRATIONS
: PAGE
1. Cross Between a Tall and a Short Race of Garden Peas........... 20
2. Cross Between White and Red Flowered Four-o’clocks............ 24
3. Cross Between Splashed-White and Black, in Andalusian.......... 26
4. Male and Female Vinegar Fly..............0. 00-0 cc cu cecueeees 28
5. Normal and Abnormal Abdomen of D. melanogaster............... 29
6. Relation of Black Body Color to Wild Type as Shown by Classes
OF PUGS pis.523 4 eek eles hon Sesawes fovievee ex gherereeseniaces 30
7. Normal, Heterozygous, and Bar Eye of the Vinegar Fly............ 31
8. Relation of Bar Eye to Normal Eye..............0. 0. .c cece eae 31
9. Relation of Andalusian to Splashed White and to Black as Shown
by Classes of Birds 35.52 eh 4 joer 32 ewes na np Wi deeaes git das wien 2 32
10. Relation of Tall to Short Peas............. 0000s cece cence eee 32
11. Relation of Normal to Abnormal Abdomen as Shown by Classes
Of PCS voces dew eedinn a Ria a a edd woe $4 daw spud ae nowadanteed 32
12. Relation of Normal to Duplicate Legs of Flies..................0. 33
13. Notch Wings in the Vinegar Fly.... 0.0.0.0... 0000 cc cece cece eee 35
14. Odcyte of Ancyracanthus; Growth Period; Nucleus with Tetrads... 40
15. Egg of Ancyracanthus .......00 000. c cc ccc ent ee ence esenes 40
16. Eggs of Ancyracanthus within Membrane...................0005 41
17. Spermatogenesis of Ancyracanthus.. 0.0.0.0... cc cece cece es 42
18. Last Spermatogonial Division of Tomopteris and Stages Before and
During Syna palsies sista th eest oaee wns ao heen anes s Aa eos 45
19. Thin-Thread Stage of Tomopteris Spermatocyte; Tetrads, and First
and Second Spermatocyte Divisions.............. 00... sees 47
20. Synaptic Stages and Those Immediately Following in Batracoseps... 48
21. Synaptic Stages and Those Immediately Following in the Egg of
Pristurus ce i Wan hodeve Age eisies Gea ates oie Boe Be yeas 50
22. Sister Blastomeres of Ascaris Preparatory to Another Division.... 52
23. Normal and Reduced Chromosomes of Bistom............-...0+5 53
24. Division Figures in Egg of Ctenolabrus Fertilized by Fundulus..... 54
25. Female and Male Chromosome Groups of Protenor................ 55
26. Reduced Chromosome Group; and Extrusion of Polar Bodies in
10
ay,
28.
29.
30.
31.
32.
33.
34.
BB.
36.
37.
38.
39.
40.
41,
42.
43.
44.
46.
46.
47.
48.
ILLUSTRATIONS
Reduced Chromosome Group of Male; and Spermatogenesis in
PHOLEION co siecle 3 5h achae Reged ie hddyhcae ton Sona sbsoh bstee Leia Ron IUUa CR Ra nh
Diploid and Haploid Chromosome Groups of Drosophila busckit and
D. melanica (neglecta)... 10... cette ene e ae
Cross Between Wingless and Ebony Vinegar Fly.................
Miniature Wing, Dumpy, and Miniature Dumpy.................
Combsiof Bowlacisssain casnneeosiar wea ead aacieaes bday wlaeeets
Eight Chromosome Groups of Twelve Chromosomes Each of
Trimerow opis» «a saws «bile 24s Ree kG a ee NE OE TAA SEBO TESS
Back-cross of Male (Out of Black Vestigial by Wild) to Black
Vestipial » ssaa.is.acanice acwikiess sad aioamds. eeoeeen 6 Hew mdkoweeataed ot
Back-cross of Male (Out of Gray Vestigial by Black) to Black
Vesti gales Jccsca es sae eine de die te sahee vetnas Haw lees Saise wae ee 2
Scheme Showing the Inheritance of the X-Chromosome in
D080 plilalssa woe va e089 teh 3e aude ORME Fe Wa bad AA Moke
Back-cross of Female (Out of Black Vestigial by Wild) to Black
Vestigial Maléin.3 at4 cei gutta oe V5 pautbas ae se eH oatintee sas
Back-cross of Female (Out of Gray Vestigial by Black) to Black
Vestigial, Maley s.0 us 9 sapien crue sos odo nna eS RA weecd hein IDA
Scheme to Illustrate Double Crossing Over Between White and
Curve Showing the Influence of Temperature on Crossing Over Control
Curve Showing the Influence of Temperature on Crossing Over....
Diagram Showing Crossing Over of Two Chromosomes at Four-strand
Stage and the Subsequent Opening Out of the Tetrad..........
Scheme Showing the Opening Out of the Strands of the Tetrad .....
Scheme Showing Crossing Over Involving Both Strands of Each
Chromosomes tan cied aay pa dtutden Meaee S285 Welt sd boo hemes
Spermatogonial Cells in the Last Phase of Division and the Following
Resting Stages saris ica 46 wie da Gag va sid da dis bos ap aed Serene vada
Cells Emerging From the Resting Stages Preparatory for the Next
Spermatogonial Division................ Peis taseuer Nd alee bie mate Se
Formation of a Thick Thread after Synapsis and the Following
Condensation of a Tetrad............ 00.0 cc cece ce cece usec cece
56
57
65
66
69
77
81
83
84
89
90
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
61.
62.
63.
64.
65.
66.
67.
68.
69.
ILLUSTRATIONS
The Same Chromosome Pair in Conjugation from Thirteen Different
Conjugation of an Unequal Pair of Chromosomes and Their Subse-
quent Separation. 00.2... ieee cece cee eee enn ees
Two Schemes Illustrating the Idea of Reduplication by Bateson
and Punnett seccus ney eyeln ail caudal ae huies sok aets
Scheme Illustrating How Double Crossing Over Between Two
Distinct Genes takes Place...........4000ceeceeceeeeeeee eee:
Chromosome Groups of Pea, Wheat, and Primula...:............
Types of Chromosome Groups Found in Drosophila...............
Haploid Group of Chromosomes of the Silkworm Moth...........
Curve Showing Influence of Crossing Over at Different Temperatures
Diagram Illustrating the Effect on Crossing Over Due to the Presence
of Crossover Genes... 0.0... 0. cece cnet nee n ete e eee ne es
Chromosome Group of @nothera Lamarckiana and O. gigas, and
Triploid Group ¢ esaicivad scab eee pt scan orks Baa ee eeu
Life Cycle of Moss .......... 0.00 cece ce eee ene cece eee n eens
Diagram Illustrating the Formation of Individuals from the Regener-
ation of the Sporophyte in a Dicecious Species..................
Diagram Illustrating the Formation of Individuals from the Regener-
ation of the Sporophyte in a Hermaphroditic Species..........
Somatic Chromosomes Groups of nothera scintillans..............
Scheme Showing the Probable Relation Between the Extra Chromo-
some Pieces of Fig. 62, and the Normal Fifteen Chromosomes of
This Mutant soos vs giesgloe ou oea ew ean cais Gea ne teas ee nares
An Egg of Ascaris bivalens Fertilized by Sperm of A. univalens....
Diploid and Haploid Groups of the Sundew Drosera...............
A Scheme Illustrating the Fertilization of the Egg of One Species of
Moth by the Sperm of Another..............0. 0606060002 ce eee
Scheme Illustrating the History of the Chromosomes, and the Back-
cross Between a Hybrid Male and One or the Other Parent... .
Scheme Showing the Relation of the Sex-Chromosome to Sex-De-
TENMINAMON: s vieiiy me iwadd haere ek ey has boule eee ed Aalee
11
110
111
116
121
135
136
137
142
143
149
152
153
153
156
158
160
160
161
162
166
Cross Between White-Eyed Male and a Red-Eyed Female of the
Vinegar: Fy .iesccis tac ad ouecas Baca aaeiand WEAR dees heen eat
168
86.
87.
88.
89.
90.
91.
92.
ILLUSTRATIONS
Cross Between White-Eyed Female and a Red-Eyed Male of the
Vinegar Flys cx ccoelen shane. {abe Haag een (ag a ete oe eee
. Cross Between a Yellow White-Eyed Female and a Wild-Type
(“Gray’’), Red-Eyed Male... 2.0... 0.0... c cece cece e eee
. The Results from the Reciprocal Cross of That Shown in Fig. 71...
. Scheme Showing the Relation of the Sex-Chromosomes of the Moth
in Sex-Determination. .......... 000 cee eeee cece ee eee ee ee rece
. Cross Between Abrazas lacticolor Female, and Grossulariata Male....
. Cross Between Abrazas grossulariata Female and Lacticolor Male. ..
. Cross Between Barred Plymouth Rock Male and Black Langshan
. Scheme Showing the Transmission of the Sex-Linked Characters....
Cross Between Black Langshan Male and Barred Plymouth Rock
. Scheme Showing the Transmission of the Sex-Linked Characters
Shown in Big. 78 .....cioe cscs en see séctue yee tae cone names OR A
. First and Second Spermatocyte Divisions in the Bee.........-....
. First and Second Spermatocyte Divisions in the Hornet...........
. Life Cycle of Phylloxera caryecaulis... 10... ccc cece ene
. Extrusion of the Polar Body from a Male-Producing Egg..........
. First and Second Spermatocyte Divisions in the Bearberry Aphid....
. Hydatina senta: Adult Female, Young Female Soon After Hatching,
Adult Male, Parthenogenetic Egg, Male-Producing Egg, Resting
Diagram Showing How a Continuous Diet of Polytoma .Through
Twenty-Two Months Yielded Only Female-Producing Females...
A, Gynandromorph of Drosophila melanogaster, that was Female on
Right Side and Male on the Left; B, Female on the Left Side
and Male on the Right................. 0.0000. cc cece eee eee
Diagram Showing Elimination of X’ at an Early Cell Division... ..
Caterpillars of the Silkworm Moth............... 00.0 cceeeeuees
Diagram Illustrating How a Heterozygous Egg With Two Nuclei
Fertilized by Two Sperms Might Produce a Gynandromorph like
that Shown in Fig. 89....... 0... cc cece ec cree teen cece cence acne
Scheme Showing the Transmission of a Lethal Sex-Linked Factor
in an X-Chromosome......... 0... ees eee eee eee eee ee ee enee
169
171
173
174
175
176
178
178
178
179
181
182
182
183
184
186
187
190
191
192
193
93.
94.
95.
96.
97.
98.
99.
100.
101.
102.
103.
104.
105.
106.
107.
108.
109.
110.
111.
112.
113.
114.
115.
116.
117.
ILLUSTRATIONS 13
Non-Disjunction. Egg Fertilized by X-Sperm.................. 201
Non-Disjunction. Egg Fertilized by Y-Sperm.................. 202
A Wingless Aphid and a Winged One..................0.00000 0s 207
Curve Showing the Non-effect of Selection for the First Twelve
Generations for Increase in Body Length...................4. 208
Curve Showing the Effect of Selection for the Second Score of
Geile raiOns inveterate reish oveateaaes on ave Gescaielghualaductine ie 209
Scheme Showing Dispermic Fertilization of the Egg of the Sea
MTT CIID 2. Svante Sal etl coe Ste Spd gas ee aaNet ae a td teat 214
First Division of a Hybrid Egg.............00. 0000 cece eee ee 215
Fertilization of an Egg Starting to Develop Parthenogenetically... 216
Larval Sea Urchin Seen in Side View...............0..00c ce eeee 217
Green Leaf and Checkered Leaf of Four-o’clock ................. 220
Pelargonium that Gave Rise to a White Branch................. 221
Diagram to Show How a Sectorial Chimera May be Produced..... 221
Diagram to Illustrate Maternal Inheritance... ...............-.. 228
Diagram to Show the Inheritance of Two Pairs of Mendelian
Characters. cies cra week a da Rain WS aa oe ae 238
A,Hen-Feathered Campine Male; B, Adult Castrated Campine Male;
C, Sebright Hen-Feathered Male, D, Adult Castrated Sebright Male 246
Diagram I]lustrating Mutation in a Nest of Genes.............. 252
Two Flies (Drosophila) with Beaded Wings..................... 258
Diagram Showing the Relation of the Chromosomes. . . 258
Diagram to Show how the Appearance of a Lethal Neve Beaded
Causes the Stock to Produce only Beaded...................5- 259
Diagram Showing the Results of Crossing Over in a Stock Contain-
ing Both Beaded and Lethal. ............ 0.000 260
Diagram Illustrating How in the Presence of a Dominant Factor,
Dichete, and a Lethal in Its Homologous Chromosome at About
the Same Level, Together with Another Factor, Peach-Colored
Eyes, Gives the Result Shown in the Squares ..............4+6+ 261
Diagram Illustrating Crossing Over of Factors in Fig. 113........ 262
Rosettes of the Twin Hybrids of the Evening Primrose........... 263
Diagram Illustrating Balanced Lethals and Twin Hybrids ....... 264
Diagram Illustrating Lethals and Four Types..........-....--. 265
THE PHYSICAL BASIS OF
HEREDITY
CHAPTER I
INTRODUCTION
Tat the fundamental aspects of heredity should have
turned out to be so extraordinarily simple supports us in
the hope that nature may, after all, be entirely approach-
able. Her much-advertised inscrutability has once more
been found to be an illusion due to our ignorance. This
is encouraging, for, if the world in which we live were as
complicated as some of our friends would have us believe
we might well despair that biology could ever become an
exact science. Personally I have no sympathy with the
statement that ‘‘the problem of the method of evolution
is one which the biologist finds it impossible to leave alone,
although the longer he works at it, the farther its solution
fades into the distance.’’ On the contrary, the evidence
of recent years and the methods by means of which this
evidence is obtained have already in a reasonably short
time brought us nearer to a solution of some of the import-
ant problems of evolution than seemed possible only a few
years ago. That new problems and developments have
arisen in the course of the work—as they are bound to
do in any progressive science, as they do in chemistry and
in physics for example—goes without saying, but only a
spirit of obscurantism could pretend that progress of this
kind means that we see the solution of our problem fading
away into the distance.
Mendel left his conclusions in the form of two general
laws that may be called the law of segregation and the
15
16 PHYSICAL BASIS OF HEREDITY
law of independent assortment of the genes. They rest
on numerical data, and are therefore quantitative and can
be turned into mathematical form wherever it seems desir-
able. But though the statements were exact, they were
left without any suggestion as to how the processes
involved take place in the living organism. Even a purely
mathematical formulation of the principles of segregation
and of free assortment would hardly satisfy the botanist
and zoologist for long. Inevitably search would be made
for the place, the time, and the means by which segre-
gation and assortment take place, and attempts would
sooner or later be made to correlate these processes with
the remarkable and unique changes that take place in the
germ-cells. Sutton, in 1902, was the first to point out
clearly how the chromosomal mechanism, then known,
supplied the necessary mechanism to account for Mendel’s
two laws.
The knowledge to which Sutton appealed, had been
accumulating between the years 1865, when Mendel’s
work was published, and 1900, when its importance became
generally known. An account of the chromosomal
mechanism may be deferred, but I have spoken of it here
in order to call attention to a point rarely appreciated,
namely, that the acceptance of this mechanism at once
leads to the logical conclusion that Mendel’s discovery
of segregation applies not only to hybrids, but also to
normal processes that are taking place at all times in all
animals and plants, whether hybrids or not. In conse-
quence we find that we are dealing with a principle that
concerns the actual composition of the material that car-
ries one generation over to the next.
Segregation and independent assortment were the two
fundamental principles of heredity discovered by Mendel.
Since 1900, four other principles have been added. These
are known as linkage, the linear order of the genes, inter-
ference, and the limitation of the linkage groups. In the
same sense in which in the physical sciences it is custo-
INTRODUCTION 17
mary to call the fundamental generalizations of the science
the ‘‘laws’’ of that science, so we may call the foregoing
generalizations, the six laws of heredity known to us at
present. Despite the fact that the use of this word ‘‘law’’
has been much abused in popular biological writing we
need not apologize for using it here, because the postu-
lates in question have been established by the same scien-
tific procedure that chemists and physicists make use of,
viz., by deductions from quantitative data. Excepting for
the sixth law they can be stated independently of the chro-
mosomal mechanism, but on the other hand they are also
the necessary outcome of that mechanism.
The theory of the constitution of the germ-plasm,
to which Mendel’s discoveries led him, not only failed to
receive any recognition for fifty years, but the principle
of particulate inheritance to which it appeals has met
with a curious reception even in our own time, leading
a recent writer to state that particulate theories in general
‘do not help us in any way to solve any of the funda-
mental problems of biology,’’ and another writer to affirm
that if the chromatin of the sperm is ‘‘pictured’’ as com-
posed of individual units that represent ‘‘some specific
unit-characters of the adult,’’ then we should expect it to
be extremely complex, ‘‘more complex indeed than any
chromatin in the body, since it is supposed to represent
them all,’’ but ‘‘as a matter of fact chemical examination
shows the chromatin in the fish sperm to be the simplest
found anywhere.’’ Were our knowledge of the chemistry
of the ‘‘chromatin’”’ as advanced as these very positive
statements might lead one to suppose, the objection raised
might appear to be serious, but there is no evidence in
favor of the statement that the sperm-chromatin should be
expected to be more complex than the same chromatin
in the cells of the embryo or adult. And even were it
different in the germ-tract and soma the criticism would
miss its mark, because heredity deals with the constitution
of the chromatin of the germ-tract and not with that of
2
18 PHYSICAL BASIS OF HEREDITY
the soma. Until physiological chemists are in position
to furnish more complete information concerning the com-
position of the chromosomes, or more illuminating criti-
cism of the situation as it exists, we need not, I think,
be over-much troubled by such views so long as we handle
our own data in a manner consonant with the recognized
methods of scientific procedure.
Other critics object for one reason or another to all
attempts to treat the problem of heredity from the stand-
point of the factorial hypothesis. It has been said, for
instance, that since the postulated genetic factors are
not known chemical substances the assumption that they
are such bodies is presumptuous, and gives a false analogy
with chemical processes. Such critics claim that the pro-
cedure is at best only a kind of symbolism. Again, it has
been said, that the factorial hypothesis is not a real
scientific hypothesis, for it merely restates its facts in
terms of factors, and then by juggling with numbers pre-
tends that something is being explained. It has been
argued that Mendelian phenomena relate to unnatural
conditions and that they have nothing to do with the
normal process of heredity in evolution that takes place
in ‘‘nature.’’ It has been objected that such a hypoth-
esis assumes that genetic factors are fixed and stable in
the same sense that molecules are stable, and that no such
hard lines are to be found in the organic world. And
finally it has been urged that the hypothesis rests on dis-
continuous variation which, it is said, does not exist.
If the implications in any or in all of these objections
were true, the attempt to explain the traditional prob-
lem of heredity by the factorial hypothesis would
appear fantastic in the extreme. An attempt will be
made in the following chapters to present the evidence
on which our present views concerning heredity rest, in
the hope that an understanding of this evidence will go
far towards removing these a priori objections, and will
show that they have no real foundation in fact.
CHAPTER II
MENDEL’S FIRST LAW—SEGREGATION
OF THE GENES
MENDEL succeeded in discovering the principle of
segregation because he simplified the conditions of his
experiments so that he had to deal with one process at
atime. Others before him had failed because they worked
with too complex a situation. In each case Mendel picked
out for study a pair of contrasted characters of a kind
that were sharply distinguishable from each other when-
ever they appeared. He chose plants that normally self-
fertilize and are little liable to accidental cross-fertiliza-
tion, which made it possible easily to obtain in the second
generation numbers large enough to give significant
results. To Mendel’s foresight in arranging the condi-
tions of his work, as much as to his astuteness in interpret-
ing the data, is due his remarkable success.
Mendel used varieties of the common edible garden
pea (Pisum sativum). Many of these varieties (races)
differ from each other in a particular character. Some
races are tall, others short; some have green peas (seeds
in the pods), others have yellow peas; some of these seeds
have a smooth surface, others are wrinkled; some of the
pods are hard, others are soft. One of the crosses made
by Mendel will serve as an illustration of his work (Fig. 1).
Pollen from a race of tall peas was put artificially on
the stigma of a plant of a short race, whose own stamens,
and therewith the pollen, had been previously removed.
The hybrid plants that came from the seed were tall.
These hybrids were allowed to self-fertilize and their
seeds collected. Some of the seeds produced tall plants,
19
t
20 PHYSICAL BASIS OF HEREDITY
Talt is) 9 S4ort. 6)
'
Gamete Gamete (,]
Nf
! =
Fic. 1.—Cross between a tall and a short race of garden peas. The Fi generation is
tall. In the second generation, F2, there are three talls to one short. (Pi, Fi and F2 were
reared from peas supplied by Dr. O. E. White.)
MENDEL’S FIRST LAW 21
others produced short plants; in the ratio of 3 tall to 1
short. In other words, the contrasted cuaracters of the
grandparents reappeared in the grandchildren in the ratio
of 3to1. The experiment was carried through one more
generation, which was necessary in order to get data
for finding out what had been taking place. The short
peas were allowed to fertilize themselves. They pro-
duced only short peas. The tall peas were also allowed to
fertilize themselves. One-third of the tall peas produced
only tall offspring; two-thirds produced both tall and
short offspring in the ratio of 3:1, as had the first genera-
tion hybrids. Evidently then the grandchildren had been
of three kinds, one kind was pure for shortness, others
were hybrids, and the remaining kind was pure for tall-
ness. These kinds appeared in the proportion of 1: 2:1.
Some factor or factors in the original tall peas must
cause the peas of that race to be always tall, and some
factor in the original short peas must cause them to be
short. The short factor may be represented by s, and
the long factor by S. When crossed, the fertilized egg
should contain both factors (sS), and since the hybrids
coming from this egg were tall, it is evident that tall must
dominate over short» Now if the two factors (sS) present
in the hybrid should separate (i.e., ‘‘segregate’’) when its
ovules and its pollen-grains are formed, half of the eggs
would contain the factor that represents the short peas
(s), and half of the eggs the factor that represents tall
peas (S); also half of the pollen grains would contain the
factor that represents the short peas (s), and half of them
would contain the factor that represents the tall peas (9).
Chance meeting between egg-cells and pollen-cells (one
ovule being always fertilized by one pollen grain), would,
on the average, give one fertilized egg containing two
factors for short (ss); to two fertilized eggs that contain
one of each kind of factor (sS) ; to one that contains two
22 PHYSICAL BASIS OF HEREDITY
factors for tall (SS). The chance combination just given
. may be represented graphically as follows:
n Ovules e3 ~~ ZA ie
Pollen ral” \ short
Tall-Short.
Tall-Tall. + Tall-Short. +Short-Short.
In the actual experiment that Mendel carried out, plants
of the tall race measured from 6 to 7 feet, and those of
the short plants three-quarters to one foot and a half.
The F, plants were as tall as, or even taller than the tall
parent. When these F’,’s were self-fertilized, the seeds
(either from the same plant or from a random collection
of seeds from different F, plants) produced 787 long
plants and 277 short plants—a ratio of 2.84 to 1.
As a fair sample of each plant, ten seeds were taken
from each of 100 tall plants of this second (or F’,) genera-
tion. Out of the 100 plants so tested, 28 plants produced
only tall plants, while 72 of them produced some tall
and some short offspring. This means that 28 plants
were pure (homozygous) tall, whilst 72 were hybrid like
the Ff, plants. Taking, then, all F, plants together, the
results show 14 were short, ?/, were hybrid, and 14 were
tall, z.e., they stand in a ratio of 1: 2:1.
This relation is illustrated in the scheme below, based
on what 16 F, plants might give. Twelve would be tall
to 4 short. If the tall plants are tested, they are found
to consist of 4 pure talls (SS) and 8 hybrid talls (s9).
Altogether, then, there are 4 talls to 8 hybrid talls to 4
short, z.e., there are three kinds of F', peas in the ratio
of 1:2:1.
12 tall + 4 short
(rte
48S + &sS + 46s
1 2 1
The process of disjunction, or separation of the mem-
bers of a pair of factors, is known technically as segre-
gation. While we sometimes also speak of the segrega-
MENDEL’S FIRST LAW 23
tion of the characters themselves, it seems better, I think,
to avoid as far as possible this application of the word.
The factor for tall and the factor for short are said to be
allelomorphic to each other. The parents are generally
designated by P,; the first hybrid generation is known
as the first filial generation, or briefly. F,. The next
generation, derived from F’, is called F,, ete. When one
member of the pair of contrasted characters appears in
F, to the exclusion of the other it is said to be dominant,
the eclipsed character is said to be recessive. The hybrid
itself is said to be heterozygous, meaning that it contains
one factor or gene of each kind, while an individual con-
taining both genes of the same sort is said to be homo-
zygous for the genes involved. Mendel did not emphasize
the idea that even in pure races each character is also
represented, as a rule, by a pair of factors or genes that
segregate in the formation of the germ-cells in the same
way as do the pair of contrasted genes in the hetero-
zygotes, but at the present time this idea is accepted by
all geneticists. It was at least implied on Mendel’s view
that the two pure classes in F, (SS and ss), formed by
the recombination of two like genes, are identical with
the two grandparental races (P;).
A crucial test of the correctness of the assumption that
segregation of the members of a pair of elements takes
place in the germ-cells of the hybrid, consists in back-
crossing the hybrid (F',) to one of the parent stock, viz.,
to the not dominant stock, here the short pea. Since short
is recessive to tall, it will not influence the height of the
offspring when a tall and a short factor are brought
together. Such a cross should show whether the germ-
cells of the hybrid are, as postulated, of two sorts, and
whether equal numbers of each sort are produced. Mendel
made such tests, and obtained equal numbers of two kinds
of offspring.
Mendel obtained results like these with tall versus
short peas for other pairs of characters, such as fasciated
versus normal stems, hard versus soft pod, yellow versus
24 PHYSICAL BASIS OF HEREDITY
green pods, gray versus white-skinned peas, yellow versus
green cotyledons (seen through the skin of the seed),
and round versus wrinkled seeds (determined by the
nature of the cotyledons within the seed coat).
The 3:1, F,, ratio characteristic for a single pair of
characters is the expectation based on the chance meeting
of either one of two kinds of eggs with either one of two
kinds of pollen grains. In actual numbers this ratio is,
of course, not always exactly realized, but only approxi-
mately. For the seven pairs of characters that Men-
del examined, the F, ratios were as follows:
Dominants|Recessives| No’s. per 4
Form of seed.................. 7,324 5,474 1,850 | 2.99: 1.01
Color of cotyledons............. 8,023 6,022 2,001 | 3.00: 1.00
Color of seed coats............. 929 705 224 | 3.04: 0.96
Form of pod................... 1,181 882. 299 | 2.99: 1.01
Color of pod...............005 580 428. 152 | 2.95: 1.05
Position of flowers............. 858 651 207 | 3.03 : 0.97
Length of stem................ 1,064 787 277 «| 2.92: 1.08
Totals ;cc5csactdiadescnads 19,959 | 14,949 5,010 |2.996 : 1.004
The following collective data for the inheritance of
color of the cotyledons of garden peas show that the
approximation to a 3 to 1 for the recessive character is
very close:
Yellow Green Total No’s. per 4 |Probable errors
Mendel........... 6,022 2,001 8,023 | 3.002 : 0.998} =0.0130
Correns........... 1,394 453 1,847 | 3.019: 0.981) 0.0272
Techermak........ 3,580 1,190 4,770 | 3.002 : 0.998} =+0.0169
Hurst............. 1,310 445 1,755 | 2.986 : 1.014) +0.0279
Bateson........... 11,903 3,903 15,806 | 3.012 : 0.988} +0.0093
Doel sii. sar ates wars 1,438 514 1,952 | 2.947 : 1.053} =+0.0264
Darbishire....... .. 109,060 | 36,186 | 145,246 | 3.004:0.996] 0.0030
Darbyshire aedmepeteciee 1,089 354 1,443 | 3.019 : 0.981] | +0.0308
MNEs eectee as 1,647 | 543 | 2190 | 3.008 :0.992| |+0.0250
Correns,.......... 1,012 344 1,356 | 2.985 : 1.015) |+0.0319
Tschermak........ 3,000 959 3,959 | 3.051 : 0.969] |+0.0186
Lock. ...-.---.--- 3,082 1,008 4,090 | 3.014 : 0.986] {|+0.0183
Darbishire. ....... 222 1,856 7,518 | 3.013 : 0.987] |+0.0135
Correns.......... “ 2,405 70 295 | 3.051 : 0.949) |+0.2151
Theis stesso avs eens 50 850 3,250 | 2.954 : 1.046] '+0.0205
Totals...... 218,425 | 50,676 203,500} 3.004 : 0.996} +0.0026
"PARENTS
ES
Fic. 2.—Cross between white and red flowered four-o'clocks (Mirabilis jalapa). In
the lower part of the diagram the large circles represent somatic conditions, the included
small circles the genes that are involved.
MENDEL’S FIRST LAW 25
That Mendel’s principles apply to animals was first
made out by Bateson and by Cuénot in 1902. Since then
many characters both in domesticated and in wild animals
and plants have been studied, and there can be no question
of the wide application of Mendel’s discovery.
During the years immediately following the re-dis-
covery of Mendel’s principles (1900) much attention was
paid to the phenomena of dominance and recessiveness.
This was due, no doubt, to the striking fact that the hybrid
sometimes resembles only one parent in some particular
trait, whereas the older observations, where many charac-
ters were generally involved in the cross, seemed to have
shown that hybrids are intermediate in regard to their
parents. Wenow know, however, that although there are
cases in which the dominance is as complete as in those
described by Mendel, yet in a very large number of forms
the hybrid is intermediate between the parents, even
when only a single pair of characters is involved. A few
examples will serve to illustrate these relations.
The common garden four o’clock, Mirabilis jalapa, has
a white-flowered and a red-flowered variety (Fig. 2).
When crossed, the hybrid has a pink flower, which may be
said to be intermediate in color between white and red.
Here neither color can strictly be said to dominate. When
the hybrid (F,) is self-fertilized the offspring (F'.) are
in the proportion of one white, to two pink, to one red-
flowered plant. The F, reds and the F’, whites breed true;
the pinks when self-fertilized give white, pink and red in
the proportion of 1:2:1. In a case of this kind the color
of the F,, plants reveals the nature of the three classes
present, so that it is not necessary to test them out, as was
the case in the F, generations of Mendel’s peas, where
the F, talls were found in this way to be of two sorts.
The F, results with the four o’clock also show that
the segregation of the genes is clean, for the F’, whites
never produce in subsequent generations anything
26 PHYSICAL BASIS OF HEREDITY
but white descendants, and the F, reds never anything
but red descendants.
In this case the color of the F, flowers is obviously
somewhere between red and white. Jn so far as the F,
flower is colored, it may be said that red is dominant; in
which case the red and the pink F, classes (1+ 2=83)
are to be counted together as contrasted with the white,
giving a 3:1 ratio. On the other hand, if one chose to
emphasize the fact that the F’, pink flower is not red, but
affected by the white-producing element in its make-up,
then not red, but white, might be said to be the dominating
character ; in which case the white and the pink F’, classes
(1+ 2=—3) would be counted together as contrasted with
the red giving an inverse 3:1 ratio. It appears then
largely a matter of choice as to what is to be called
dominance (see below). The essential fact of segrega-
tion is not affected by the decision, and it is this. that is
fundamentally important.
Another example of failure of complete dominance is
shown in the race of Andalusian fowls. In this race there
are blue, splashed-white, and black birds; the blue birds
going under the name of Andalusians. When splashed-
white is mated to black, all the offspring (F,) are blue
(Fig. 3); when these blues are bred together they give
1 splashed-white : 2 blues : 1 black. Evidently the blue
birds are the heterozygous type. Their feathers show
under the microscope less black pigment, somewhat dif-
ferently distributed from that in the black birds. The
intermediate blue color is due in this case to the less dense
distribution of the pigment in the heterozygote. Lippin-
cott, who has recently examined this cross in greater detail
than heretofore, states that the colored areas or splashes
in the white males are either blue or blackish according
to the part of the body on which they occur, and that this
corresponds with the distribution of the color on the Anda-
lusian, for while the latter is said to be blue, this applies
4
Fic. 3.—Cross between splashed-white and black, giving in 7: Andalusian, and in F2 one
splashed-white, two Andalusian, and one black.
MENDEL’S FIRST LAW 27
strictly only to the hen and to the lower parts of the body
in the cock whose upper surface is very dark blue or
even, black.
In this case neither black nor white can be said to be
dominant. The blue brought in as splashes by the
splashed-white might indeed be regarded as dominant over
the black of the other (black) parent, but if so, then the
uniform distribution of the blue must be determined by
dominance of the allelomorphic gene brought in by the
black parent. Each parent then would contribute at the
same time a dominant and a recessive effect, each the
product of one member of the same pair of allelomorphs.
There are other cases in which the hybrid is inter-
mediate in color, and, in addition, its range of variation
is so large that the extremes overlap one or even both
of the two parental types. For example: In the vinegar
fly, Drosophila melanogaster, there is a race with ebony
wings and another race with sooty wings. When such
flies are crossed to each other, the wings of the F’, fly are
intermediate in color, ranging from wings like those of
sooty to wings as black as ebony. When the Ff’, flies are in-
bred they give rise to a series that at one extreme has gray
wings and at the other black wings. Separation into three
classes is difficult or impossible. Here it may appear that
the two original characters have completely blended in
F, and in F,, but that there are in reality three classes
of flies in F, can be demonstrated by suitable tests. If,
for instance, we pick out a sufficient number of F’, males
to give a fair sample of the population, and mate each
male first to an ebony female of pure stock, and then to
a female of sooty stock, we shall find that one-quarter of
the males mated to ebony give only ebony, one-quarter
mated to sooty give only sooty, while the remaining two-
quarters give, both in the back-cross to sooty, and in that
to ebony, a wider ranging group, which is darker on the
whole when mated to ebony, and lighter when mated to
28 PHYSICAL BASIS OF HEREDITY
sooty. These and other tests show that in the F’, hybrid
segregation of the same kind as in the preceding cases has
taken place, but the results are obscured by the wide
variability of the hybrid flies. In other words, evidence
can be obtained that the segregation of the genes has been
clean cut, even although this is obscured by the character
of the heterozygous flies.
Fia. 4.—Male and“female vinegar fly (Drosophila melanogaster).
In the preceding illustrations the character difference
between the two races is supposed to show itself in the
same environment. It has been found in a few other
cases that the dominance of one character over the other
may depend on the environment. For example, in the
normal vinegar fly the black bands of the abdomen show
great regularity (Fig. 4), but in a mutant race called
‘“‘abnormal abdomen’’ (Fig. 5) the bands may be irregu-
larly broken up, or even absent. In cultures with abund-
ance of fresh food and moisture, all the individuals have
very irregular bands, but as the culture gets old, and the
MENDEL’S FIRST LAW 29
food and moisture become less and less, the bands become
more and more regular until at last the flies are indistin-
guishable from normal flies. If a cross is made between
a female with abnormal bands and a wild male, the off-
spring that first hatch under favorable conditions are all
very abnormal. Here abnormal completely dominates
normal bands. But as the culture dries up, the hybrid
offspring become more and more normal, until finally they
are allnormal. At this time it might be said that normal
dominates abnormal. Both statements are correct, if we
add that in one environment abnormal banding dominates,
Fig. 5.—Normal and abnormal abdomen of D. melanogaster.
in another environment normal banding dominates. The
genetic behavior of the pairs of genes is the same here
as in all other cases of Mendelian behavior, but this is
revealed only when the environment is one in which the
abnormal gene produces one effect, the normal a different —
one. That the gene is not itself affected by the environ-
ment can be shown very simply. If a female from the
abnormal stock be picked out, at a time when the stock
has only normal bands, and crossed to a wild male, the
offspring will all be as ‘‘abnormal’’ as when the mother
herself is abnormal, provided the food and moisture
conditions are of the right kind. The late hatched normal
flies of abnormal stock may be bred from for several
30 PHYSICAL BASIS OF HEREDITY
generations, but as soon as a generation hatches under
favorable conditions they are as abnormal as though all
their ancestors had been of this sort. Thus it is evident
that no fundamental importance is to be attached to domi-
nance of characters. On the other hand, it is equally
obvious that it would be entirely unwarranted to suppose
that incompleteness of dominance is due to failure of
segregation of the genes that stand for the characters.
While the problem of segregation can be studied to
greatest advantage where the characters of a pair are
sharply separated, yet even where the pair does not
possess this advantage, the cleanness of the segrega-
tion process can be just as definitely, though more
laboriously, demonstrated.
In cases where there is an overlap between the hetero-
zygous type and one of the parental types it may, simply
as a matter of convenience, be advantageous to call that
character that gives the more continuous F, group the
dominant, thus leaving the smaller more sharply defined
group as the recessive. For example, the F, group from
black by wild-type Drosophila may be represented by
such a scheme (Fig. 6) as the following:
Fic. 6.—Relation of black body color to wild type as shown by the classes of F2 flies.
The heavy outline includes the mutant class, the lighter line the wild type, and the dotted
line the heterozygous class.
Here the heterozygous flies are typically intermediates,
but their variability overlaps that of the wild type to
such an extent that separation of the intermediate from
the wild type is practically impossible. On the other hand,
there is no difficulty in making a complete separation
between the heterozygous class and the homozygous black.
MENDEL’S FIRST LAW 31
Black is accordingly treated as a recessive in nearly
all experiments.
Fia. 7.—Normal eye, a, a’, heterozygous eye b, b’, and bar eye c, c’, of the vinegar fly.
A mutant eye shape of Drosophila, called ‘‘bar’’ (Fig.
7, a), has an intermediate hybrid type (Fig.7,b). The F,
group may be represented (lig. 8)in the following scheme:
Fia. 8.—Relation of bar eye to normal eye, as shown by the F% classes. .
In this case the hybrid, intermediate type, overlaps the
bar type, so that in F, these two latter types give a nearly
continuous class. At the other end of the F, series, the
round eyed normal (or wild) type can be distinguished
without difficulty from either of the other classes. Bar is
therefore normally treated as a dominant.
32 PHYSICAL BASIS OF HEREDITY
The case of Mirabilis, or of the Andalusian fowl, might
be represented (Fig. 9) in the following scheme:
Fic. 9.—Relation of Andalusian to splashed white and to black as shown by classes
of F2 birds.
Here all three types are fully separable, in which case
either homozygote might be considered the dominant.
Finally, to return to the case of the tall and short
peas, the following scheme (Fig. 10) represents the F,
Fic. 10.—Relation of tall to short peas as shown by F2 classes.
group: Here the tall and the heterozygous group are
alike, and inseparable by ordinary inspection, even at
the extreme end of their variation curves, and short is
‘‘completely’’ recessive.
In cases in which the environment enters more
obviously into the result (as in ‘‘abnormal abdomen,”’ Fig.
5), the following scheme (Fig. 11) represents the relation:
Dry Wet
Fig. 11.—Relation of normal to abnormal abdomen as shown by classes of F2 flies. ‘‘Dry"’
signifies conditions that make for normal; wet for abnormal.
In this case both the heterozygous and the parental
‘‘abnormal’’ type may show ‘‘normal’’ abdomen like the
MENDEL’S FIRST LAW 33
wild type. The abnormal type is treated as the dominant
although only when the conditions are favorable to its
appearance is the hereditary phenomenon seen. In
another case (duplicate legs) only the homozygous form
may show the duplications (in a special environment).
The following scheme (Fig. 12) represents this relation,
reduplication of legs being treated as a recessive:
Fre. 12,—Relation of normal to duplicate legs.
There are still other relations that affect the dominance
of characters. For example, there may be internal fac-
tors, which when present, determine that a character shall
be dominant over its allelomorph, or recessive to it. In
this connection might be mentioned what has been called
‘‘reversal of dominance.’’ An example from Davenport
will illustrate what is meant. Ina certain strain of fowls
there is a tendency for the toes to be united by a web at
the base. Crossed to birds with normal feet, no birds
with united toes (syndactyls) appeared in F,. The F,
birds inbred gave in F’, only about 10 per cent. of syndactyl
birds. It would appear that the latter character is reces-
sive, and that the recessive type overlaps largely the
dominant heterozygous type.
Davenport interpreted, however, the syndactyl as the
dominant type, because ‘‘two syndactyls may give nor-
mals, but no true normals give syndactyls.’’ In other
words, he defines the dominant type as the one that can
carry the other type, because he says dominance is due to
presence of factors, recessiveness to absence. ‘‘Now
dominance may fail to develop but recessiveness never
can do so.’’ For this reason two syndactyls may give
3
34 PHYSICAL BASIS OF HEREDITY
normals, because a dominant character may fail to develop,
even though its factors be present. Since normal feet
never give syndactyls, the normal type must be recessive.
But Davenport’s definition of a recessive type as one
that never shows in the heterozygous condition is in my
opinion based on an arbitrary distinction of what is the
cause of dominance and recessiveness. The evidence may,
I think, be better interpreted as indicated in the same
diagram as that for abnormal abdomen (Fig. 11) in that
part marked ‘‘dry,’’ in which the syndactyl condition
would be represented as recessive (heavy line). In the
hybrid the character is usually seen only in a few individ-
uals, z.e., it is intermediate, overlapping both parent types.
While this case shows that it is often only a convention
as to which type is called the dominant and which the
recessive, I can see no special reason why in these cases
of syndactylism the usual convention may not be followed
which recognizes the small F, class as the recessive.
Mendelism rests on the theory of a clean separation
of the members of each pair of factors (genes). In
every heterozygote the factor for the dominant and that
for the recessive are supposed to come into relation to
each other and then to separate at the ripening of the
germ-cells. If we think of the two genes coming together
and afterwards separating, it would seem that a favor-
able situation might exist for the two to become mixed,
and one ‘‘contaminate’’ the other. If any extensive
process of this kind occurred the Mendelian phenomena
would be so irregular and erratic that they would have
little interest. But even those who are inclined to appeal
to contamination as an exceptional phenomenon, grant
that clean separation of the genes is the rule. The best
critical evidence against contamination is in cases in which
for many successive generations breeding has taken place
from heterozygous forms only (which creates a favorable
situation for contamination to take place were it possible).
No influence of contamination has been found in such cases.
MENDEL’S FIRST LAW 35
Marshall and Muller kept flies heterozygous for three re-
cessive mutant factor for about seventy-five generations,
and at the end of that time found that these factors had
not been weakened in any way as a result of juxtaposition
Fig. 13.—Notch wings in the vinegar fly, extreme condition, a; average condition, b;
nearly normal condition, c.
with their normal dominant allelomorphs. I have kept
a stock of notch-winged flies under selection for twenty-
five generations. Notch (Fig. 13) is a character varying
in the direction of normal wings (Fig. 13, c); in every
generation of notch, many notch flies have normal wings.
The character is dominant, and exists only in heterozy-
36 PHYSICAL BASIS OF HEREDITY
gous condition, since a fly homozygous for notch dies. The
race is therefore necessarily maintained in a hetero-
zygous state. In each generation females that were
genetically notch, but had normal wings, were selected
and bred to normal males. The selection was away from
notch (i.e., toward normal). After a time more than half
of the notch flies had normal wings. The effect produced
proved to be due not to a change in the notch gene through
contamination, but to modifying genes; for at the end of
the selection the original notch could be recovered at any
time by removing the influence of the modifying factor.
It has been sometimes stated, usually by the opponents
of Mendel’s theory, or by advocates of doctrines of evolu-
tion that appeared to be compromised by the Mendelian
conception of ‘‘unit factors,’’ that Mendelism deals only
with such superficial characters as the color of flowers
or the hair color of mammals. This statement contains
an element of truth in so far as it covers most of the
kinds of characters that students of heredity find most
convenient to study; but it contains an entirely false
inference as to the limitations of Mendelism. The issue
involved is this: changes in superficial characters are not
so likely to affect the ability of the organism to survive
as are changes in essential organs; hence they are the best
kind of hereditary characters for study. But there is no
evidence that such superficial characters are inherited in
a different way from ‘‘fundamental’’ characters, and
there is evidence to the contrary. A common class of
characters showing perfect Mendelian behavior are
so-called lethals that destroy the individual when in homo-
_zygous condition. There can be no question as to the
fundamental importance of such factors. Between these
extreme cases and the superficial shades of eye color,
for example, all possible gradations of structure, physio-
logical and pathological, are known. The only possible
question that might be seriously raised is whether these
characters are all losses or deficiencies, while progres-
- MENDEL’S FIRST LAW 37
sive advances may belong to a different category. This
may be a serious question for the evolutionist, but has
nothing to do with the problem that concerns us here.
In recent years an entirely unexpected and important
discovery in regard to segregating pairs of genes (allelo-
morphs) has been made. In an ever-increasing number of
cases it has been found that there may be more than
two distinct characters that act as allelomorphs to each
other. For example, in mice, yellow, sable, black, white-
bellied gray, and gray-bellied gray (wild type) are allelo-
morphs, %.e., any two may be present (as a pair) in an
individual, but never more than two. In Drosophila the
eye colors white, eosin, cherry, blood, tinged, buff, milk,
ivory, coral and the normal allelomorph form a series of
multiple allelomorphs. In the grouse locust, Paratettix,
there are nine types that may be allelomorphic, all of
which exist in the wild state (Nabours). In Drosophila,
again, there are as many as twelve other series of allelo-
morphs known at present; in rats there is a small allelo-
morphic series, also two in guinea pigs and two in rabbits.
In plants there are a few cases known, especially in corn.
In all these series it is the same organ that is mainly
affected by the different allelomorphs, which seems ‘‘natu-
ral,’’ but was not necessarily to have been expected. The
chief interest of these series is that they appear to demon-
strate that the normal (wild type) allelomorph, and its
mutant mates need not be due to presence and absence,
but rather represent modifications of the same unit in the
hereditary material; for, taken literally, only one absence
is thinkable, and yet in Drosophila there are eight such
‘‘absences’’ in one series.
As has been stated, Mendel did not make it clear that
there exists in the normal animal or plant the same dual-
ity that comes to light when a hybrid is produced; never-
theless this condition is implied, at least, in his paper,
and has been taken for granted in practically all of the
modern work on heredity. The demonstration that such
38 PHYSICAL BASIS OF HEREDITY
is the case is, however, not a simple matter. It could not
have been made by Mendel or in the earlier days after the
rediscovery of Mendelism (1900). An attempt to furnish
this demonstration is given in Chapter XX. Assuming
the demonstration to be satisfactory, we reach the highly
important conclusion that segregation is not something
peculiar to hybrids, but something most readily demon-
strated by means of hybrids, and that in all probability the
germ-plasm is at first made up of pairs of elements, but
at the ripening of the germ-cells these elements (genes)
separate, one member of each pair going to one daughter
cell, the other member to the other cell. The mechan-
ism by means of which such a process might take place
had been known for several years before its relation
to Mendel’s principles of segregation was realized. This
mechanism is to be found in the conjugation and reduc-
tion processes that take place in the maturation of egg-
and sperm-cell. An account of this process is given in
the next chapter.
CHAPTER III
THE MECHANISM OF SEGREGATION
Onz of the most secure generalizations of modern work
on the cell is that every cell of the individual contains a
constant number of self-perpetuating bodies (called chro-
mosomes), half of which are traceable to the father and
half to the mother of the individual. No matter how
specialized cells may be, they contain the same number
of chromosomes. Equally important is the fact that after
the eggs of the female and the sperm-cells of the male
have passed through the ripening or maturation divisions
the number of chromosomes is reduced to half. Lastly,
there is convincing evidence that the reduced number of
chromosomes is brought about as the result of a separa-
tion of such a kind that each mature germ-cell gets only a
paternal or a maternal member of each chromosome pair.
The reduction takes place in the female at the time
when the polar bodies are given off from the egg; and in
the male just prior to the formation of the spermatozoa.
A characteristic process is seen in the odgenesis and sper-
matogenesis of the nematode worm Ancyracanthus cysti-
_dicola (a parasite in the swim-bladder of fresh-water
fishes) described by Mulsow. The young eggs contain
twelve chromosomes (Fig. 14, a). As the result of the
later union of these twelve in pairs, six short threads
appear in the nucleus of the egg just before it extrudes its
polar bodies. The threads contract to six short rods
(split in two planes at right angles to each other), the
tetrads (Fig. 14, c). With the dissolution of the nuclear
wall these tetrads are set free in the protoplasm, and a
spindle develops about them (Fig. 15, a). They pass to
the equator of the spindle, and there dividing lengthwise,
1Exceptions occur in certain cases of parthenogenesis.
39
40) PHYSICAL BASIS OF HEREDITY
half of each goes to one pole, and half to the other pole
of the spindle (Fig. 15,b). One end of the spindle pro-
trudes from the egg, and around it the protoplasm con-
Fra. 14 —Odcyte of Ancyracanthus, a; growth period, 6; nucleus with tetrads, c. (After
Mulsow.)
stricts off (Fig. 15, c) to form the first polar body. About
the six ovoidal chromosomes left in the egg a new spindle
develops; and these chromosomes become drawn into
its equator, where they divide again, half of each going
Fig. 15.—Egg of Ancyracanthus with six tetrads, a; egg with first polar spindle, b;
egg after extrusion of first polar body, c; egg with second polar spindle, d; egg after the
extrusion of both polar bodies, e.
to one pole and half to the other (Fig. 15, d). :
ll. . «| OKRRe
Y.2>.2 _
. g, “= =m -¢ e
oi te aN ar A\ |
sr.3,> 6 OY
C20: = 1) «YD
R: « AN AR en a Ai
Fic. 54.—Types of chromosome groups found in Drosophila. A-H female groups;
I-L female and male groups. In , F, I,J, K, and L, the X-chromosome can be identi-
fied, because, in the male (Alex. Metz), the Y-chromosome has a different shape from the X.
It should be emphasized that it is to be expected for
new types that the number of characters that may seem
to give independent assortment will be found at first
greater than the number of chromosomes, because wher-
ever two genes in the same chromosome are far apart they
will appear to assort independently until the discovery
LIMITATION OF THE LINKAGE GROUPS 137
of intermediate genes shows their true relation. This will
be especially the case when crossing over occurs in both
sexes ; when it occurs only in one sex, the linkage relations
are more quickly determined. Moreover, in some cases
where several genes are known the mutant characters
have not yet been tested out against each other but against
different ones. Such information does not furnish the
data that are needed. '
Fig. 55.—Haploid group of chromosomes of the silkworm moth (Yatsu) a. Haploid
group of chromosomes of mouse (Yocom) b. Haploid group of chromosomes of man
(Guyer), ¢ and (von Winnewarter) d.
There are several forms in which there are two or
more chromosomes that come together in a group at the
time of segregation and move collectively to one pole.
Such groups should be expected to count as a single chro-
mosome so far as segregation is concerned, although the
crossing over relations may turn out to be something
different from anything as yet known.
138 PHYSICAL BASIS OF HEREDITY
An extension of the principle of agreement of linkage
groups and chromosomes (if they are thought of only as
a linear order of genes) is found in the case of ‘‘duplica-
tion’’ described by Bridges, where a short series of linked
genes appears to lie at one end of the regular series, dupli-
cating their number for this region of the chromosome.
Obviously this is not to be looked upon so much as an
exception to the principle but rather as a special case due
to an accidental change in the mechanism. The number
of linkage groups is not changed, but one of them has
its genes duplicated for a short part of its length.
CHAPTER XII
VARIATION IN LINKAGE
Crossine over is not absolutely fixed in amount, but
is variable. This statement does not refer to variability
in the number of crossovers due to random sampling,
but to variability due to fluctuation in environmental
conditions, or due to internal changes in the mechanism of
crossing over itself. For example, it has been shown
that the amount of crossing over in Drosophila is different
at different temperatures, and it has also been shown that
there are factors (genes) carried by the chromosomes
themselves that affect the amount of crossing over. These
questions, that have already been touched upon in other
connections, may be taken up here in more detail.
The work of Plough on the influence of temperature
on crossing over in Drosophila, that has already been
utilized, was concerned with the influence of different tem-
peratures on the number of crossovers obtained. It may
be recalled that he found that when the eggs were sub-
jected to a given temperature during a certain stage in
their maturation the amount of crossing over that took
place, as shown in the kinds of flies produced, was definite
in the sense that the average results were predictable for
each specific temperature, and that there are values for
different temperatures which, when plotted, give the curve
drawn in Fig. 56.
Further details of one of the experiments may serve
to make its significance clearer. Three points (or loci)
were made use of that involved three mutant genes (and
their diagnostic characters, of course). Males, pure for
the three mutant characters, black body color, purple eyes,
139
140 PHYSICAL BASIS OF HEREDITY
and curved wings were crossed to wild-type females. The
F, female produced in this way would be heterozy-
gous for the three mutant factors involved in the cross.
Such an F’, female was then bred to a male pure for the
three recessive genes, black, purple, curved; and her
offspring were kept at a given temperature until they
emerged as flies, and then if necessary for some days
longer in order that as many eggs as possible might have
matured under the specified temperature. Controls of
sisters and brothers were made in each case and kept at
average ‘‘normal’’ temperature. In the table that fol-
lows crossing over between black and purple is indicated
as ‘‘Ist crossover,’’ and between purple and curved as
‘‘2nd crossover,’’ and between both as double crossover.
Ten different temperatures were tested. At 5° C. the
eggs did not hatch, and at 35° C. the females were sterile.
In the seven intermediate temperatures the results were
those recorded in the next table.
b — pr —cl
Female parents hatched at temperature indicated below, Weiehted
alue for
Num-| Temp.| Total Non- Ist and Double Ist and b—pr
ber cross- cross- cross- cross- cross- | cross-' | Region
over over over over over over
per cent|per cent| per cent
9° 995 643 95 218 | 39 | 13.5 | 25.8 | 13.6
13° 2,972 | 1,854] 310 716 92 | 13.5 | 27.2 | 17.5
17.5°| 2,870] 2,021] 189 610 50 8.3 | 23.0 8.2
15,000 | 11,318 | 735 2,775 172 6.0 | 19.6 6.0
29° 4,269 | 2,993] 315 898 63 8.8 | 22.5 8.7
31° 3,547 | 2,265 | 333 785 164 | 14.0 | 26.7 | 18.2
32° 4,376 | 2,701 | 513 984 178 | 15.7 | 26.5 | 15.4
© OONT OTR Co OD
bo
i]
°
At the two lower temperatures the crossover value is
high, 7.e., little crossing over occurs. At the next three
temperatures (17.5°, 22°, 29° C.) the crossing over value
is much less, while at the last two temperatures 29° and
VARIATION IN LINKAGE 141
31° C., it is high again. The control values for sister
flies, at normal temperature (22° C.), are given in the
next table. -
Controls—female parents hatched at 22° C.
Ist and Non- Ist 2nd |Double
cross- | cross- | Total | cross- | cross- | cross- |. cross-
over | over over | over | over | over
per cent |per cent
6.1 | 19.2 904! 683] 47 | 166 8
7.8 | 20.1 | 3,622| 2,655] 231 | 685 | 51
5.9 | 19.5 | 2,219] 1,678] 108 | 409 | 24
5.9 | 20.3 | 4,822 | 3,608| 231 | 927 | 56
The figures given in this table were obtained as a con-
trol for the last results, and from these data the results
of crossing over are reduced to the same scale. These
weighted crossing-over values for the first regions give
the curve drawn in Fig. 56. The curve begins at a high
level and drops rapidly. The first maximum is reached at
about 13° C., and then falls to 17.5° C., where the level
remains nearly constant for ten degrees more (27° C.).
It rises rapidly at about 28° and reaches a second maxi-
mum at 31° to 32° C. Afterwards it is seen to fall until
sterility occurs at 35° C.
The temperature curve of crossing over seems to show
that the phenomenon is not a simple chemical reaction,
for if it were we should expect for every rise in 10° C. the
amount of change in crossing over to be approximately
tripled. It would appear, therefore, that the phenomena
might be due to the physical state of the materials involved
in crossing over. Plough calls attention to the similarity
of this curve to that shown by the amount of contraction
of a frog’s muscle. Here there is an increase from zero
to 9° C., when a maximum is reached. After this, the
amount of contraction decreases, reaching a low point
142 PHYSICAL BASIS OF HEREDITY
between 10° C. and 20°C. It then rises rapidly, reaching
a higher maximum than the first at about 28°C., after
which it decreases until rigor sets in at 38° C.
The results of crossing over between purple and curved
gave similar results, but the ‘‘distance’’ here is so great
that double crossing over complicates the results; there-
fore they need not, for the present, be analyzed further.
Attempts to change the crossing over value by starvation,
moisture, increase in fermentation of the food, iron salts,
etc., gave no results that seemed significant. On the other
a
3
eo 26 «@ 3 8 § & & LEP
ng
°
egret
| q "9 ns 7) ay t £5
Fia. 56.—Curve showing influence of crossing over at different temperatures. (After Plough.)
hand, Bridges had already noted that a decrease in the
amount of crossing over is found in second broods as
compared with first broods—ten-day periods. What
change in the environment is behind this ‘‘age’’ dif-
ference is not clear, but since most of the eggs pass
through this early prematuration stage in the larve
and some of them may reach the maturation stage
in the pupa, it is possible that prevailing conditions in
one or the other of these physiological states may be
responsible for the difference between these states and
those that prevail after the fly has hatched.
VARIATION IN LINKAGE 143
Not only external factors but internal factors, and
these genetic ones, may influence the amount of crossing
over that takes place. Sturtevant has discovered two such
genes in the second chromosome of a certain stock of
Drosophila. A female from a wild stock from Nova
Scotia was crossed to a male showing the characters ves-
tigial and speck. One of the daughters was tested and
gave no crossovers in 99 offspring, though the vestigial,
speck hybrid usually gives about 37 per cent. of crossing
over. All of the descendants of this female that were
& > ow ow os ep
+
0a 379 442 55.9 66.0 94.2
\ PF sé ¢ 8p.
fs. is : “i
06 os 13.4 © 210 56.3
0.0 424 486 5.
ear ?
os” dike ie
Br ad sp
0.0 sésals eke
Fie. 57.—Diagram illustrating the effect on crossing over due to the presence of crossover
genes. (After Sturtevant.)
known, through linkage relations, to have the Nova Scotia
second chromosome, gave the same result, while those of
her descendants that did not have the particular chromo-
some did not show such a change in linkage. These rela
tions held regardless of whether the chromosome involved
had come from the father or the mother.
A number of experiments were made with females hav-
ing a Nova Scotia second chromosome, while the other
second chromosome bore the mutant genes for black, pur-
ple, curved, and in other experiments other mutant genes
were present. In Fig. 57 (upper line) all the genes stud-
ied, viz., star (S), black (b), purple (pr), vestigial (vg),
144 PHYSICAL BASIS OF HEREDITY
curved (c), and speck (sp) are indicated in their relative
locations, i.e., spaced in proportion to the usual amount
of crossing over between them. Correspondingly, the
short second line is based on the crossover relations of
these factors when the female is heterozygous for the two
Nova Scotia genes.
Further experiments were made with females
(obtained by crossing over) in which only the ‘‘left half”’
of a Nova Scotia chromosome was present (third line),
the other half being derived from an ordinary chromo-
some. The offspring of such a female showed that cross-
ing over was decreased only in the left half. .
When the right half of the Nova Scotia chromosome
was present (fourth line) that half was ‘‘shortened.’’ It
follows that there are two (or possibly more) factors
present, one in each half of the second chromosome of the
Nova Scotia stock, each inhibiting almost completely
crossing over in its own region, but not in the other region.
An equally surprising result was obtained from a
female so constituted that the right halves of both mem-
bers of this pair of second chromosomes were present, 1.¢.,
when she was homozygous for the ‘‘right hand’’ pair of
factors for little crossing over. Under these circum-
stances, the crossing over was normal for this end (last
two lines). How such results are produced (quite aside
from the nature of the factor producing them) is unknown.
Almost inevitably, however, we think of the cause as a
difference in the length or shape of the chromosome con-
taining these factors, so that corresponding levels do not
come together, hence failure of interchange. When, how-
ever, both chromosomes are affected in the same way their
corresponding regions might be expected to come to-
gether and‘cross over.
The preceding results of Sturtevant’s suggest the
possibility that all genes may have an effect on crossing
over—possibly one might think that in some mysterious
way the crossing-over values shown by the genes are a
VARIATION IN LINKAGE 145
function of their nature. It may be well to point out that
in the only cases where the evidence suffices to give an
answer to such a question, that answer is very clearly
against such a view. For instance, if we determine the
linkage between two factors 4—M and then exchange one
of the intermediate genes for its allelomorph, we find that
in general the change has no effect on crossing over
between 4 and M. If we exchange factors outside of
A and M—either near them or far away—still no effect on
crossing over between A and M is observed. If we sub-
stitute one allelomorph for another, in cases where more
than two are known, we find no change in the crossing
over for that level. This and other evidence shows that
crossing over is quite independent of such genes, never-
theless there are other specific genes, as shown above,
whose sole effect, or main effect at least, is to change the
crossing-over values.
One highly important and significant result of Sturte-
vant’s work on crossing-over factors should be noticed.
The order of the factors is not in any way changed by
the ‘‘shortening’’ process, as shown by the experiments
in which three or more loci are followed at the same time.
The most remarkable fact connected with crossing
over is that no crossing over at all takes place in the
male of Drosophila, and this applies not only to sex-
chromosomes (XY) but also to the other pairs or auto-
somes. When the absence of crossing over was discovered
for sex-linked genes, it seemed probable that this was due
to the presence of only one X-chromosome in the male, for
at this time Steven’s work had led us to conclude that the
male Drosophila, like some other insects, is XO. Later,
when failure to cross over in the male was found in other
chromosomes as well, it was evident that some more gen-
eral relation was behind the phenomenon in these chromo-
somes at least. It is true that other genetic evidence
has shown that the Y-chromosome is ‘‘empty’’ (7.e., con-
tains no genes dominant to any of the mutant genes as yet
10
146 PHYSICAL BASIS OF HEREDITY
discovered) and on this account one might still ascribe
failure to cross over in this pair to its peculiar condition.
The interest in the situation became even greater when
it was found that in the silkworm moth (in which the sex
formula is reversed, so to speak) crossing over is again
absent in the sex that is heterozygous for the sex fac-
tors—here the female. The female moth is apparently
ZW, at least in two cases.
In one of the flowering plants, Primula sinensis, cross-
ing over occurs in both sexes (Gregory, Altenburg), but
the amount of crossing over in the pollen is somewhat dif-
ferent from that in the ovules. Gowen has examined
Altenburg’s data statistically and finds that the differ-
ence is probably significant.
That crossing over should take place in the sex that is
homozygous for the sex-chromosomes (the female in
Drosophila, the male in the silkworms) but in both sexual
elements in the hermaphrodite plant (Primula) may
appear to have a deeper significance, but more recent dis-
coveries seem to deprive the results of any such meaning.
Castle, for instance, gives data that show crossing over
in the male rat (the male is probably heterozygous for
the sex-chromosome), and Nabours gives data for crossing
over in the male and female grouse locust, Apotettia
(in which the male is presumably heterozygous). Until
more cases are forthcoming it must seem doubtful, there-
fore, if any such relation as that mentioned above is a
general one.
CHAPTER XIII
VARIATION IN THE NUMBER OF THE CHROMO.
SOMES AND ITS RELATION TO THE TOTAL-
ITY OF THE GENES
Tuer theory that the chromosomes are made up of inde-
pendent self-perpetuating elements or genes that compose
the entire hereditary complex of the race, and the impli-
cation contained in the theory that similar species have an
immense number of genes in common, makes the numeri-
cal relation of the chromosomes in such species of un-
usual interest. This subject is one that could best be
studied by intercrossing similar species with different
numbers of chromosomes, but since this would yield sig-
nificant results only in groups where the contents of the
chromosomes involved were sufficiently known to follow
their histories, and since as yet no such hybridizations
have been made, we can only fall back on the cytological
possibilities involved, and on the suggestive results that
cytologists have already obtained along these lines.
A good deal of attention has been paid in recent years
to the not uncommon fact that one species may have
twice as many chromosomes as a closely related one. So
frequent is this occurrence that it seems scarcely possible
that it is due to chance. The implication is that the num-
ber of the original chromosomes has either become
doubled, or else halved. If the number is simply doubled
there would be at first four of each kind of chromosome
from the point of view of genetic contents. This is what
T understand by tetraploidy. There is some direct evi-
dence that doubling may occur. If a new race or species
is ever established in this way, we should anticipate that
in the course of time changes might occur in the four iden-
tical chromosome groups so that they would come to differ
147
148 PHYSICAL BASIS OF HEREDITY
and form two different sets... Theoretically, the number
of different genes in a species might in this way be in-
creased. If changes in the same gene in the same direction
sometimes occur, as the evidence indicates that they do,
then identical new mutant genes, derived from the same
kind of original ones, might later arise in different pairs.
There is, however, another way in which the number
of chromosomes may be doubled without doubling the
number of genes. If the chromosomes break in two,
double the number will be produced. It is not easy to
explain how this could occur in all of the chromosomes at
the same time if the process is supposed to be accidental.
If it be supposed that the break first occurred accidentally
in one member of the pair, it is not clear why such a
broken chromosome could establish itself on the theory
of chance, for the intermediate condition of one broken
and one intact chromosome would seem of no apparent
advantage. The same reasoning applies to the converse
process, viz., the coming together of chromosomes end
to end which would reduce the number by half. Such a
process would not increase the number of genes in
the total complex. Until we know more about the
physical or chemical] forces that hold the genes in chains,
and more about the way new genes arise, it is not worth
while to speculate about the causes or probabilities
of such occurrences.
What has just been said in regard to doubling and
halving of the whole set of chromosomes applies also to
doubling in one pair of chromosomes. If doubling
occurred in one pair of a ten-chromosome type, a twelve-
chromosome type would result; if in two pairs, a fourteen-
chromosome type, etc. Unless tetraploidy is the simpler
procedure we should a priori suppose that increasing (or
decreasing) in pairs would, on the theory of chance alone,
1The question as to whether the four chromosomes involved would or
' would. not mate at random introduces a difficulty (as shown in the
primula case).
VARIATION OF CHROMOSOMES 149
be the more common procedure. A few examples will
illustrate what has been found out so far concerning some
of these possibilities. ,
The evening primrose, Enothera lamarckiana, has 14
chromosomes as its full or somatic number, and 7 as its
reduced number (Fig. 58, a), and these numbers charac-
terize most of the mutant types that De Vries found. But
there is one mutant known as gigas, that has 28 chromo-
somes as its full number, and 14 as its reduced number
(Fig. 58, b). Stomps estimates that gigas appears about
9 times in a million cases, 2.¢., in 0.0009 per cent. Gigas is
distinguished from Lamarckiana in many details of struc-
ture, but chiefly in its thick stem, etc., which is associated
with larger cells.
¢,
We IU re .
Se % Dd Isis
ZN K we’
a Ob C
Fic. 53.—Chromosome group of Cnothera lamarckiana, a; chromosome of group of O.
gigas, b; triploid group, c.
The type breeds true, z.e.,it does not revert to Lamarck-
iana; thus De Vries grew a family of 450 individuals from
his original gigas, only one being a dwarf gigas, viz.,
nanella. The way in which gigas originates has been
much discussed, but no conclusion reached. De Vries
suggested that it is produced by an egg with 14 chromo-
somes (diploid), being fertilized by a sperm with 14
chromosomes, both of these diploid cells originating by
the suppression of a cytoplasmic division in the develop-
ment of the gametes. It has also been suggested that a
tetraploid condition might arise in a spore mother cell
that developed without fertilization (by apospory). Gates
pointed out that by suppression of the first division of the
egg, after fertilization, the tetraploid condition would
arise. The only objection to this last view, that seems
150 PHYSICAL BASIS OF HEREDITY
the simplest one since such suppressed division has been
seen and can be induced in animal eggs, is that the follow-
ing division might be expected to be into four parts owing
to the doubling of the centres.
Gregory has described two tetraploid races of Primula
stmensis,? one of which arose from ordinary plants in the
course of his experiments. Since known genetic factors
were present an opportunity was given to examine into the
relation between the members of the four chromosomes
of a set. The possibilities involved are these: Assuming
the parents to be AJ’, and aa’, and that conjugation of
chromosomes takes place in twos only, then if any one
of the four (4 A’ aa’) chromosomes of a set may mate with
any other member, there will be six possible unions, viz.,
AA’, Aa, Aa’, A’a, A’a', aa’. Tf the two derived from
the same parents were the only ones that can mate, only
two combinations are possible, 4A’, aa’, and if the two
derived from the opposite parents were the only ones that
mate only two (but different ones) could form, viz., Aa,
A’a’. The genetic expectation is somewhat different
for each of the three cases, since the number of different
kinds of gametes produced is different in each. The data
obtained by Gregory are not sufficient to give convincing
evidence in favor of any one of these possibilities, although
as Muller has shown by an analysis of the evidence, they
are more in favor of the first possibility, viz., that of ran-
dom assortment. Gregory, without committing himself to
the chromosome view, follows the second possibility in his
analysis of the case. There is, however, nothing in the
chromosome theory that would support the view that
restricts the conjugation of homologous chromosomes
according to their parental origins.
There are two other species of primose, Primula flori-
bunda and P. verticillata, each with 18 chromosomes that
have, after crossing, produced tetraploid types. In a
2 Other giant races of P. sinensis examined by Keeble and by Gregory
are diploid.
VARIATION OF CHROMOSOMES 151
cross between these two, a hybrid called P. kewensis was
produced, which Digby has shown has also 18 chromo-
somes. It produced only thrum flowers, and was therefore
sterile. Five years later, after this plant had been multi-
plied by cuttings, one pin flower appeared which was pol-
linated by a thrum flower. It gave rise to the fertile race
of P. kewensis, that had 36 chromosomes. What connec-
tion there may have been between thé hybridization and
the subsequent doubling, if there is any connection, is by
no means clear. It may be noted that in the reciprocal
cross between P. verticillata and P. floribunda, a hybrid,
P. kewensis, with 36 chromosomes also appeared.
The most interesting results on tetraploidy are those
of Elie and Emile Marchal on certain mosses, for they
have been able to produce tetraploid types experimentally.
It may be recalled that in mosses there is an alternation
of generations. The diploid (2N) generation is known
as the sporophyte (Fig. 59) that develops out of and
remains attached to the other haploid generation, the
gametophyte or moss plant (1NV). The sporophyte pro-
duces a large number of spores, each containing the half
number of chromosomes (1N) as a result of reduction that
has taken place in their formation, and from each spore
a young moss plant develops, beginning as a protonema of
loose threads. When the moss plant produces its heads or
' flowers the sexual organs appear—archegonia (?) and
antheridia (4). Thus the ‘‘sexes’’ are here represented
by the haploid generation.
The egg-cell, contained in the archegonium, is ferti-
lized by a sperm-cell, the antherozooid. The fertilized
egg-cell (2N) develops im situ into the straight stalk
imbedded at its lower end in the tissue of the moss plant,
expanding at its upper end into the cup containing the
spores. The mother-cells of the spores—like the tissue of
the sporophyte itself—contain the 2N number of chromo-
somes, which, by two divisions (similar to these already
described for the animal cells during reduction), reduces
152 PHYSICAL BASIS OF HEREDITY
the number to 1N. It is at this time, too, in mosses with
separate sexes, that sex differentiation takes place, for as
the Marchals have shown, each spore gives rise to a male
Gametophyte
yin @xIn)
Fie. 59.—Life oycle of moss. The mycelial thread and the moss plant constitute
the In, or gametophyte generation; and the stalk and capsute (with its contained spores),
arising after fertilization out of the moss plant, constitutes the 2n or sporophyte generation.
or to a female thread that produces archegonia or else
antheridia regardless of the condition under which the
young plants are reared. Allen has recently shown in
related plants—the liverworts—that during the reduction
division (that gives rise to the spores) an unpaired sex-
VARIATION OF CHROMOSOMES 153
chromosome is present that goes to half only of the spores.
Presumably then in liverworts, and mosses, also, there
is an internal mechanism for producing the two ‘‘sexes.’’
The Marchals have worked both with species having
separate sexes and with hermaphrodites. We may con-
oyin)
exyirm
Fie. 60. Fia. 61.
Fie. 60.—Diagram illustrating the formation of 2n individuals from the regeneration of
the sporophyte in a diccious species. (According to Marchal.
Fia. 61.—Diagram illustrating the formation of 2n individuals from_the regeneration of
the sporophyte in a hermaphroditic species. (According to Marchal.)
sider the former first. If the sporophyte is removed and
cut across, its cells regenerate a tangle of threads (pro-
tonema), which is the beginning of a new moss plant (Fig.
60). Since the sporophyte had the double number (21)
of chromosomes, it is to be expected that the young moss
plant that regenerates from its tissue (sporophyte) will
also have the double number, and such proves to be the
154 PHYSICAL BASIS OF HEREDITY
case. The new moss-plant is therefore 2N (or diploid)
instead of being 1N, as in the normal mode of propaga-
tion. Since no reduction has taken place into male- and
female-producing individuals, it would seem possible that
such a plant might. produce either or both sexes. Such
is the case, for when the 2N moss plant produces its
‘‘flowers’’ some contain archegonia, others spermato-
gonia (with their contained germ-cells) and other flowers
contain both. The hermaphroditism here produced would
seem to be the sum of both the contrasted elements. The
expectation from such a 2N plant would be that its germ-
cells (2N) would produce a 4N sporophyte—unfortunately
the plants proved sterile. Imperfect germ-cells were
present incapable of fertilizing or of being fertilized,
so that it was not possible to perpetuate the 2N plant by
sexual reproduction.
The results with the 2N plants derived from the regen-
erating sporophyte of the hermaphroditic species (Fig.
61) is different in one important respect. When, as
before, a diploid (2N) plant is obtained by regeneration
from the sporophyte it produces hermaphroditic flowers,
i.e., flowers containing both odgonia and spermatogonia,
and these are fertile. The sporophyte that they produce
is tetraploid (4N), due to the union of a diploid anther-
ozooid with diploid egg. Regeneration from the tetraploid
sporophyte (4N) should produce fertile gametes, which
might give rise by their union to an octoploid sporophyte
(8N). So far the Maréchals have not been able to produce
such plants, for although in a few cases the 4N sporophyte
regenerated it failed to produce flowers.
The difference then between the results from mosses
with separate sexes and mosses that are hermaphrodite is
that the 2N plant of a race with separate sexes does not
form normal gametes, while a 2N plant of hermaphroditic
races forms fertile gametes. It may appear more or less
plausible that the failure of the former is due to failure
in the reduction of the spores into two alternative types,
VARIATION OF CHROMOSOMES 155
while in the latter case, since there are presumably no
such types found, there is no conflict. Some other dif-
ference would have to be appealed to to explain why the
octoploid forms fail to develop.
A triploid condition (3N) has been found to occur in
certain types of the evening primrose (Stomps, Lutz,
Gates). De Vries has found in crosses in which Lamarck-
tana was the mother and some other species (muricata,
cruciata, etc.), the father, that triploid types appear three
times in 1000 cases. He interprets the results to mean
that three in 1000 times the egg-cell of Lamarckiana has
the double number of chromosomes (14), which being fer-
tilized by a normal pollen grain with seven chromosomes,
gives the triploid number, viz., twenty-one chromosomes.
The same result would be reached if a diploid pollen grain
fertilized a normal egg. That such pollen grains appear
is as probable a priori as that diploid eggs occur. It
may be recalled that one explanation of the tetraploid
evening primrose (gigas) is that it arises from a 2N pollen
grain meeting a 2N egg-cell. How reduction takes place
in the triploid cenotheras is uncertain, since the accounts
of the process are different. Geerts states that, as a rule,
only seven chromosomes conjugate (7+ 7), while the
remaining seven chromosomes are irregularly distrib-
uted in the dividing germ-cells. On the other hand, Gates
finds in a 21-chromosome type that the chromosomes
separate into groups of 10 and 11, or occasionally into
9 and 12. The former account fits in better with results
of the same kind obtained by others, and is more easily
understood from a general point of view, because seven
homologous pairs would correspond to the normal conju-
gation, while the seven chromosomes left over would have
no mates and fail to divide at the reduction division, hence
their erratic distribution.
It has also been shown in Ginothera that there are
three 15-chromosome types. If the 15th chromosome is
156 PHYSICAL BASIS OF HEREDITY
sometimes one, sometimes another chromosome, there may
be genetically several types, but as yet evidence on this
point is lacking.
Irregularities in the germ-cells of Enothera have been
observed by Gates of such a kind that one cell gets 6, the
Fia. 62.—Somatic chromosomes groups of @nothera scintillans, showing variable numbers
of chromosomes. (After Hance.)
other 8 chromosomes. A pollen grain with 8 chromo-
somes fertilizing an egg with 7 would give a 15-chromo-
some type. When such a 15-chromosome plant forms its
egg-cells the supernumerary chromosome having no mate
may go to either pole of the spindle, hence eggs of two
VARIATION OF CHROMOSOMES 157
sorts would result, viz., 7- and 8-chromosome cells.2 Such
a plant if crossed to a normal plant should give half nor-
mal (14), half 15-chromosome types. Such plants have
been shown, in fact, to be produced (Lutz). Other com-
binations that would give 22, 23, 27, 29 chromosomes have
been reported.
A variation in the number of the chromosomes of a
somewhat different kind has been described by Hance for
Gnothera scintillans, one of the 15-chromosome types of
O. Lamarckiana. No variation in number was found in
the germ-tract of the same individuals that consistently
gave two types of pollen grains, one with 7 and the other
with 8 chromosomes. The number of chromosomes in the
somatic cells was found to vary from 15 to 21. Some of
the groups are shown in Fig. 62. When the 15 chromo-
somes of the type-group are measured, it is found that
they can be arranged in respect to length in 7 pairs, with
one odd one (marked a in the figures). There is also
found a constant length difference between the pairs. In
those cases where there are more than 15 chromosomes in
a cell, measurements show that the pieces can be assigned
to particular chromosomes. When this is done, Fig. 63, the
lengths of the chromosomes come out as in the typical
cells. There can be no doubt that the extra chromosomes
in these cases represent pieces that have broken off from
typical chromosomes. This process of fragmentation
does not destroy the ‘‘individuality of the chromosomes’’
since the increase in this way of the number of chromo-
somes would not lead to any immediate change in the
number of the genes. The peculiarity of the mutant O.
scintillans is not connected with the increase in the number
of its chromosome bodies, but rather to the presence of a
15th chromosome.
Bridges has called attention to a peculiar case in
Drosophila (1917) in which an individual behaves as
*No pollen is produced by most of the lata plants.
158 PHYSICAL BASIS OF HEREDITY
though a piece of the X-chromosome (recognizable from
its genes that normally lie in the’ middle of the chromo-
some) had become attached to one end of the other X-chro-
mosome. Owing to this piece (including the region that
contains the normal allelomorphs of vermilion and sable)
the individuals give unexpected results in relation to domi-
nance or recessiveness of certain factors. For example,
ABC
ABCDEPGHISKLMNO BBR AGES SRC MONO
(Te sat
BCOEFGHTIRULMNO Weltae.
a
ef? n
Bp J wy
o
3
Wi ki
GA fii
Fie. 63.—Scheme showing the probable relation between the éxtra cooks pieces of
Fig. 62, and the normal 15 chromosomes of this mutant. (After Hanse.)
a male that contains the recessive genes for vermilion and
for sable, normally located, and having attached to this
chromosome the duplicated piece (containing the normal
allelomorphs of vermilion and sable) is in appearance a
wild-type fly, instead of being vermilion sable as it would
otherwise be without the piece. On the other hand, a
female having one such chromosome and a normal ver-
milion sable chromosome is in appearance not wild type
VARIATION OF CHROMOSOMES 159
(as might have been expected), but shows vermilion and
sable, because in this case the two recessive genes for
vermilion and for sable dominate the single normal allelo-
morphs. But a female having two such duplicated chro-
mosomes (1.e., tetraploid for the genes of certain regions
of the sex-chromosome) is now wild type in appearance,
because the two dominants dominate the two recessives.
Such a female crossed to a vermilion sable male gives wild-
type sons and vermilion sable daughters, which is criss-
cross inheritance in an opposite sense from that ordinarily
met with in Drosophila.
A second instance discovered by Bridges, but not yet
reported, seems best explained on the assumption that a
piece taken from the second chromosome has become
attached to the middle of the third chromosome. This
condition makes possible the linkage of mutant characters
to genes in both the second and the third chromosome at
the same time. The second chromosome that lost a piece,
and the third chromosome that gained the piece (both were
of course in the same cell), have been easily kept together
in the same stock ever since, because in those cases where
they become separated through assortment every zygote
that receives the deficient (2nd) chromosome dies unless
the same zygote has received the third chromosome with
the duplicate piece.
The preceding results show that chromosomes may
not only gain genes by the attachment of pieces
(duplication), but also that chromosomes may lose
pieces (deficiency). :
Other instances of deficiency have been reported by
Bridges which can be explained either as total losses of
certain regions, or due to their inactivation. Unless the
lost pieces happen to have been retained as in the last
case, the distinction between these possibilities is difficult.
A study of one case has shown that no crossing over takes
place in the region of deficiency, although the rest of the
chromosome was little or not at all affected. As a result
160 PHYSICAL BASIS OF HEREDITY
the chromosome is ‘‘shortened’’ by an amount correspond.
ing to the ‘‘length’’ of the deficient region.
It is not without interest to notice that in the first
case the duplicating piece is attached to that end of the
first chromosome where the spindle fibre is attached. In
the other case the duplicating piece is attached to the
2
Fig. 64.—An egg of Ascaris bivalens fertilized by sperm of A. univalens, a; later stage
of same, b.
middle of the third chromosome, and in this chromosome
the spindle fibre is attached to the middle.
An interesting case of triploidy has been reported in
the threadworm Ascaris (Boveri). Two varieties occur,
one with four chromosomes (haploid two), and one with
two (haploid one). Rarely a female of one variety is
@ De
ee0,.% Ree
@ 4,2, ‘ Dal ev
6%
“es. <0 2
Fie. 65.—Diploid and haploid groups of the sundew Drosera. (After Rosenberg.)
found that has mated with a male of the other variety.
The fertilized eggs have each three chromosomes (Fig.
64). As yet no triploid adults have been met with, so that
the method of conjugation of the chromosomes in the
triploid types is not known.
Rosenberg crossed two species of sundew, Drosera
longifolia, with 40 chromosomes (haploid 20), and D.
rotundifolia, with 20 chromosomes (haploid 10), Fig. 65.
VARIATION OF CHROMOSOMES 161
The hybrid had 30 chromosomes (20-+10). He found that
when this hybrid produces its germ-cells they show, after
reduction, 20 chromosomes, which he interprets as due to
10 of the rotundifolia conjugating with 10 of the longi-
folia. This leaves 10 without mates. At the following
maturation division Rosenberg describes the 10 paired
chromosomes as reducing, sending one member of each
dyad to one pole, the other member to the other; but the
Conjug ation. Reduction. Gamete.
@e
Egg. Sperm.
-O
© 0?
Fig. 66.—A scheme illustrating the fertilization of the egg of one species of moth by
the sperm of another, with reduction in I, with no reduction in II, and with partial reduc-
tion in III. :
10 unpaired chromosomes are irregularly distributed at
this division. If the account is confirmed, the situation
is peculiar, for if the 20 (haploid) chromosomes of longi-
folia correspond to the 10 (haploid) of rotundifolia it is
not obvious why all 20 might not find a place alongside
of the 10, unless chance or some difference of length, etc.,
makes this impossible. This assumes, however, that longi-
folia is not tetraploid—if it is, then a further question
arises as to which chromosomes of each set of three would
be the ones most likely to conjugate, ete.
Crosses between three species of the moth Pygera,
11
162 PHYSICAL BASIS OF HEREDITY
having different chromosomes, were made by Federley.
The hybrids showed intermixed characters of both
parents, and their chromosome number was the sum of
the haploid numbers of their parents (Fig. 66).
No reduction in number of the chromosomes takes
place in the hybrid at the synaptic stage (except perhaps
for one or two small ones), so that the 1st spermatocytes
contain nearly the sum of the haploid number of the
Egg. Sperm. Zygote. Conjugation. Reduction. Gamete.
0 80\ @ le
C:) 08 @0
FLXF, are) Meda ro)
N
Fig. 67.—Scheme illustrating the history of the chromosomes, and the back-cross between a
hybrid male and one or the other parent; also between two such hybrid F; individuals.
parents (A and B) after division of each chromosome
(Fig. 67). A second maturation division follows in which
each chromosome again divides. As a result each sperm
contains the full number of chromosomes, half paternal,
half maternal (A and B). The hybrid female is sterile,
but the male is fertile. If he is back-crossed to a female
of the A race his sperm, carrying both sets of chromo-
somes, will produce a 3N individual, d+ B+ A. It will
have two sets of the A genes to one set of B. In appear-
ance the moth is practically the same as the F, hybrid,
because both contain both sets of chromosomes—the
VARIATION OF CHROMOSOMES 163
double set 4A with B not producing any striking differ-
ence from the single set A+B. When this second hybrid
(3N) matures its germ-cells, the two homologous series
(A + A) mate with each other, and then segregate at the
first division, while the unmated B-series simply divides.
At the second division both the A- and the B-series divide,
thus giving to each sperm a haploid set of chromosomes
(4-+B). The sperm then is the same as the sperm of
the first hybrid. So long as the back-crossing continues
the outcome is expected to be the same.
If, instead of back-crossing the first hybrid to parent
A, it is back-crossed to parent B, the same result as
before takes place, except that the second hybrid is now
A+B+B. When it matures its germ-cells, the B’s
unite and then separate, giving AB sperm as before.
Here then we find a kind of inheritance that super-
ficially appears to contradict the generality of Mendel’s
law of segregation. On the contrary, a knowledge of the
chromosomal behavior shows that the results are different
because the mechanism of conjugation of the chromosomes
is changed, and changed moreover in such a way that on
the chromosome theory itself the results are what are to
be expected.
These crosses are so important that some further
details may be added. The whole (2N) and half (1N)
number of chromosomes of the three species studied by
Federley are as follows:
Whole Half
Pygaera anachoreta.............-.----- 60 30
Pygaera curtula ............-.2--+ ee eee 58 29
Pygaera pigra........ 66. eee e eee eee 46 23
In the hybrid between the first two species the number of
spermatocyte chromosomes was found to be 59 (30+ 29).
No union between any of the maternal and paternal chro-
mosomes could have taken place. But in the hybrid
formed by the union of the two more nearly related spe-
cies, curtula and pigra, the number of spermatocyte chro-
164 PHYSICAL BASIS OF HEREDITY
mosomes was found to be as a rule somewhat smaller than
the sum of the parental haploid numbers, indicating that
one or more had conjugated. To the extent to which such
union, and the consequent reduction, takes place, the
‘characters of the second hybrid generation may differ
from those of the first—at least if the conjugating pairs
have different factors in them.
A similar behavior of the chromosomes has been
described by Doncaster and Harrison for two species of
moth of the genus Biston (Fig. 24). The hybrids were
sterile, and no further generations were raised.
Federley later made similar crosses with three other
moths. Zw
1 Barredé Barred 9
Z? PP Pw Pw
Barredd Barredd Barredg Black 9
Fia. 77,—Scheme showing the transmission of the sex-linked characters B =barred, and
b =black in the cross shown in Fig. 76.
daughter cell gets both Z’s (8+ 2). This cell then divides ©
again, the Z’s presumably separating so that two second
spermatocytes are produced, each with 9 chromosomes
(8+1), including the Z. These become the functional
sperm. The other spermatocyte, the one without a Z,
may divide again, but it, or its products, degenerate, and
never produce sperm. According to Guyer, there are 17
chromosomes in the female, including one Z. Presum-
ably, then, after reduction half of the eggs will contain a
Z (8 +1), the other half will be without it (8). The egg
that carries a Z (8 + 1), fertilized by a sperm (each sperm
earries a Z (8+ 1)), will make a male with 18 chromo-
Fic. 76.—Cross between Barred Plymouth Rock male and Black Langshan female.
Fig. 78.—Cross between Black Langshan male and Barred Plymouth Rock female.
SEX-CHROMOSOMES AND INHERITANCE 179
somes, including two Z’s. The egg that lacks a Z (8), fer-
tilized by a sperm (8 + 1), makes a female with 17 chro-
mosomes, including one Z.
__ This scheme gives consistent results for sex-linked
inheritance in birds. Since the daughter gets her single
Z-chromosome from her father, she will show any sex-
linked characters carried by his Z-chromosome. If the
father carries a sex-linked dominant gene his sons and his
daughters will be alike. It should be noticed that while
Guyer’s scheme gives the same results so far as sex-link-
Black& Barred 2
: Se
22° Ow
Barredd —_— Black ce)
ZZ ZF Zw 2w
Barredd Black & Barredg Blackg
Fig. 79.—Scheme showing the transmission of the sex-linked characters B =barred, and.
b =black in the cross shown in Fig. 78.
age is concerned, as the one described by Seiler for some
moths, the machinery in the male is different in the two
cases, while that in the female is presumably the same. In
both the female is heterozygous for Z; in the moth the
male is homozygous (ZZ), but in the bird the two Z’s
described by Guyer both go to one pole at one of the
maturation divisions, and reduce at the other—a proce-
dure not known in any other animal.
In the reciprocal cross (Fig. 78) a black cock is bred
toa barred hen. The sons are barred—like their mother—
the daughters are black—like their father, criss-cross
inheritance. When the barred F, cock and the black hen
180 PHYSICAL BASIS OF HEREDITY
are inbred, there are four F, classes with sex taken into
account in the proportion of 1:1:1:1; or ignoring sex,
1 barred to 1 black. The barred and the black races
differ by one factor difference (Fig. 79), viz., barred Z*
and its normal recessive allelomorph Z>. This seems to
mean that the Barred Plymouth Rocks is a black race
with an additional dominant factor for barring. The
Black Langshan is the same black race but without the
barring factor.
Until quite recently no cases of crossing over had been
observed in forms having the Abraxas type of sex-linked
inheritance, for, except in one or two cases in poultry,
only a single pair of sex-linked genes were known, and two
at least must be studied together in order to demonstrate
linkage. Goodale has recently studied two sex-linked
characters in poultry, and states that crossing over occurs
in the male, but whether or not in the female is not stated.
SEX-DETERMINATION AND NaTuraL PaRTHENOGENESIS
Variations in the ordinary sex-determining mechanism
account in some cases for the normal output of males and
females produced by parthenogenesis, and determine the
exceptional sex-ratios of such species. The honey bee
furnishes the best known example. The queen comes
from a fertilized egg, and has therefore the double (2)
number of chromosomes. Her eggs give off two polar
bodies, hence have the reduced, or single number of
chromosomes. Any egg that is not fertilized develops
parthenogenetically into a male. If there are two X-chro-
mosomes in the bee, as in some of the other insects, the
egg is expected to contain only one of them after the
extrusion of the polar bodies. Hence, if it develops with-
out doubling its chromosomes, it should give rise to a
male. That the male has the single number of chromo-
somes is also borne out by the evidence from a peculiarity
of the first spermatocyte division in which the cytoplasm
divides, but the chromosomes do not separate into two
SEX-CHROMOSOMES AND INHERITANCE 181
groups. Several stages in the maturation of the sperma-
tozodn of the bee are shown in Fig. 80. In a, the spindle
for the first spermatocyte division has appeared. A small
piece of the cytoplasm cuts off, but the chromosomes do
not separate, and they return again (b and ¢) to a resting
f
Fie. 80.—First spermatocyte divisions a-e, and the second spermatocyte division d-g
in the bee. (After Meves.)
stage. Another spindle forms (d), and the chromosomes
separate into two groups, one of which is pinched off
as a rudimentary cell that never becomes a spermatozoon.
Hence only one, and not four spermatozoa as in ordi-
nary cases, is formed from each spermatocyte. In the
hornet (Fig. 81), the spermatogenesis is Similar to that of
the bee in that the first divisionis abortive. It is different
182 PHYSICAL BASIS OF HEREDITY
Fig. 81.—First spermatocyte division a-c, and the second spermatocyte division d-f in
the hornet. (After Meves.) 5
in that the second division produces two functional
sperms, both female producing.
Since the male comes from an unfertilized egg, the
Fic. 82.—Life cycle of Phylloxera caryecaulis.
SEX-CHROMOSOMES AND INHERITANCE 183
queen must transmit to him all her characters, thus giving
rise to a form of inheritance that has a superficial resem-
blance to sex-linked inheritance. A queen of a pure race,
bred to a male of another race with a dominant factor,
produces daughters all showing the dominant’ character
of the father, and sons all showing the recessive character
of the mother. Since the son gets his entire chromosome-
complex from his mother, he must necessarily be like her,
whether the character in question is in the sex-chromo-
some, or in some other one.
Fie. 83.—Extrusion of the gee! body from a male-producing egg with lagging chro-
mosomes on the spindle, a; and extrusion of the polar body from a female-producing
egg,b; in Phyllozera.
In the phylloxerans there are two parthenogenetic
generations followed by a sexual one (Fig. 82). In the sec-
ond parthenogenetic generation two whole chromosomes
leave certain eggs (Fig. 83) passing into the single polar
body which is given off from the egg. Such eggs have two
less sex-chromosomes and develop parthenogenetically
into males. In other eggs of the same generation all four
sex-chromosomes are retained after the polar body is
produced. These eggs also develop parthenogenetically,
but produce females. Similar changes take place no doubt
in the aphids, for the males have been shown to have one
less chromosome than the female, although the loss of one
184 PHYSICAL BASIS OF HEREDITY
chromosome in the polar body has not yet been observed
in the group.
In both phylloxerans and aphids there are two classes
of sperm produced in the males as in other insects, one
with X, one without it. The latter degenerates, and only
the X or female-producing sperm remains functional. A
few stages in the spermatogenesis of the bearberry aphid
Fic. 84.—First and second spermatocyte division in the bearberry aphid with the formation
of one rudimentary cell.
are shown in Fig. 84, a-g. In b, the chromosomes have
divided and moved to opposite poles while the sex-chromo-
some is drawn out but has not moved yet to either pole.
In c, the sex-chromosome has been drawn into the.larger
of the two cells that is produced. Ind, the division into a
larger and a smaller cellis completed. In e, preparations
for another division are taking place in the larger cell, and
in f and g this is completed. The smaller cell does not
divide, and later degenerates. The two spermatozoa from
SEX-CHROMOSOMES AND INHERITANCE 185
the two larger cells each contain one X-chromosome and
two autosomes. They correspond obviously to the female-
producing sperm of other insects. Hence only females
arise from fertilized eggs.
The rotifers, especially Hydatina senta, are the only
animals in which the transition from parthenogenetic to
sexual reproduction has so far been gotten under con-
trol by regulating the environment, and although the
evidence that the environment causes part of its effects by
influencing the chromosomal mechanism is not yet estab-
lished, there is, in my opinion, some indication that such
is the case. The common method of reproduction in
Hydatina is as follows: A parthenogenetic female (Fig.
85, A) lays eggs (D), each-of which, after giving off a
single polar body, develops at once (i.e., without fertiliza-
tion) into a female like the mother. The whole number
of chromosomes is retained in the eggs. Several or many
generations may be produced in this way. Whitney has
shown that if such females are fed on a green alga,
Euglena, daughters appear (structurally like the others)
that produce smaller eggs (E). If these eggs develop
without fertilization they become males (C). Examina-
tion of these small eggs show that they give off two polar
bodies, and retain a reduced number of chromosomes. This
process is the same by which the male bee is produced.
If the female, that produces the small eggs just
described from which the males develop, should have been
impregnated by a male soon after she hatched, her eggs
would then grow larger and surround themselves with a
thick-walled coat. They become the winter or resting
eggs. Hach such egg, after the sperm enters, gives off two
polar bodies, reducing in this way the number of its chro-
mosomes. By the addition of the sperm nucleus the full
number of chromosomes is recovered.
Whitney has recently shown that there are two classes
of spermatozoa produced by the male, large and small;
186 PHYSICAL BASIS OF HEREDITY
for, owing to the few sperms produced by each male their
actual number can be counted. There are twice as many
large as small spermatozoa, if, as may be the case, only
the large ones contain chromosomes and are functional,
Fic. 85,—Hydatina senta, adult female, A; young female soon after batching, B; adult
male, C; parthenogenetic egg, D; male-producing egg, E; resting egg, F. (After Whitney.)
the conditions here would appear to be like those in the
hornet, provided there are no chromosomes in the small
spermatozoa. This would also explain why all fertilized
eggs produce females.
So long as the ordinary parthenogenetic females are
fed on the poor diet of Polytoma, they continue to produce
SEX-CHROMOSOMES AND INHERITANCE 187
parthenogenetic females like themselves (Fig. 86), and
this non-sexual process continues indefinitely. If on the
contrary, parthenogenetic females are fed abundantly on a
rich diet of the green alga Euglena, their eggs develop into
individuals which, if early fertilized as explained above,
become sexual females, i.e., they lay fertilized eggs, but if
not fertilized, produce small eggs that, developing par-
thenogenetically, become males. In other words, the same
female becomes either a sexual female, or a female that
inn
LT
PERCENTAGE OF MALE-PRODUCING FEMALES
PPPPPPPPPPPPPPPEPPPPPPPPPPPPPPPPPPPO PP
22 Months 9-4days
Fie. 86.—Diagram showing how a continuous diet of Polytoma (P-P) through twenty-
two months yielded only female-producing females, but when the diet was suddenly changed
to Chlamydomonas (at C), male-producing females appeared at once. (After Whitney.)
gives birth to males. Some recent writers, misunderstand-
ing these relations, have tried to make it appear that the
change here is one that is sex-determining, using this
expression to all appearances as it is ordinarily employed
in other cases, but in fact using the term in such a way
as to obscure the one important fact that the results really
show, viz., that an environmental change of a specific kind
produces a new kind of female that is either a producer
of eggs that become males (after or because two polar
bodies are extruded), or becomes a sexual female, should
she early meet a male.
188 PHYSICAL BASIS OF HEREDITY
SEX-DETERMINATION AND ARTIFICIAL PaRTHENOGENESIS
Many interesting questions concerning sex-determina-
tion might be studied were it as easy for man, as it appears
to be for nature, to make eggs develop without fertiliza-
tion. Only three cases are known in which eggs developing
under artificially induced conditions have reached matur-
ity. Delage raised one sea urchin that had been produced
artificially to maturity, and determined that it was a male.
Tennent has shown that the male is heterozygous for the
sex-chromosomes. Hence, if the artificially produced
urchin has the half number of chromosomes it should, if
like the bee, be a male, but if, as Herlandt has shown, the
number of chromosomes may double before development,
a female would be expected.
In the frog, Hertwig, and later his pupil Kuschake-
witch, found that the number of males is increased up to
100 per cent. if the eggs are detained in the uterus for
one to three days before adding sperm to them. Hertwig
has attempted to explain the result as due to a relative
change in the size of the nucleus that takes place in conse-
quence of the delay, but since the chromosomes are at this
time in the metaphase of the second polar spindle, it is not
obvious how such an enlargement could be brought about,
quite aside from the question as to whether the result
imagined would follow even after such a change. I have
suggested that these eggs with deferred fertilization may
develop parthenogenetically, due either to the egg nucleus
alone giving rise to the nuclei of the embryo, or to the
sperm alone giving rise to these nuclei, in the latter case,
the polar spindle of the egg having been caught at the sur-
face and prevented from taking part in the development.
The possibility of the nuclei of the frog arising in one
or the other of these ways is shown by the work of Oscar
and Gunther Hertwig who have found evidence that after
treatment with radium, the sperm-nucleus alone may give
rise to the somatic nuclei of the embryo. Packard also
SEX-CHROMOSOMES AND INHERITANCE 189
has shown that such kinds of androgenetic embryos
may arise in the eggs of Chetopterus treated with radium,
and by following every stage in the process he has
determined also that the embryos have the reduced
number of chromosomes.
Other work on the egg of the sea-urchin had seemed
to show that while in most cases the egg, that begins to
develop parthenogenetically, starts with, and continues
to maintain the half number of chromosomes, yet accord-
ing to a recent observation of Brachet, a parthenogenetic
tadpole, eighteen days old, that he produced, had the
double number of chromosomes. Whether it may turn out
that when the egg nucleus gives rise to the nuclei of the
parthenogenetic individual it may sometimes double its
number of chromosomes (by failure of the first cytoplas-
mic division, for example), and that when a sperm gives
rise to these nuclei the half number is retained, cannot be
stated. Until we have farther information on these points
the expectation as to what the sex of parthenogenetically
produced frog individuals will be can only be speculative.
Loeb has raised seventeen adult, or nearly adult male
frogs and three nearly adult female frogs from eggs devel-
oping after Bataillon’s puncture method of inducing par-
thenogenesis. One male frog had more than the half num-
ber of chromosomes (at least 20 and presumably the
whole number, 26?). The number of chromosomes in the
females was not determined.
GYNANDROMORPHS AND SEX
In the group of insects especially, it has long been
known that individuals occasionally appear that are part
male, part female. In the most striking cases the line
of division runs down the middle of the body, but there
are also antero-posterior gynandromorphs, and individ-
uals with only a quadrant or even a small piece of the body
different from the rest in its sex character. Several
hypotheses have been advanced to explain these rare com-
190 PHYSICAL BASIS OF HEREDITY
binations of the two sexes, and it is probable that gynan-
dromorphs may arise in more than one way, but in Droso-
phila it can be demonstrated that the great majority of
gynandromorphs result from dropping out of one of the
sex-chromosomes at some early division of the fertilized
egg. The demonstration is made possible by using sex-
linked characters that are known to be carried by the sex-
chromosomes. For example: Yellow body color in Droso-
phila is due to a recessive gene carried by the X-chromo-
some. Its allelomorph (wild type) lies also, of course, in
the normal X-chromosome. If yellow is crossed to wild,
and a bilateral gynandromorph should arise, it may be
yellow on the male side (as seen in the yellow wings and
yellow hairs over half the body) and wild type on the
female side (Fig. 87).
Since the male characters arise when only one sex-
chromosome is present, it must be the yellow-bearing
chromosome in this case that gives the male side. Since
the female characters arise when two X’s are present,
both must be present in the female side, which will here
be the wild type, since the gene for wild type domi-
nates the yellow-producing gene. The gynandromorph
must have arisen, therefore, at a very early nuclear divi-
sion in the egg in which one daughter X-chromosome failed
to pass into one of the daughter nuclei. The diagram
(Fig. 88) shows how such a result might be supposed to
have come about.
The diagram indicates that one daughter chromosome
X’ (bearing the gray gene) has failed to become incor-
porated in its proper nucleus, which is therefore left with
only one X. rom this nucleus the nuclei of the male half
are produced, while from the XX nucleus the nuclei of the
female half arise. That both of these nuclei, the XX and
the X nucleus contain other chromosomes derived from
both parents has been shown by making one of the original
parents homozygous for some recognizable autosomal
_, Fie, 87 A.—Gynandromorph, Left side of thorax and abdomen and left wing and legs are yellow in color and male.
Right side of thorax and abdomen and wing and legs are gray and female. All of head is gray and female with “white-
eosin-compound”’ eyes, Genitalia are female. The mother of this gynandromorph was a ‘‘white-eosin-compound,” i. e.,
she had one X-chromosome with an eosin gene and one with a white gene. The father was yellow white, i.e., his single
X-chromosome carried the genes for yellow and for white. Elimination consisted in the loss of one of the maternally
derived chromosomes, viz., that one bearing eosin, leaving only the yellow white chromosome to produce the male side
of the thorax and abdomen. aie 2
Fic. 87 B.—Gynandromorph, | Left side is female, Left eye red and wing long, like that of a normal female. Right
side, except abdomen, is male with eosin eve and miniature wing. Sex comb on right fore-leg only. The mother of this
gynandromorph had one vermilian-bearing X-chromosome; and another X-bearing eosin and miniature genes; the father
carried an X-chromosome bearing eosin and miniature.
SEX-CHROMOSOMES AND INHERITANCE 191
character. It, or its normal allelomorph, should therefore
be present in both nuclei if all the chromosomes of the
fertilized egg have divided normally except the X-chromo-
somes. This, in fact, has been found to be the case (Mor-
gan, Bridges, Sturtevant).
Nearly all of the many hybrid gynandromorphs of
Drosophila can be explained as above. In a few cases,
when the abdomen of the fly was sufficiently female to
make mating possible, it has been found that the eggs give
the results expected for a female having the sex-linked
factors that entered the cross.
Fig. 88.—Diagram showing elimination of X’ at an early cell-division, so that the nucleus
to the right gets X and X’ and that to the left only X.
In a few cases in Drosophila the explanation of chro-
mosomal dislocation will not cover the results. Some of
these cases can, however, be accounted for by another
hypothesis. Should an egg arise with two nuclei (there
are several possible ways for this to occur), one nucleus
having one set of factors, the other the other set (the
parent being heterozygous), then if each nucleus is sepa-
rately fertilized a different combination of factors is pos-
sible from that possible on the elimination theory. A
gynandromorph, described by Toyama, appears to belong
to this category. Toyama found two gynandromorphs
of the silkworm (Fig. 89) whose mother belonged to a race
with banded caterpillars, and whose father belonged to a
192 PHYSICAL BASIS OF HEREDITY
race with pale caterpillars. One of these was banded on the
left side (which side was also female) and pale on the right
side (which was also male). The sex of the two sides was
only apparent after the moth had appeared. The banded
character of the worm is known to be dominant to the pale
character, but neither is sex-linked. The case can be
explained, if as the evidence indicates, the mother was
striped gynandromorph
Fia. 89.—Caterpillars of the silkworm moth. A striped one to the left, a plain one to
the right, a hybrid gynandromorph in the middle.
heterozygous for a not sex-linked character, banded, and
if she produced an egg with two nuclei (Fig. 90). Don-
caster has found such eggs in Abraxas, and has shown
that each nucleus extrudes separately polar bodies, and
that each reduced egg nucleus is fertilized by a separate
spermatozoon. If as shown in the next diagram one
reduced nucleus has a W-chromosome, and a factor for
banded carried in one of the autosomes, and the other
reduced nucleus has a Z-chromosome, and in one of the
SEX-CHROMOSOMES AND INHERITANCE 193
autosomes a factor for pale, and if a spermatozoén, carry-
ing the factor for pale, fertilizes each nucleus, the two
zygotic nuclei will be ZW female and banded, and ZZ
male and pale. This gives at least a formal explanation
of the results, and helps us to see how such a rare event,
the appearance of two gynandromorphs in the same brood,
happened to occur at the same time; because, as Doncas-
ter’s evidence shows, a double nuclear condition may be
characteristic of the eggs of certain females.
Fie. 90.—Diagram illustrating how a heterozygous egg with two nuclei fertilized by two
sperms might produce a gynandromorph like that shown in Fig. 89.
‘In TERsEXES’’ anD Sex GENES
The quantitative relation of one X for male and two
X’s for female that has been found to hold in many of the
groups of animals might seem from a purely a priori
point of view capable of being modified in such a way that
an intermediate condition might be realized, but whether
such conditions should be expected to give rise to her-
maphrodites or to non-sex-somethings (intermediates) —
or to a mosaic of both sexes, or should rather be expected
to die could scarcely be foretold. There are three cases
in which individuals called ‘‘intersexes’’ have been found,
or produced; and since their interpretation has led to a
view that has appeared to contradict the ordinary sex-
determination scheme, these cases must be briefly referred
to here. Goldschmidt has studied very thoroughly ‘‘inter-
13
194 PHYSICAL BASIS OF HEREDITY
sexes’’ that arise when the European and Japanese race
of gypsy moths, Lymantria dispar and L. japonica, are
crossed. Riddle has described doves obtained by crossing
the white ring dove (Streptopelia alba) and the Japanese
turtle dove (Turtur orientalis) that are intersexual in
their mating habits. Olga Kuttner and Banta have found
that certain lines of Cladocerans (Simocephalus) may
produce (parthenogenetically) ‘‘intersexual individuals’’
in the sense that an individual may possess some of
the secondary sexual differences of one sex and some
of the other.
Some of Goldschmidt’s combinations between different
races of gypsy moth produce only intersexual females, 2.e.,
individuals that are mostly female, but have also, in spots,
male characters. In the most extreme cases they are
almost like males, not only in color, but even in the partial
production of testes. Other racial combinations give male
intersexes, 7.¢c., individuals that are for the most part
males, but show, in spots, some of the characteristics of
the female. Goldschmidt explains these results by the
assumption that the sex factors have different quantita-
tive values in the different races. He represents the
female by FF Mm, and the male by FFMM. If the FF ‘‘fac-
torial set’’ is represented by 80 units, and the ‘‘present”’
male factor, M, by 60 units, then the above formula for the
female becomes 80-60=—-+ 20, and the male formula
becomes 80—(60 + 60)—-40. In the former, female units
‘‘dominate,’’ in the latter, the male. Values like these
can be arbitrarily set for all the different races. For
instance, to the ‘‘weak’’ European race and the ‘‘strong’’
Japanese the following values are assigned:
Weak European Race Strong Japanese Race
Q FF Mm FF Mm
80, 60 100, 80
3d FF MM FF MM
80, 60,60 — 100, 80, 80
SEX-CHROMOSOMES AND INHERITANCE 195
If a Japanese female is crossed to a European male,
the F', female and male may be represented in the fol-
lowing formula:
F,9 FF Mm F, 9 FF MM
100, 60 100, 80, 60
Both ‘‘normal’’ female and male offspring are expected
in equal numbers. The reciprocal cross gives a different
result, vdz.:
80—(80-+60) 80, 80, 60
The F, female is FF -M =O; and is therefore repre-
sented as intersexual. It will be observed that the so-
called ‘‘female factors’’ in these formule are supposed
to be inherited entirely through the mother.
By assigning different values to FF and M in the dif-
ferent races it is possible to express the results in such
a way that the sexes obtained by various crosses have
different minimal values—those less or more than any
assigned value for a given sex are interpreted as inter-
sexes. In the example cited, an exact balance (=O)
between the conflicting factors produces an individual that
is represented as neither male nor female. It is not
obvious, however, why it should be made up of parts each
of which is strictly comparable to the same part in a male
or a female.
While the assignment of arbitrary values to sex fac-
tors is a legitimate procedure, it is not a quantitative
analysis in the ordinary sense, since the quantities
are not referred to some external measure, but are
purely arbitrary.
How far an erratic elimination of sex-chromosomes
in later stages of cell-division might account for the result.
cannot be stated, since there are at present no facts to
go upon—the chromosome count in somatic cells of the
hybrid has not yet been reported, but Goldschmidt thinks
196 PHYSICAL BASIS OF HEREDITY
that the mode of development of the embryo precludes
this interpretation.
Riddle obtained his intersexual hybrids by causing
their mother to produce many more eggs than she would
ordinarily produce. This was done by removing the eggs
from the nest as soon as they were laid. Towards the end
of a series obtained in this way an overworked female
produced an excess of males. Some of these males Riddle
regards as females that have been changed into males—
the completeness of the change being shown in their sexual
behavior towards other males, etc. But there is involved
in the cross a sex-linked factor that behaves, as R. M.
Strong had already shown several years ago, as do sex-
linked factors in other birds. It is thus possible to identify
the chromosoma] make-up of Riddle’s intersexual hybrids.
His own results show that the hybrids have the expected
combination of chromosomes for males. It appears, there-
fore, that whatever it may be that affects their behavior
their sex is determined by their possessing the ordinary
chromosome constitution for males.
HERMAPHRODITISM AND SEX
As has been shown, the sex-mechanism, whether XX-
XY or WZ-ZZ, gives rise to two kinds of individuals—
males and females. There are, however, many groups and
species of animals where both eggs and sperm are found
within the same individuals, and in typical cases there are
in such individuals special] ducts that are outlets for the
male germ-cells and others for the female germ-cells. In
these hermaphrodites ‘‘sex-chromosomes’’ are not known
to be present, or if present as in Ascaris nigrovenosa, they
act as sex determinants only in alternate generations.
The usual interpretation of the determination of the
sex-cells of hermaphrodites is that their differentiation
is determined by the same kind of specific influences that
determine, for example, that certain cells of the primitive
gut develop into liver cells, others into lung cells, still
SEX-CHROMOSOMES AND INHERITANCE 197
others into pancreas cells, etc. There is nothing inconsist-
ent in such a view with the theory that in other cases a
different mechanism produces different kinds of germ-
cells. Logically, this viewpoint is consistent, but I can
sympathize with efforts that are continually being made to
find an explanation that makes use of the same kind of
process in genetic segregation and in embryonic differen-
tiation. In fact, in 1902, while still under the influence of
the then recent advances. in the field of experimental em-
bryology (developmental mechanics), I suggested that one
might attempt to treat the phenomenon of segregation
from the same theoretical standpoint (viz., the realization
of alternative states) as was then appealed to for embry-
onic differentiation. It soon became apparent to me, how-
ever, that (1) the two kinds of results depended upon
entirely different situations, and therefore need not have a
common explanation; (2) that the genetic evidence showed
the improbability of explaining segregation and differ-
entiation in the same way; (3) that special tests that I
carried out failed to support the supposition of a common
explanation; (4) that while no detailed explanation is
possible at present for the general phenomena of specific
differentiation, yet for Mendelian segregation the reduc-
tion division supplies all that the results call for.
Sex Ratios
The theory of sex-determination has been deduced
from the evidence of equality of males and females as
well as from the cytological evidence. It remains to
explain why in some cases the machine fails to give
equality of the two sexes; why, for example, all fertilized
eggs of phylloxerans and aphids, or daphnians, or roti-
fers, or bees, are female; why certain mutant races of flies
give twice as many daughters as sons; why other races
of flies produce nearly all sons; why the sex ratio in man
is about 106 males to 100 females.
It is perhaps needless to point out that if, in a species
198 PHYSICAL BASIS OF HEREDITY
in which sex is determined by a chromosome mechanism,
it were possible to change the sex by other agencies in
spite of the chromosome arrangement, the latter relation
would be entirely thrown out of gear and males would
transmit sex-linked characters and sex itself like females,
and females like males. As no such cases have been
found, it is futile to discuss such a possibility.
It has been shown that only the female-producing
sperm in phylloxerans and aphids becomes functional,
hence it is obvious why all the fertilized eggs develop
into females. In daphnians and other crustacea it is not
known whether one class of spermatozoa degenerates, but
the results are explicable on such a view. In rotifers
the production of males only by certain females is due to
the eggs developing by parthenogenesis with the haploid
number of chromosomes and this explains also the case of
the bees, wasps and other hymenoptera. If a queen bee
is unfertilized or if her supply of sperm gives out she
produces only males. If she contains sperms, then any
egg that is fertilized produces a female, and as Petrunke-
witch showed several years ago, spermatozoa are to be
found in eggs laid in worker cells—such eggs being known
to produce workers (? °). In rotifers, too, the presence
of a large and a small class of sperm suggests that only
the former is functional.
Certain females of Drosophila give a sex ratio of two
females to one male. By making such a female hetero-
zygous as to her X-chromosomes (each carrying different
factors) it can be determined that the half of the expected
sons that die are the ones containing one of these two
chromosomes. It is easily possible by means of linked
genes to locate a factor in the sex-chromosome (Fig. 91)
and to show that whenever it goes to a male the fly dies.
All the daughters survive because the lethal factor being
recessive does not harm a female whose other chromo-
some comes from a normal father. The scheme is shown
on the next page.
SEX-CHROMOSOMES AND INHERITANCE 199
As many as 20 different lethals have been found in
the X-chromosomes of Drosophila. Their occurrence in
these chromosomes is first noticed by the appearance of
such exceptional sex ratios. Lethal factors like these
need not be thought of as different in kind from any
other mutant factors. They may mean only that the
changes that they cause are of such a kind, structural
or physiological, that the affected individual cannot
develop normally. Some of the lethals may be fatal in
g
K
X xX
y
Fia. 91.—Scheme showing the transmission of a lethal sex-linked factor in an X-chromosome
the black one in the diagram.
ee anes
the egg stages, others are known to cause the death of
the larve, others probably act on the pupx, and a few
even allow an affected male to occasionally come through.
In man and in several other mammals there is at birth
a slight excess of males over females. Since male babies
die oftener than females, the difference has been said to be
an ‘‘adaptation,’’ with the implication that it calls for no
further explanation. Several possible solutions suggest
themselves. The male-producing sperm bearing the sex-
chromosome may more frequently develop abnormally
than the female-producing sperm. Again, since the sper-
_ matozoa must, by their own activity, travel the entire
200 PHYSICAL BASIS OF HEREDITY
length of the oviduct to reach the egg as it enters the
tube, the greater size or weight of the female-producing
sperm may give a slight advantage to the male-produc-
ing sperm in the long trip up the tube. This would lead
to an excess of males. There are still other possibilities,
which if realized, would suffice to slirhtly change the equal-
ity of the output of the machine.
Non-DISJUNCTION
Females of Drosophila are occasionally found that
give exceptional breeding results which have been
explained by Bridges on the view that these females are
FEMALE MALE
AN IN
XXY FEMALE
Bice
Fria. 92.—Normal female and ae groups of chromosomes of the vinegar fly, with the
XY female group below.
XXY individuals (Fig. 92). It has been shown by cyto-
logical examination that such females do actually contain
an additional Y-chromosome. The four possible ways in
which these three chromosomes might be expected to
behave at the reduction division when the polar bodies
SEX-CHROMOSOMES AND INHERITANCE 201
are given off by the egg are shown in the next diagram
(Fig. 93). One X may go out of the egg, and the other X
and the Y stay in; or one X may stay in the egg and the
other X and the Y go out. In these two cases, X and X
may be thought of as members of a pair that conjugate, as
in the normal female, and then separate, and chance alone
determines whether the Y stays in or passes out. Again
¥ may go out of the egg and X and X stay in; or X and X
ms
ECGS
46 ay 46 .
Zon! °) 2K .°) XOX IRED ( XY
1 2 3 4
sit itiatoy Nemeth SORE Qin Sean he SSaB et thee chi
zation by an X-bearing sperm of the male is shown below.
go out and Y stay in. Here X and Y may be supposed to
be members of the conjugating pair, and the free X goes
to the same pole as the X that conjugated.
In the diagram, each of these four types of eggs is
represented as fertilized by an X-bearing sperm. In order
to make the outcome more apparent the original XXY
female may be supposed to have had white eyes (clear
X’s) and the male that fertilized her red eyes (here repre-
sented by the black X carrying the gene for red eyes).
Four classes of individuals are expected: (1) Red-eyed
females (X XY) ; (2) red-eyed females (XX) ; (3) red-eyed
202 PHYSICAL BASIS OF HEREDITY
females (XXX) that die, and (4) red-eyed males (XY).
The last are exceptional, since white-eyed females nor-
mally never produce anything but white-eyed sons. Here
the exceptional male is due to an egg without an X, being
fertilized by a ‘‘female-producing’’ (or X-bearing) sperm.
The three X individuals have never been found, and
undoubtedly die, presumably from too many X’s. The
remaining red females are of two kinds, one normal XX
a OO CA MY (mO
mw =6«OUMlllCOKSC‘N
N a
z
EGGS
~~
467 469 4% AZ
SPERM VY
vy ~
MYY kY Oy OV
WHITE do WHITE db WHITE @ (excerrion) DIES
5 6 7 8
Fria. 94.—Non-disjunction. In the upper part of the figure the four possible modes
of elimination of the sex-chromosome from the XXY eggs are shown, and the results of
their fertilization by a Y-bearing sperm of the male is shown below.
and the other (XXY), which is expected to repeat the
exceptional behavior of her mother. In fact, this is what
she does.
In the next diagram (Fig. 94) the fate of the same four
kinds of eggs is shown if they are fertilized by a Y-bearing
sperm. Four classes of individuals are expected (5) white
males (XYY); (6) white males (XY); (7) white females
(XXY); and (8) YY individuals. No individuals having
the last make-up have ever been found, and there can
be no doubt that an individual without at least one X dies.
The white-eyed females are exceptional, since white-eyed
SEX-CHROMOSOMES AND INHERITANCE 203
mothers by red-eyed fathers have normally only red-eyed
daughters. These exceptional white-eyed females (XXY)
must repeat the phenomena of non-disjunction, and it has
been found that they do so invariably. The white-eyed
male XY is normal; the other male should produce some
XY sperm and thus transmit both X and Y to some of his
daughters. Such daughters as get both X and Y from
the entering sperm should show non-disjunction. This
has been proven-to occur.
An analysis of the data has shown that two of the four
types of eggs are more common than the other two. As
indicated in both diagrams the types of eggs that result
after X and X have united occurs in 92 per cent. of the
cases, and since in this type the unmated Y has a random
distribution, the XY egg is found in 46 per cent. of cases
and the X egg in 46 per cent. The more uncommon type
of egg would be expected to result if X and Y united and
then separated while the other X had a random distribu-
tion! Eight per cent. of such cases occur, giving XX eggs
in 4 per cent., and Y eggs in the other 4 per cent. of cases.
These results not only furnish very strong proof of
the chromosome theory of sex, but serve also to show how
a knowledge of the actual mechanism involved leads to
the discovery of how a change in the mechanism gives a
new output. The conclusion that females behaving in
this way must contain a Y-chromosome was confirmed
by the cytological demonstration that showed in them two
X’s anda Y.
1 Since this was written it has been found that after XY synapsis the free
X always goes to the same pole as the synapsed X.
CHAPTER XV
PARTHENOGENESIS AND PURE LINES
In so far as parthenogenetic reproduction takes place
without reduction in number of the chromosomes, the
expectation for any character is that it will have the same
frequency distribution in successive generations, because
the chromosome group is identical in each generation.
There are a few cases where parthenogenetic inheritance
has been studied. The results conform to expectation.
The only difference between a species reproducing by
diploid parthenogenesis and one propagating vegetatively
is that in the latter a group of cells starts the new genera-
tion and in the former only one cell, viz., an egg, that no
longer undergoes reduction, or needs to be fertilized. In
both, the chromosome complex remains the same as in the
parent. Strictly analogous to the two foregoing methods
of propagation are the cases of sexual reproduction in a
homozygous group of individuals, composed of males and
females or in a group of hermaphroditic forms that are
homozygous. Successive generations are here also
expected to have the same frequency distribution, whether
selected or not, because they have the same germ-plasm.
Johannsen’s pure lines furnish an example of the last
case, for, in principle, pure lines, parthenogenetic repro-
duction, and vegetative propagation, are concerned with
nearly the same situation.
Johannsen worked with one of the garden beans
(Phaseolus vulgaris) taking the weight of the seeds, in
some cases, and measuring their sizes in other cases. It
is known that this bean regularly fertilizes itself. Asa
consequence of self-fertilization there is a tendency for
the descendants of any form to become in time homozy-
gous, even when heterozygous forms were present at first.
204
PARTHENOGENESIS AND PURE LINES 205
In fact, in a few generations perpetuated by self-fertiliza-
tion with chance elimination of individuals, a homozygous
race will result. This comes about as follows: Starting
with a heterozygous hermaphroditic individual, some of
its offspring will, through recombination of factors,
become homozygous, and if self-fertilization prevails they
will continue homozygous; other offspring will be hetero-
zygous. From the latter both homo- and hetero-zygous
offspring will again be produced, the former remaining
such in later generations, the latter continuing the process
of splitting. Since only a part of each generation sur-
vives, there is in the long run a better chance that the
homozygous individuals will be the survivors, because
those that have become such in each generation are fixed,
and those that are not will continue to produce some
homozygotes. There will be in consequence a steady proc-
~ ess of recurrence of homozygotes which, on chance alone,
will sooner or later win out.
The beans that Johannsen worked with had apparently
reached a homozygous condition, and at the start there
must have been several such lines. He studied nineteen
of them. The offspring of any one plant produced beans
that gave the same frequency distribution as the beans
of the last generation. This condition continued through
all successive generations. It is to be noted that the beans
on any one plant differ in size, but any one will give the
same frequency distribution as the beans of the preceding
generation. It made no difference whether the larger
or the smaller beans were chosen for planting—they gave
the same group in the next generation.
It is interesting to compare this result with what would
have happened had the beans been propagating by cross-
fertilization at the time when Johannsen began his work
with them. If this had been their normal method of
reproduction they would probably have been heterozygous
at the start, and would have given different genetic types
for several generations, even if self-fertilized. Pure lines
206 PHYSICAL BASIS OF HEREDITY
would have appeared only after the beans had become
homozygous through repeated inbreeding. But Johann-
sen, starting with homozygous beans, was able to obtain
his extremely important results, because if selection could
bring about any change it would have to be due to a
change in the genes themselves. Here, by means of a
crucial experiment, he exposed an error that had been
accepted by selectionists from 1859 to 1903. It would have
been difficult, almost impossible, to give this demonstra-
tion on any plant or animal in which self-fertilization or
asexual reproduction was not the rule; for, if the material
had been heterozygous either for the main factors for a
character, or for modifying factors for that character,
selection in one or another direction would be expected
through recombination of factors to change the original
frequency distribution. It is true that any stock, even
such as reproduces by males and females, may be made
homozygous by inbreeding brother and sister for ten or
more generations, but even such stock would have to be
constantly watched for mutation.
Johannsen defined a pure line as a race or family of
individuals descended through an unbroken series of self-
fertilizations from an ancestor homozygous in all its
genes. By making this definition precise he made clear
the essential point of his demonstration. Now that his
point is made, it seems no longer necessary or even desir-
able, I think, to narrow the definition of a pure line to
races that self-fertilize, since this is only one form of
inbreeding, resulting in the production of homozygous
individuals. By extending the definition of a pure line
to all forms whose genes are the same in all individuals
(whether the pairs are homozygous or not), the definition
covers all cases of parthenogenesis that do not undergo
reduction, and all cases propagating by non-sexual means,
for, in these cases the same complex of genes is present
in successive generations.
Many plants are propagated by offshoots, stolons,
PARTHENOGENESIS AND PURE LINES 207
tubers, cuttings, etc. Hast has studied the effect of selec-
tion of tubers of certain races of the common potato. A
race was first grown from a single tuber. By boring holes
into the tubers enough material could be obtained for a
chemical test of the amount of nitrogen in them. The
rest of each tuber could, if desired, be cut into pieces of
standard size and planted. Ten tubers, high in nitrogen,
and ten, low in nitrogen, were selected. The tubers of the
next generation showed that there was no relation found
between the amount of nitrogen in the original tuber and
in those that came from it. A repetition of the experi-
Fra. 95.—A wingless aphid to the left and a winged to the right, both belonging to
the same species. (After Webster and Phillips.)
ment in another generation gave only meagre results
owing to drought. As far as the facts went, this genera-
tion, too, showed no effect of selection.
Most of the protozoa propagate by dividing into equal
or nearly equal parts—i.e., by a process of cell-division.
Jennings has studied the effect of selection in a culture
of paramecium, all members of which had descended from
a single individual. No change was induced. Later, how-
ever, working on another protozoén, Difflugia corona,
Jennings found that selection brought about changes in
the direction of selection. In this case, the method of
division may possibly include irregular distribution of the
chromatin material, and the recent work of Hegner indi-
cates that such an interpretation is not improbable. Pos-
208 PHYSICAL BASIS OF HEREDITY
sibly, too, the irregular distribution of chromatin par-
ticles (chromidia) in the cytoplasm—aside from the
nuclear phenomena, or in connection with them—may
make the results similar in certain aspects to the distri-
bution of plastids in certain plant cells.
Many species of plant lice—aphids—(Fig. 95, a)
propagate throughout the summer by parthenogenesis.
There is no chromosomal reduction during the develop-
ment of the egg. Hach egg gives off only one polar body,
49 44 45 46 47 48 49 SO G1 §2 53 SH SS 56 57 58 59 60 Gf 62 4&3
2.2
21
in ZN Za NEA eS
inZ 1 NESTS N
SRS NIZIT SINE SENAY SSS
pl al" AYN 7 SS)
1 cs Po Lp
: 7
1a ard Zo
1.10) .
1.6 \
0.90
0.80
6 $? 6 5 4 5 8 2 B22 Fu 3 0 8B 5 G6 H bb B
Fie. 96.—Curve showing the non-effect of selection for the first twelve generations
for increase in body length, the heavy solid lines represent the fluctuations of the fraternal
means; the light solid line the fluctuations of the longest variant; the broken line the
fluctuation of the shortest variant. (After Ewing.)
each chromosome splitting into two daughter chromo-
somes, so that the egg retains the whole number of chromo-
somes. Ewing has carried out an extensive experiment
with Aphis avene, selecting individuals through a num-
ber of generations for the length of the cornicles (honey-
dew tubes), for the length of the antenne, and for body
length. Considering here only the last, individuals were
selected for forty-four generations in a plus and in a
minus direction. The graph for the fourty-fourth to the
sixty-third generation is shown in Fig. 96. The heavy
solid line represents the fluctuations of the longest vari-
PARTHENOGENESIS AND PURE LINES 209
ants, the broken line the fluctuations of the shortest
variants. It was found that much of the fluctuation
observed was connected with temperature. The tempera-
ture was therefore kept constant at about 65° F. for the
next twenty generations, and as shown in Fig. 97, the
fluctuation in the fraternal line was cut down. No in-
fluence of the selection is observable in the chart. This
evidence, in conjunction with that for other characters,
shows that no change takes place in the characters of
63 64 65 66 67 68 69 70 71 72 73 74 95 76 77 78 79 80 Q2 BR 83
Pee i
va — AN ~ N IN LA
cs ZINN RN EAS SOIREE EN
tANZ_T SIN TSA TSS TNR ESERNTSS
sik a2 Z VI \ = cs sy
1140p c <= NY aa
B 6b 8 BI 8B 1 8 7 1 7 1 2 HB 0 5 6 8 HB BR F
Fie. 97.—Curve showing the effect of selection for the second score of generations.
(See Fig. 96.)
the insect so long as the same group of chromosomes
remains. It would be difficult to find a better example
than these parthenogenetic insects to test the claim that
selection can change the germ-plasm, for here the con-
ditions. are even simpler than in unisexual forms unless
they have first been made homozygous.
The aphids also furnish favorable material to illus-
trate how the environment may cause very great changes,
even when the genetic complex remains the same. The
parthenogenetic aphids appear often as winged individ-
uals (Fig. 95,b). There is an entire change in structure
involving practically every part of the body. The winged
14
210 PHYSICAL BASIS OF HEREDITY
and wingless individuals may differ more strikingly than
do species of the same genus. The winged forms arising
from the wingless produce wingless forms again in the
next generation that may be identical with those from
which they came. It has long been believed that environ-
mental influences bring about these transitions in aphids,
but only recently has critical evidence been obtained. The
clearest evidence is that of Shinji, with the rose aphid.
By sticking twigs of the rose in sand and flooding the sand
with water containing substances in solution—a method
first suggested by W. T. Clarke—the fluid being drawn
up into the leaves is sucked out by the aphids on the leaves.
As the following table shows, young aphids reared on the
Winged Apterous
Individuals. Individuals
ABNO) iasten-n2 sues so eee ee TEE SE Wee Re Somes 61 0
CaSO g i esc e se see 096 soinkss. Fee DRA Re OES HAY POOR 34 1
POLS coe soagull te etieesnate tinae eda nes ie tomd wea ea eios 31 6
INTS Op ie eckies Scien ce So steele ics cevarts Nive te gate ace ella nau eeweaae canoes tee 955 5
IS DOME sites tides 25.3 aplenty a ade ead soe teed ee aaa 41 5
PHO sinc caw edavlng hak gis SF Wem Neg WER ees eemuece 12 2
Sn@h vase ken cyen kat ose Bas ame Rs Siw eo aes 579 8
TC gs ose os sag dean aptie es ee acai “Seavatanbua sus avacd ca spa obarae ee eeaient 49 2
Mig BLES: jeans nx goeie tie Guanes, oat ines aca iSong trader tree ueh el aes 840 9
UOT sg: sista tia sg tetscg aoe slags Wang jnlgcan wi i fo pr algiada laine 365 160
Alcohol sec sa.ssceaistey ernie eee a eee eee Barlse mene 2 288
AlOm, 2% giss ces 24 dees) saoe.us Base eee ee ES x 3 34
ACOCiG BCI cise dcise 8 widens SA RR ea ANS SE OS 0 67
IN Ge a LERE feiss setts coped tah estes arth alo tare fal NOP ayacives aide Oyaaeaade soaeanens 2 1029
Cas palltig acc is scpee ines bene Ha dae Naa celpriee os epnte ana 1 433
KG Ba lt6. isaciaiie os cidade dors Wate oa ee a 3 ea kee 3 324
Sri Salta vec <¢ wee ees ee gs Sees ae Shae ee aek Bee tenes 1 220
TOMMIN. 42059 oie ege $58 AOR Soe PaaS SS MRE RSA TES 1 14
UPC: snot de haiss, aah a echiee Deanne at auuueoncunes 5 153
Water, distilled .......... ee re en 0 394
Water, tap and creek ........ re eo 17 461
Peptone. 5 i605 03 o60-605 sie eae nen say we 64 BWW eee ROS 15
salts of the heavy metals as well as on magnesium salts
and sugar became winged, while those reared on the other
substances in this list remain apterous. Here we have
an excellent example of how in one environment a given
germ-plasm produces one result, and in another environ-
ment a different result without any intermediate forms.
PARTHENOGENESIS AND PURE LINES 211
The change from wingless to winged aphids is far greater
than most mutational changes that we know, yet must
involve a different kind of change because the result is
reversible, while a mutation, having once taken place, is
relatively irreversible.
Summing up, it may be said that the evidence shows
that whenever the same chromosomal complex containing
the same genes is found, the measurements of any charac-
ter in successive generations show the same frequency
distributions of the measurements, and the form may be
said in a general sense to belong to a pure line. The
evidence shows that whether the chromosomal complex is
heterozygous or homozygous, the results are the same,
so far as the pure line is concerned; but it is also obvious
that in most animals and plants, where redistribution
(reduction) of the chromosomes takes place in each gen-
eration, only forms already homozygous will give pure
lines. This was the special feature of the material that
Johannsen worked with, but aside from its practical value
in studying the selection problem, the limitation of the
definition of pure lines to such an exceptional situation
leaves out of sight the wider bearing of the evidence.
CHAPTER XVI
THE EMBRYOLOGICAL AND CYTOLOGICAL EVI-
DENCE THAT THE CHROMOSOMES ARE THE
BEARERS OF THE HEREDITARY UNITS
Lone before the genetic evidence brought forward its
abundant data that are explicable on the theory that the
chromosomes carry the genes, embryologists had already
found other evidence that led them to regard the chromo-
somes as the bearers of the hereditary factors. Taken
as a whole, this evidence makes out a very strong case
for the chromosomes, but since it did not establish the
relation beyond question, the genetic evidence was all
the more welcome.
The earliest evidence, sometimes cited in favor of
chromosomal inheritance, was based on the statements
that in some cases at least, only the head of the spermato-
zoon enters the egg. Since it was then thought that the
head is composed almost entirely of the nucleus, and since
the child inherits equally (in the older parlance) from its
father and from its mother, it followed that the nucleus
carries the hereditary elements. When later it became
known that the head of the sperm represents almost
exclusively the mass of condensed chromatin, it was sup-
posed that the chromosomes, in particular, must be that
part of the nucleus that is the bearer of hereditary charac-
ters. Such a conclusion received indirect support from
the facts, then becoming known, that the chromosomes
remain constant through successive generations of cells,
whereas the nuclear sap becomes lost in the gen-
eral cytoplasm each time that the nuclear wall is dis-
solved. It was also found that the spindle fibres disappear
in the resting stages, while the nuclear reticulum (chro-
matin) remains.
212
BEARERS OF HEREDITARY UNITS 213
This evidence failed, however, in so far as there might
be present a certain amount of nuclear plasm in the sperm-
head that is carried in with the head, and if so, would be
later mixed with the egg cytoplasm. The discovery that
at the base of the sperm-head there is present in some eggs
a centrosome that becomes, through division, the dynamic
centre of the next division, opened the door to suspicion
that the sperm might bring in other things than the chro-
mosomes to influence development, and hence heredity.
In conclusion then, while it may be said that the evi-
dence that the sperm-head alone enters the egg may be
claimed as favorable for the chromosome view, it cannot
be accepted as critical proof, because it is uncertain
whether other things also may not be brought in besides
the chromatin of the sperm.
Boveri’s evidence for chromosomal heredity from di-
spermic sea urchin eggs was open to less objection. It was
known that when two sperms enter the sea urchin’s egg
simultaneously, the first division of the egg is into three
or into four parts, because four (instead of two) division-
centres appear in these dispermic eggs. It was also known
‘that these eggs rarely produce normal embryos or larve.
Boveri, studying the mode of division of the dispermic
eggs, found that there was an irregular distribution of
the chromosomes to the three or four poles that appear,
and consequently to the three or four resulting cells (Fig.
98). The abnormal development of the whole egg that
generally follows might be ascribed to the irregular dis-
tribution of chromosomes to different regions; for, quite
apart from the specific nature of each chromosome or
group of chromosomes, the activity of one region being
quantitatively different from that of a corresponding
region in another part of the egg might be responsible for
the failure to develop normally. But Boveri went further
in his analysis. He shook apart the three or four blasto-
meres coming from dispermic eggs (by using Herbst’s
calcium-free sea-water method), and compared the num-
214 PHYSICAL BASIS OF HEREDITY
ber that developed into normal plutei with the number
of plutei from one-fourth normally fertilized blastomeres.
From the latter a large proportion give rise to normal
embryos, from the former normal embryos are rarer.
Their greater rarity, Boveri thought safe to attribute to
the chromosomal deficiencies present in most of such iso-
Fia. 98.—Scheme showing dispermic fertilization of the egg of the sea urchin with the
subsequent irregular distribution of the chromosomes. (After Boveri.)
lated blastomeres. He suggested that the chance of a
blastomere developing normally depends on its having
at least one full set of chromosomes. For these triploid
sea urchin eggs with three times 18 chromosomes, the
chance of one full set of chromosomes getting into each
blastomere is, according to Boveri’s calculation, only one
to 10,000. The chance of getting at least one chromosome
of each kind in one cell is greater. He concluded that the
few embryos he obtained came from quadrants that had at
least one haploid set of chromosomes. There is, however,
BEARERS OF HEREDITARY UNITS 215
to-day some uncertainty concerning the assumption that
normal development is to be expected if in addition to
one haploid set of chromosomes other chromosomes are
also present, because while one set alone might permit
normal development, it is by no means certain that if
there were one, two, or more additional chromosomes, the
balance might not be upset and abnormal development fol-
low. On chance distribution alone the isolation of just one
set and no more would seem a very remote possibility,
‘ Mints
ee
8 adie
a 6b
Fic. 99.—First division of a hybrid egg showing the elimination of chromosomes at the
equation of the spindle, a. The reciprocal cross, b, shows no such elimination. (After Baltzer).
but if there is to some degree a tendency for a group of
daughter chromosomes to move off together as a result
of their method of division, there might be a better chance
of such a group getting into one of the three or four
blastomeres than by chance distribution alone. At pres-
ent it is not possible to make any calculation based on such
an assumption. While, therefore, Boveri’s argument can-
not be accepted as demonstrative, yet it has probability
in its favor.
Baltzer has found a different kind of evidence of
chromosomal influence. When the eggs of one sea urchin,
216 PHYSICAL BASIS OF HEREDITY
Strongylocentrotus, are fertilized by the sperm of another
sea urchin, Sphaerechinus, the segmentation nucleus,
formed by the union of the egg- and sperm-nucleus shows
irregularities in the movements of the daughter chromo-
somes to the poles of the spindle. While some of the
chromosomes after dividing pass normally to the poles,
others become scattered irregularly between the two poles
and fail to become incorporated in the two-daughter nuclei
(Fig. 99,a). They appear to become lost and take no
Fre. 100.—Fertilization of an egg that had started to develop parthenogenetically.
The belated sperm unites with one of the daughter chromosomes groups only, a; an
earlier condition of the same proeedure. (After Herbst.)
part in the further development. Counts of the chromo-
some plates in the later divisions of the egg give about
21 chromosomes, whereas 36 are expected as the whole
number. It appears that 15 chromosomes are lost, and
presumably they belong to the foreign sperm. Many of
these eggs develop abnormally, but those that reach the
pluteus stage show a maternal skeleton only. This seems
to mean that the sperm has done no more than start the
development. It has contributed nothing, or little, to the
embryo, and it seems reasonable to attribute this to the
BEARERS OF HEREDITARY UNITS 217
loss of the paternal chromosomes, especially in the light
of the reciprocal cross.
In this reciprocal cross, the egg of Spherechwnus
is fertilized by the sperm of Strongylocentrotus. All
the chromosomes of the segmentation nucleus divide
and pass regularly to the two poles (Fig. 99,b). The
hybrid embryo shows characters of both parental species.
600
# §9
i ce
/ Qo
He) OO
( @) RO
00.0
(0 O OO!
fO ads
i O
t 6 OO
\Oo O =
-
wee”
Fre. 101.—Larval sea urchin seen in side view. On one side it shows hybrid characters,
on the other side it is maternal. The sizes of the nuclei on these two sides, as seen in the
figure, coincide with the view that the hybrid side is diploid and the maternal side haploid.
After Herbst.)
The difference in the two cases can be safely attributed
to the observed differences in the fate of the chromosomes,
rather than to unrecognized differences in other elements
brought in by the sperms.
Herbst’s experiments contribute further evidence in
favor of the chromosome interpretation. He caused the
unfertilized eggs of a sea urchin to begin to develop
parthenogenetically by adding a little acid to the sea
water. After five minutes the eggs were removed to pure
sea water, and sperm of another species, Strongylocen-
218 PHYSICAL BASIS OF HEREDITY
trotus, was added. The sperm entering the egg after its
nucleus had started to divide, failed to reach the egg
nucleus until the latter had divided (Fig. 100). The sperm
nucleus then formed a nucleus of its own, that passed into
one only of the daughter cells. This cell got two nuclei.
The other cell had only one of the daughter nuclei. Such
half-fertilized eggs give rise to larve that are maternal
on one side, and hybrid on the other—or at least larve
of this kind are sometimes found in such cultures (Fig.
101), and Herbst believes it is safe to refer them to the
half-fertilized eggs. If so, there can be little doubt that
the hybrid half owes its peculiarities to the presence of
both sets of chromosomes in its cells, while the maternal
half owes its peculiarities to its single set of maternal
chromosomes. This in itself, however, shows little more
than do whole hybrids and whole parthenogenetic eggs
themselves, for the demonstration that it is the chromo-
somes and not other constituents of the sperm-nucleus
that make the difference in the two sides rests on the
unproven inference that if other things than the nucleus
are involved they would be distributed equally throughout
the cytoplasm, but produce no effects. There is no reason
to suppose that they would be so distributed, and no evi-
dence that they are. Hence the proof is not cogent, how-
ever probable it may seem that only the sperm-nucleus is
responsible for those cases where there is a difference
in the two sides.
On the whole, then, while I am inclined to give much
weight to this evidence from experimental embryology as
very favorable to the hypothesis that the chromosomes
carry the hereditary characters, it is the genetic evidence
that furnishes convincing evidence in favor of this view.
CHAPTER XVII
CYTOPLASMIC INHERITANCE
In the preceding pages so much emphasis has been
laid on the chromosomes as bearers of the hereditary
material that it may appear that no very important réle
is left to the rest of the cell. Such an impression would be
quite misleading; for the evidence from embryology
appears to show that the reactions by means of which
the embryo develops, and many physiological processes
themselves, reside at the time in the cytoplasm. Further-
more, there is also genetic evidence to show that certain
forms of inheritance are the outcome of self-perpetuating
bodies in the cytoplasm, most of which go under the name
of plastids. Recognition of plastid inheritance carries
with it the idea that if there are such materials in the
cytoplasm that are self-perpetuating they will have to
be taken into account in any complete theory of heredity.
In the case of certain chlorophyll characters there is
excellent genetic evidence to show that a peculiar kind of
inheritance is due to the mode of transmission of plastids
in the cytoplasm. There is a race of four-o’clocks known
as Mirabilis Jalapa albomaculata, whose leaves are made
up of patches of green and white. Such leaves are said
to be checkered (Fig. 102, b). The amount of green, or of
white, varies on different leaves, and on such plants there
frequently appear leaves and entire branches that are
green and others that are white. The white is due to the
absence of green in the chlorophyll grains. Some cells
have only green chlorophyll bodies, and others only white,
still others may have the two mixed in various amounts.
Correns has shown that ‘if the flowers on a green
branch are self-fertilized they produce only green plants,
and these again only green plants. Flowers on white
219
220 PHYSICAL BASIS OF HEREDITY
branches give only white offspring. Flowers on the check-
ered branches give some checkered plants, some white
plants and some green plants. The proportions in which
these different types arise varies according to the amount
of green in the branch from which the self-fed seed came.
When the ovary of a flower on a green branch is fertil-
ized by pollen from a white branch, the plant produced
is green like the maternal branch. If the ovary of a
flower on a white branch is fertilized by pollen from a
a
Fig. 102.—Green leaf and checkered leaf of four-o'clock. (After Baur.)
green branch the offspring is white like the maternal
branch. These and other combinations show that this
color inheritance is only through the mother. The results
are explicable on the assumption that there are normal
(green) chlorophyll bodies and abnormal chlorophylless
bodies, both kinds propagating in the cytoplasm by divi-
sion, and that these two kinds are transmitted only through
the egg-cell. The green or white color of the leaves of a
given branch is an index of the kind of chlorophyll body
that the ovaries will probably contain. At each division
of the body-cells the chlorophyll grains present in it are
sorted out more or less at random—hence from a cell that
CYTOPLASMIC INHERITANCE 221
contains both kinds, more white granules than green ones
may at times get into a cell, and at other times only white
granules will get into one daughter cell, so that a white
branch arises.
In other species of plants that have white leaves and
branches and green leaves and branches, the cross may
give a different result. Thus in Melandriwm and Antirrhi-
num, green by white gives green F', (whichever way the
cross is made), in F’, there are 3 green to 1 white plant.
In this case the results can be explained as due to the
action of genes in the chromosome on the production of
chlorophyll in the cytoplasm—an action of such a kind that
Fic. 103.—Pelargonium that gave rise to a white branch. (After Baur.)
the granules do not develop green color unless the (nor-
mal) gene is present, in single dose at least. In this case,
even if the eggs only transmit plastids, the F’, individual
from a white-leaved mother by a green-leaved father is
green, because the paternal nucleus introduces a gene
that causes the green color to develop in the plastids. It
is the segregation of the genes in the germ-cells of the F,
individual that leads to the 3:1 ratio in F’,, and not the
distribution of the plastids as in the preceding case.
The most peculiar case is that of Pelargonium de-
scribed by-Baur. White leaves and branches, and green
leaves and branches occur on the same plant (Fig. 103).
Self-fertilized seeds from each breed true to color of
branch. White to green gives a different result, viz.,
222 PHYSICAL BASIS OF HEREDITY
mosaic seedlings with patches of green and white on stems
and leaves (Fig. 104). When these seedlings grow into
plants, the color of the leaves will depend on the color
of that part of the stem from which the terminal bud, and
lateral buds grow out. If a bud lies in a green part of the
stem the new part will be green (Fig. 104, a): if the new
bud lies in a white part of the stem the new part will be
white (Fig. 104, c): and if it lies in a partly green, partly
white region the new part will have some white, some
a
Fia. 104.—Diagram to show how a sectorial chimera may be produced. If the ter-
minal bud has come from a region of the seedling entirely green, all of the future leaves
will be green, a; if from aregion without chlorophyll, all the future leaves will be white, c;
but if the terminal bud lies partly in one, partly in the other region, some white and some
green leaves will arise, b. (After Baur.)
green parts (Fig. 104,b). The only explanation that is
suggested by Baur is that in this plant the plastids are
transmitted both by the egg and by the pollen. The white
plant with defective plastids contributes part of the plas-
tids in the fertilized egg, the green plant with normal
plastids the other part. The fertilized egg contains there-
fore both kinds of plastids. During division of the egg and
embryo, the granules become irregularly distributed in
the cells. Whenever a cell gets only defective granules,
it and its descendants are white, producing white parts:
when a cell gets mostly or only green granules, it and its
descendants are green, producing green parts. Hence
CYTOPLASMIC INHERITANCE 223
arise the checkered seedlings from which white or green
branches grow out.
The preceding facts and theories relating to plastid
inheritance show that if any element outside the nucleus
has the power to propagate itself it may be transmitted
through the egg, and even possibly through the sperm
(pollen) also. There is no contradiction here in any
sense to Mendelian inheritance but only an additional type
of inheritance that can be studied by as exact methods
as those used in Mendelian work. The chief difference
between chromosomal and plastid inheritance lies in the
orderly sequence of the distribution of the genes in all
divisions by means of the mitotic figure, whereas the plas-
tids are supposed to be shuffled about at random to the
daughter cells (partly because their division period does
not correspond with that of the cell). This haphazard
distribution of the plastids at any and all divisions is in
striking contrast to the sorting out of the genes that occurs
only at one specific cell-division when the germ-cells pass
through the maturation stage. Hence the orderliness of
Mendelian inheritance as contrasted with the more irregu-
lar procedure in plastid inheritance.
To embryologists familiar with the fact that differen-
tiation of the egg is closely associated with the cleavage
pattern, it was a natural inference that in the cytoplasm
lay the inherited characteristics that gave form to the
embryo, and even to all of its essential features. Little
room would seem to be left for the action of the chromo-
somes except to fill in the details of the characters already
outlined by cytoplasmic activity. This view might be la-
conically referred to as the theory of the ‘‘Embryo in the
Rough, ”? or more generally as the ‘‘Theory of the Organ-
ism as a Whole.’’ Boveri discussed some such view
(1903), and at first considered it favorably. It has since
been seriously discussed by others. Boveri pointed out
that when a horse is crossed to an ass it makes no differ-
ence which way the cross is made, for both egg and sperm
224 PHYSICAL BASIS OF HEREDITY
bring in the characteristics that make the organism first
a bilateral one, then a vertebrate, then a mammal, and,
lastly, a perissodactyl. In all these aspects, both parents
agree, and beyond these limits hybridizing is impossible.
Whatever the germ develops into must contain these com-
mon characters. The important point to determine,
Boveri thought, is whether the species characteristics are
or are not in the nucleus. He concluded, after discussing
the pros and cons, that it is doubtful if these preformed
qualities of the egg-protoplasm extend beyond the larval
periods, but that in general all characteristics that distin-
guish the individual from all others of its species and
from the characteristics of related species are inherited
through the chromosomes. Later he restated his con-
clusion as follows: ‘‘All essential characteristics of the
individual and of the species are epigenetic, and the deter-
mination is brought about through the nucleus.’’ Conklin
at one time expressed even more sharply the idea that
group characteristics may be inherited in a different way
from specific characters in the following paragraph:
We are vertebrates because our mothers were vertebrates and pro-
duced eggs of the vertebrate pattern; but the color of our skin and
hair and eyes, our sex, stature, and mental peculiarities were determined
by the sperm as well as by the egg from which we came. There is
evidence that the chromosomes of the egg and sperm are the seat of
the differential factors or determiners for Mendelian characters, while
the general polarity, symmetry and pattern of the embryo are deter-
mined by the cytoplasm of the egg.
In another statement, however, Conklin takes what
seems to me to be more nearly a correct view in regard to
the question, viz., that ‘‘There is no doubt that most of the
differentiations of the egg cytoplasm have arisen during
the ovarian history of the egg, and as a result of the
interaction of nucleus and cytoplasm; but the fact remains
that at the time of fertilization the hereditary potencies
of the two germ-cells are not equal, all the early stages of
development, including the polarity, symmetry, type of
CYTOPLASMIC INHERITANCE 225
cleavage, and the pattern, or relative positions and pro-
portions of future organs, being foreshadowed in the cyto-
plasm of the egg-cell, while only the differentiations of
later development are influenced by the sperm. In short,
the egg cytoplasm fixes the general type of development,
and the sperm and egg nuclei supply only the details.’’
If, as implied, the egg nucleus at first has already pro-
duced its effect on the cytoplasm, it’has done something
more than supply the details; and as to the sperm nucleus
I should substitute nearly all the stages of development
later than the gastrula. Moreover, sex is certainly one
of the fundamental characters of the organism, yet it
appears to be determined at fertilization by the chromo-
somal combination formed at that time. Conklin later
abandoned his earlier interpretation.
Quite recently, in his book on ‘‘The Organism as a
Whole,’’ Loeb has discussed the question as to whether
the protoplasm of the egg is ‘‘the future embryo in the
rough,’’ the sperm furnishing only the ‘‘individual charac-
ters.’’ Loeb suggests that the ‘‘specificity of the species’’
must be due to their proteins, and that the ‘‘heredity of the
genus is determined by proteins of a definite constitution
differing from the proteins of other genera. This consti-
tution of the proteins would therefore be responsible for
the genus heredity. The different species of a genus have
all the same genus proteins, but the proteins of the species
of the same genus are apparently different again in chemi-
eal constitution and hence may give rise to the specific bio-
logical or immunity reactions.’’ The possible relations of
these considerations to heredity are summed up in the
following paragraph:
It is thug doubtful whether or not any of the constituents of the
‘nucleus contribute to the determimation of the species. This in its
ultimate consequences might lead to the idea that the Mendelian charac-
ters which are equally transmitted by egg and spermatozoon determine
the individual or variety heredity, but not the genus or species heredity.
It is, in our present state of knowledge, impossible to cause a spermato-
15
226 PHYSICAL BASIS OF HEREDITY
zon to develop into an embryo, while we can induce the egg to develop
into an embryo without a spermatozodn. This may mean that the
protoplasm of the egg is the future embryo, while the chromosomes of
both egg and sperm nuclei furnish only the individual characters.
The evidence from Mendelian heredity is adverse to
any such distinctions as those made by the three authors
referred to above. We find in them, I think, an echo of an
old and somewhat mystical conception of fundamental dis-
tinctions between order, family and generic characters
of animals and plants—distinctions that even most syste-
matic writers recognize to-day as little more than conven-
tions that change from group to group. In the second
place, since the cytoplasm of the egg has been under the
influence of its own nucleus with a paternal and a maternal
group of chromosomes there is no direct means of deter-
mining whether its characteristics are due to such an
influence or have always been free from it. The fact that
sperm of a foreign species does not change the cytoplasm
. of the egg at once is to be expected even from a chemical
viewpoint. Mendelian workers can find no distinction
in heredity between characteristics that might be called
ordinal or specific, or fundamental, and those called ‘‘indi-
vidual.’’ This failure can scarcely be attributable to a
desire to magnify the importance of Mendelian heredity,
but rather to experience with hereditary characters. That
there may be substances in the cytoplasm that propagate
themselves there and that are outside the influence of the
nucleus, must, of course, be at once conceded as possible
despite the fact that, aside from certain plastids, all the
Mendelian evidence fails to show that there are such char-
acters. In a word, the distinction set up between generic
versus specific characters or even ‘‘specificity’’ seems at
present to lack any support in fact.
CHAPTER XVIII
MATERNAL INHERITANCE
Tuere is a kind of inheritance shown by eggs and
embryos, sometimes called maternal inheritance, that is
not the same as plastid inheritance, even although the lat-
ter is maternal in another sense. Nor is this so-called
maternal inheritance to be confused with cases of inheri-
tance in which all or some of the paternal chromosomes
fail to function, leaving the embryo at the mercy of its
maternal set alone. Nor should it be confused with sex-
linked inheritance where the son gets certain characters
only from the mother, because he gets his single sex-
chromosome from her.
‘‘True’’ maternal inheritance relates to peculiarities
of the egg or larva that are due to materials already pres-
ent in the egg-cytoplasm when the egg is laid. For exam-
ple, if pigment is scattered in the egg, it may collect in
certain regions after fertilization, and produce color in
them, as does the yellow pigment in the egg of Cynthia,
studied by Conklin. In this ascidian, much of the yellow
pigment is carried at the moment of fertilization to that
part of the egg that later goes into the muscle of the tail.
If the sperm used to fertilize such an egg should come
from a species without pigment in the egg, the inheritance
of color of the young embryo would obviously be entirely
maternal. In cases like this one, the formed material,
or any substance producing such materials, is already
present in the cytoplasm, but whether it has always been
free from nuclear influence must be shown by other tests.
In only one cross, viz., in the silkworm, has a third genera-
tion been raised, and until this has been done in
others we cannot know whether we are dealing in them
with plastid or with deferred nuclear influence (‘‘ma-
ternal inheritance’’).
227
228 PHYSICAL BASIS OF HEREDITY
In certain races of the domesticated silkworm moth,
Toyama has shown that pigment develops in the em-
bryonic membrane (serosa) which, seen through the
egg-shell, gives a specific color character to the embryo.
It is not clear from Toyama’s account whether the pig-
ment is present at first, scattered in the cytoplasm, and
collects later at the surface, or whether it develops only
after the embryo develops. When races are crossed with
characteristic but different embryonic membranes, the
color of the hybrid is like that of the maternal race only.
Pi @ op vy ¢ RO Oor vy ¢ D@
Se a
Eggs and embryo,
®@ O
Genetic DR DR
constitution. | |
Eggs and embryo. D D
Po @ ©
Genetic consti- DD DR RR DD DR RR
tution of Fo
individuals.
@ @® oO e 4 Q
8 D D R
Be oe 1 2 1 1 2 1
Fie. 105.—Diagram to illustrate maternal inheritance. The black circle stands for a dom-
inant character affecting the serosa coat of the embryo.
If adults (Ff) are raised from these eggs, it is found
when they in turn produce embryos, that the color of their
embryonic membrane is determined by the dominant char-
acter of the preceding generation that had been carried
in the chromosomes, irrespective of whether it came in
from the father or the mother (Fig. 105). That the result
is really chromosomal is shown by still another generation
in which some of the females show the dominant character
in the membranes of their embryos and others no color
in the ratio of 3:1.
It appears therefore in this case, the only one known
that furnishes critical evidence, that maternal inheritance
MATERNAL INHERITANCE 229
does not differ in any essential respect from ordinary
Mendelian heredity.
A peculiar case that in some respects and in certain
combinations appears to be maternal inheritance, is
shown in the character of the seed of corn (Zea mais).
The endosperm of maize is produced, as in some other
plants, at the time of fertilization—one pollen nucleus
unites with the egg to form the embryo, another pollen
nucleus unites with two nuclei in the embryo-sac to pro-
duce the endosperm whose cells, therefore, are triploid.
Floury corns have an endosperm, that is almost wholly
made up of cells containing ‘‘soft’’ starch, while flint corns
have only a small amount of soft starch in the centre of
the seed which is surrounded by a large amount of hard
‘‘corneous’’ starch. Hayes and East have shown that if
a floury corn be used as the mother and a flint corn as the
father, the seeds are floury like those of the pure race of
floury corn. If a flint corn be used as the mother and a
floury corn as the father, the seeds are flinty. In both
cases there is apparently maternal inheritance, at least
as far as the endosperm is involved, which is not, how-
ever, a part of the embryo proper. If the seeds from races
of the foregoing crosses are sown and the plants allowed
to self-fertilize, the following results are obtained: The
F, derived from floury ? by flint produces both floury
and flint in F’, in the ratio of 1:1. The F, flinty reciprocal
cross gives exactly the same result. The explanation of
the F, and F, results is as follows: If the factor for flinty
be F, and that for floury be f, then in the first cross the
endosperm is fF and in the reciprocal cross FF. Since
fF is floury and FFf flinty it follows that two doses of
floury dominate over one dose of flinty, and conversely
two doses of flinty dominate over one dose of floury.
- The F, embryo, however, in each of the crosses has
only one F' and one f factor (Ff). Its gametes are F and
f, and so are its endosperm nuclei which, as shown by
Weatherwax have the same reduced formula as the ovules
230 PHYSICAL BASIS OF HEREDITY
in the embryo sac. Hence half the embryo sacs are F' and
half f. The former, F (+f), fertilized by F pollen gives
FFF endosperm, by f pollen give fF; the latter, f (+/),
fertilized by F pollen gives fF endosperm, by f pollen
{ff endosperm. The four kinds of endosperm fall into two
classes, soft and hard, in the ratio of 1:1 in the F, seeds.
There are races of maize with yellow dominant endo-
sperm and others with recessive white. If the mother
belongs to a yellow race and the father to a white one,
the F, endosperm is yellow like the mother. In the recip-
rocal cross it is also yellow. If, however, races with floury
seeds are used, the F’, yellow endosperm in the former
cross is somewhat paler than the pure yellow of the yellow
race. Races with purple or with red endosperm crossed to
white give the same results, except that in these crosses the
quantitative effects seen in the floury flint crosses are not
observed, for, one dose of the dominant (purple) to two
doses of white gives the same color as two doses of purple
to one dose of white.
There are two kinds of maize with white endosperm.
These when crossed together give F', colored endosperm.
In these cases one race has one of the factors for color, and
the other, another complementary factor—like the two
white sweet peas. There is also a race with a dominant
white endosperm factor. The occurrence of these kinds of
whites led to some confusion in the earlier experiments of
Correns on endosperm inheritance. The word Xenia, that
had earlier a different meaning, is used to-day for these
eases of double fertilization in which the pollen has an
influence on the seed (the endosperm) that is not a part of
the F', plant itself. Hast and Hayes sum up the results
given above (exclusive of the floury-flint cross) as follows:
When two races differ in a single visible endosperm character in
which dominance is complete, Xenia occurs only when the dominant
parent is the male; when they differ in a single visible endosperm
character in which dominance is incomplete, or in two characters both
of which are necessary for the development of the visible difference,
Xenia occurs when either is the male.
MATERNAL INHERITANCE 231
In cases in which a foreign sperm may start develop-
ment but take no further part in it, the resulting embryo
is like the maternal race. Here we are dealing not so much
with maternal inheritance, but rather with a special kind
of parthenogenesis. Such eggs, however, rarely go beyond
the cleavage stages.
The rate of cleavage of an egg fertilized by foreign
sperm usually coincides with that of the species to which
the egg belongs. Since the cytoplasm of the egg has, prior
to fertilization, always been under the influence of its own
nucleus, this relation is what might be expected. It is
necessary to study eggs from an F, generation in such
eases in order to judge how far paternal chromosomes
may influence the cleavage. It is thinkable, for example,
that a spermatozoén might bring in a factor dominant for
rate of cleavage, but because this factor had not had time
to influence the cytoplasm its effect would not show in the
P, cross. In the Fj, on the other hand, the paternal char-
acter might prove dominant. Both Driesch and Boveri
have shown in the sea urchin that the rate of cleavage,
the pigmentation, and the kind of gastrulation are entirely
or largely determined by the egg—they differ in opinion
only as to how soon the influence of the sperm can be seen.
At the time when the larval skeleton is formed most
observers agree that the influence of the foreign sperm
makes itself felt. Most of the accounts of the skeleton of
hybrid sea urchins describe it as intermediate in struc-
ture, but one that varies widely under different external
conditions. Tennent has shown, in fact, that the character
of the hybrid larval skeleton is so greatly influenced by
the alkalinity or acidity of the sea water that it can be
artificially thrown towards one or the other side—mater-
nal or paternal. Loeb, King and Moore have attempted to
determine whether the larval skeleton has dominant char-
acters in certain parts and recessive ones in other parts.
They crossed the sea urchins, Strongylocentrotus Francis-
canus and 8. purpureus. Both the straight cross and its
232 PHYSICAL BASIS OF HEREDITY
reciprocal showed neither a great predominance of the
characters of the paternal race, nor of the maternal race,
but rather certain characteristic features of purpuratus
and others of Franciscanus. The larval characters
appeared to be dominant or recessive taken singly. Until
an F’, generation can be raised it is obviously hazardous
to speak here of Mendelian dominance and recessiveness
of characters that are based on F, observations alone,
especially since it is becoming more and more apparent
that many F’, characters are more or less intermediate,
and there are no general grounds for expecting pure domi-
nance or recessiveness.
Many crosses have been made between different species
of fish, and in some of these the young, at the time of hatch-
ing, are maternal. It has generally been supposed that
such cases are due to the absorption of the paternal chro-
mosomes at the first or at later cleavage stages. Loss of
chromosomes has in fact been recorded in several of these
cases of maternal inheritance. On the other hand, Miss
Pinney’s observations, summarized in the following table,
Cross Development Results Chromosomal Behavior
Ctenolabrus 9 X Fundulug 1 Development cases Early mitotic behavior
during gastrulation. is prevailingly nor-
mal.
Ctenolabrus @ X Stenoto-
MUS Ch sci se Sen scase es Many hatching em- Early mitoses are nor-
bryos of the mater- mal,
nal type.
Ctenolabrus 9 X Menidia o Advanced development. Early mitoses are nor-
mal.
Ctenolabrus 7 X Fundulus@ One hatching embryo Abnormal nuclear be-
reported. Many ad- hayior occurs.
vanced embryos—
maternal type.
Ctenolabrus (3 X Stenoto-
MUS Go ev sek ee ie eee Development ceases Abnormal mitosis pre-
during gastrulation. dominant.
Ctenolabrus gt X Menidia 9 Two hatching embryos Abnormal mitosis is of
reported. Maternal frequent occurrence.
type.
show that the maternal type may appear not only when the
early mitoses are abnormal, but in one case at least when
they are normal. Itis quite possible, therefore, that while
MATERNAL INHERITANCE 233
early loss of the paternal chromosomes may account for
some of the cases of maternal embryos, there may also
be cases where the chromatin may divide normally but
fail to produce any conspicuous effects on the cytoplasm
sufficiently soon to become apparent in the young fish.
In this connection the tobacco crosses described by
Goodspeed and Clausen may be recalled. In these cases
it was a particular group of chromosomes, regardless of
whether it was of paternal or of maternal origin, whose
‘reaction system’’ dominated in the F’, hybrid.
CHAPTER XIX
THE PARTICULATE THEORY OF HEREDITY AND
THE NATURE OF THE GENE
Tue attempt to explain biological phenomena by means
of representative particles has often been made in the
past. The superficial resemblance of the theory of the
gene to some of the older theories, long since abandoned,
has furnished the opponents of the Mendelian theory of
heredity an opportunity to injure the latter by pretending
that the modern idea of the gene is the same as the older
ideas of Herbert Spencer concerning physiological units,
of Darwin relating to pangenes, and especially of Weis-
mann about biophors. There is no need for such con-
fusion, for even a little knowledge of the evidence on which
the old and the new views rest ought to have sufficed to
make evident some important and essential differences.
It need not be denied, however, that there is an historical
connection between the medieval theory of preformation
and the particulate theory of heredity. Bonnet, one of the
best known adherents of preformation, believed at first
in ‘‘whole’’ germs, but later admitted that pieces of germs
might be stowed away in regions of the body likely to be
injured. Weismann, also, the most prominent modern
adherent of preformation, held that whole germs, ids, are
present in the germ-plasm, each standing for a whole
organism—each (or most or one?) becoming unravelled as
the embryonic development proceeded. In fact, Weis-
mann’s entire theory was invented primarily to explain
embryonic development rather than genetics. Its connec-
tion with the modern idea of the germ-plasm is little more
than an analogy—for reduction in Weismann’s original
234
PARTICULATE THEORY OF HEREDITY 235
sense meant the sorting out of the wholes of ancestral
germ-plasms with which he peopled the chromosomes.
The danger of any appeal to a theory of representative
particles obviously lies in the ease with which by its means
any phenomenon might be accounted for, if the theorizer
is allowed to endow the particles with any and all the
attributes that he wishes to use in his explanation. It
was because Bonnet, Spencer, and Weismann assigned
arbitrarily attributes to the ultimate particles of living
matter, that these views appear to-day highly speculative.
The different kind of evidence to which the modern theory
of the gene appeals is what I wish to emphasize here.
Tue Evipence ror THE GENE
The evidence that Mendelian inheritance rests on the
distribution of separate elements has already been given.
The numerical results obtained in the second generation
from any Mendelian cross involving a pair of contrasted
characters, find their explanation on the assumption that
the two original germ-plasms (or some element in them)
separate cleanly in the germ-cells of the F, hybrid. Tested
by back-crossing the assumption is verified. Recombining
the P,, F',, F, individuals in all possible ways also gives
results consistent with the very simple assumption that
whatever it is that causes one race to produce one charac-
ter, and another race another character, the two separate
in the hybrid in such a way that equal numbers of germ-
cells of each kind are produced. Up to this point the
results do not tell us whether the two germ-plasms separ-
ate as wholes—one from the other—or whether only some
part or parts behave in this way. But when two or more
1The nominal adoption (1904) toward the end of his career. of heredi-
tary units in the Mendelian sense did not go deep. Weismann still adhered
to his view of dissociation of the ids as their most characteristic feature—
the only one in fact for which they were originally invented. The evidence
on which Mendelian units rest has nothing whatever to do with this
cardinal doctrine of Weismann’s teaching.
236 PHYSICAL BASIS OF HEREDITY
pairs of contrasted characters are involved in the same
cross, we get further information as to the situation.
For example, Mendel showed that when peas that are
both yellow and round are crossed, to peas that are both
green and wrinkled, there appear in the F’, generation not
only the original combinations, but also recombinations of
these, viz., yellow and wrinkled; and green and round
(Fig. 106). Here also the numerical results 9:3:3:1
can be explained on the theory that the representatives
of each pair of characters separate in the germ-plasm,
and that the separation of each pair is independent of
what takes place in the other pair. Obviously it can no
longer be whole germ-plasms that separate, but there
must be different pairs of elements in the germ-plasm that
assort independently of each other. It has been found that
this principle of independent assortment may apply to a
considerable number of pairs of characters segregating at
the same time. The only restriction that is found is in
the case of linked pairs of characters. This relation will
be considered later.
The independent assortment of the pairs of characters
proves that the elements that stand for the characters in
the two original germ-plasms may separate from each
other. If each such pair of characters represented one
of the pairs of homologous chromosomes, the evidence, so
far considered, would be in accord with the view that the
chromosomes were the ultimate units involved in the proc-
esses of segregation and assortment. The chromosomes
are, as has been shown, independent units in the germ-
plasm. But as Drosophila shows, there are many more
pairs of characters than there are pairs of chromosomes.
It is obvious that if the chromosomes are the ulti-
mate units involved, and remain intact, there could be no
more independent pairs of characters than there are pairs
of chromosomes. In animals and in plants there are
no cases known where there are more independent pairs
than there are chromosomes, so that, as has been pointed
PARTICULATE THEORY OF HEREDITY 237
out in another connection, this evidence may also be
appealed to as favorable.
The behavior of linked pairs shows, however, that the
analysis must be carried further, because, despite linkage,
the elements that went in together may be separated. The
evidence shows that while some linked genes separate
almost as freely as do independent genes, so that their
linkage to each other can only be safely determined by
their relation to certain other genes, other linked genes
may separate not once in a hundred times, or even less
often. Between these extremes all intermediate linkage
values are found. These results indicate that the chromo-
somes do not represent the ultimate elements that may be
separated out of the original complex (germ-plasm).
We are led, then, to the conclusion that there are ele-
ments in the germ-plasm that are sorted out independently
of one another. The Drosophila evidence shows at least
several hundred independent elements, and as new ones
still appear as frequently as at first, the indications
are that there are many more such elements than those
as yet identified.
These elements we call genes, and what I wish to insist
on is that their presence is directly deducible from the
genetic results, quite independently of any further
attributes or localizations that we may assign to them.
It is this evidence that justifies the theory of partic-
ulate inheritance.
So far as representative elements in the germ-plasm
are concerned, we might be content to rest the case on the
preceding analysis of the results; but recent work has now
advanced far enough to tempt us to assign further attri-
butes to the genes than those deducible from the preceding
analysis alone. Some of these attributes may appear
better established than others, but, all together, they give
a consistent body. of data, and have therefore a certain
value and use.
238 PHYSICAL BASIS OF HEREDITY
It has been pointed out that the evidence shows not
only that the genes are carried by the chromosomes, but
that there may be interchanges between paternally-
derived and maternally-derived chromosome pairs. The
evidence shows that this interchange is a normal] feature
of the germ-cell, and not peculiar to hybrids, or to a
heterozygous condition of the pairs.
This analysis leads then to the view that the gene is
a certain amount of material in the chromosome that may
separate from the chromosome in which it lies, and be
replaced by a corresponding part (and by none other)
of the homologous chromosome. It is of fundamental sig-
nificance in this connection to recognize that the genes
of the pair that interchange do not jump out of one chro-
mosome into the other, so to speak, but are changed
by the thread breaking as a piece in front of or else
behind them, but not in both places at once, as would
be the case if only a single pair of allelomorphs were
involved each time.
That the gene does not stand for the whole length of
the chromosome between two other known genes is shown
by the fact that new genes arising by mutation in the inter-
' mediate region do not affect the character of the gene
already known. This fact recurring continually in Droso-
phila, where new mutations frequently appear, reassures
us that the idea of the gene as a very small part of the.
thread is a legitimate conclusion, even if we can not tell
how large or how small that region is.
1. Tue Mantrotp Errects or Hach Gene
If we examine almost any mutant race, such as the
race of white-eyed Drosophila, we find that the white eye
is only one of the characteristics that such a mutant race
shows. The productivity of the individual is also much
affected, and the viability is lower than in the wild fly. All
of these peculiarities are found whenever the white eye
emerges from a cross, and are not separable from the
9 a Lo
Fic. 106.—Diagram to show the inheritance of two pairs of Mendelian characters, viz.,
yellow versus green peas, and round versus wrinkled skin in garden peas.
PARTICULATE THEORY OF HEREDITY 239
white-eyed condition. It follows that whatever it is in
the germ-plasm that produces white eyes, also produces
other modifications as well, and modifies not only such
‘‘superficial’’ things as color, but also such ‘‘fundamen-
tal’’ things as productivity and viability. Many examples
of this manifold effect are known to students of heredity.
It is perhaps not going too far to say that any change
in the germ-plasm may produce many kinds of effects on
the body. Clearly then the character that we choose to
follow in any case is only the most conspicuous or (for
purposes of identification) the most striking or convenient
modification that is produced. Since, however, these
effects always go together, and can be explained by the
assumption of a single unit difference in the germ-plasm,
the particular difference in the germ-plasm is more sig-
nificant than the character chosen as its index.
2. Tur VARIABILITY OF THE CHARACTER IS Not DUE To THE
CoRRESPONDING VARIABILITY OF THE GENE
All characters are variable, but there is at present
abundant evidence to show that much of this variability
is due to external conditions that the embryo encounters
during its development. Such differences as these are not
transmitted in kind—they remain only so long as the
environment that produces them remains. By inference
the gene itself is stable, although the character varies ; yet
this point is very difficult to establish. The evidence is
becoming stronger nevertheless that the germ-plasm is
relatively constant, while the character is variable.
3. CHaracters THat are [NDISTINGUISHABLE May BE THE
; Propuct oF DIFFERENT GENES
We find, in experience, that we cannot safely infer
from the appearance of the character what gene is pro-
ducing it. There are at least three white races of fowls,
produced by different genes. We can synthesize white
240 PHYSICAL BASIS OF HEREDITY
eyed flies that are somatically indistinguishable from the
ordinary white-eyed race, yet they are the combined prod-
uct of several known color-producing genes. The purple
eye color of Drosophila is practically indistinguishable
from the eye colors maroon and garnet. In a word, we are
led again to units in the germ-plasm in our final analysis
rather than to the appearance of a character.
4, Inrerence Tuat Hacnh CHaARacTER Is THE PRopUCT oF
Many GENES
We find that any one organ of the body (such as an
eye, leg, wing) may appear under many forms in different
mutant races as a result of changes of genes in the germ-
plasm. It is a fair inference, I think, that the normal
units—the allelomorphs of the mutant genes—also often
affect the same part. We have found about 50 different
factors that affect eye-color, 15 that affect body-color, and
at least 10 factors for length of wing in Drosophila.
If, then, it is a fair inference that the units in the wild
fly, that behave as Mendelian mates to the mutant genes,
also affect the same organ that the mutant gene affects, it
follows that many genes, and perhaps a very large num-
ber, are involved in the production of each organ of the
body. It might perhaps not be a very great exaggeration
to say that every gene in the germ-plasm affects several
or many parts of the body; in other words, that the whole
germ-plasm is instrumental in producing each and every
part of the body.
Such a statement may seem at first hearing to amount
almost to an abandonment of the particulate conception
of heredity, but on the contrary, the statement conveys a
very important idea in the modern conception of the
nature of the genes and the way they act.
The essential point here is that even although each of
the organs of the body may be largely a product of the
entire germ-plasm, yet this germ-plasm is made up of
units that are independent of each other in at least two
PARTICULATE THEORY OF HEREDITY 241
respects, vie., in that each one may change (mutate) with-
out the others changing, and in segregation and in crossing
over each patr is separable from the others.
5. ‘THE Organism as a WHo.s,’’? on Tur CoLLEctTIvEe
ACTION oF THE GENES
Several writers have stated their objections to the
particulate theory: of heredity on the grounds of their
belief that the organism is a ‘‘whole.’’ If this phrase is
intended to mean that there is some sort of an entity or
entelechy that directs all processes that go on in each
living thing, there is little to be said here, except that
this very old idea has not been found profitable as a
working hypothesis. It is improbable, however, that
many biologists mean to appeal to any such vitalistic
agency when they speak of the ‘‘organism as a whole,”’
but have rather some other idea in mind. I am inclined to
think that certain phenomena of embryonic development
are responsible for the slogan of the ‘‘organism as a
whole.’’ In the segmentation of the egg the entire chromo-
somal complex is distributed to every cell in the body.
Each cell inherits the whole germ-plasm. How then it
may be asked can the result depend on the particular
make-up of its chromosomes rather than on the action of
the whole material?
Granted that we know very little about the interactions
between the cells that cause some of them to differentiate
in one direction, others in other directions, yet if one fer-
tilized egg should begin its development with one kind of
material, and another egg with a different material, should
we not expect the end products to be different, irrespective
of the way in which the materials were present in the
original egg? No matter where the differences may lie,
i.e., whether in the nucleus or in the cytoplasm, there is
nothing here in any way inconsistent with this particulate
theory of the composition of the germ-plasm. On the
contrary, the only conclusion that seems at all reasonable
16
242 PHYSICAL BASIS OF HEREDITY
is that if differences are present at the beginning, the end
product is expected to be correspondingly different. So
much is clear. But why, it may still be asked, are not two
organisms that are different at the start, if only in some
one difference, different later in every part, rather than
in only some one small part such as in a red or in a white
eye. The answer is, of course, that the first difference
was such that it affected principally a particular process,
vig., the formation of the red pigment of the eye, and to
a less degree, or not at all, other chemical processes. hts
seems to me an entirely consistent view.
Perhaps the difficulty in accepting the ‘aeonlnte
theory lies in the erroneous idea that the specific effect
comes into action only at the moment when the red pig-
ment is about to form. But no one has, so far as I know,
made such a claim. It may be true, but it has not been
proven, and is moreover not in any way essential to the
assumption of the particulate theory. On the contrary,
as our knowledge of Mendelian heredity has increased
many cases have been found where a special factor-differ-
ence affects not only one part of the body but many parts.
It is true that the particulate theory as held at one time
by Roux and for a long time by Weismann was used to
explain the differentiating changes in the segmenting
egg and embryo in the sense that development was looked
upon as a process that resulted immediately in the sorting
out of the inherited chromosomal particles to the differ-
ent parts of the organism. Differentiation resulted in the
sorting out of particular genes to particular groups of
cells whose development they controlled. But the cyto-
logical evidence in regard to the chromosomes gave no
evidence in support of the view, and the evidence from
the experimental study of embryology seemed to entirely
disprove any such basis for the developmental phe-
nomena. In fact, Roux himself abandoned this view
in the light of the brilliant experiments of Driesch and
of other embryologists.
PARTICULATE THEORY OF HEREDITY 243
Our present conception of the relation of the germ-
plasm to developmental phenomena has then only a most
superficial resemblance to the older theories. The newer
point of view may be summed up in a few words, and has
in fact been stated already. First, that each gene may
have manifold effects on the organism, and second, that
every part of the body, and even each particular character,
is the product of many genes. The evidence for these two
conclusions has been so repeatedly referred to in the pre-
ceding pages that it is not necessary to go over it again.
but it may be worth while to emphasize that these two
conclusions are not pure speculations, but derived from
the evidence itself. It may also be well to point out that
even if the whole germ-plasm—the sum of all the genes—
acts in the formation of every detail of the body, still
the evidence from heredity shows that this same material
becomes segregated into two parts during the maturation
of the egg and sperm, and that at this time individual
elements separate from each other largely independently
of the separation of other pairs of elements. It is in this
sense, and in this sense only, that we are justified in speak-
ing of the particulate composition of the germ-plasm and
of particulate inheritance.
There is a further idea deducible from well-known
facts of physiology that may at first sight seem to give
an impression that the organism is a ‘‘whole.’’ This
is the action of one part of the body on other parts by
means of substances set free in the blood, called hor-
mones. Many of them arise through the action of certain
so-called endocrine glands. But the relation here is so
obviously different from the problem dealt with as par-
ticulate inheritance that it calls for little more than
passing notice. It may, however, not be without interest
to refer to one case of the kind in which an endocrine
secretion depends on a genetic factor inherited in the
same way as are other genetic factors. There is a race
of poultry known as Sebrights ( Fig. 107, a) in which the
244 PHYSICAL BASIS OF HEREDITY
males are always hen-feathered. This means that the
feathers of the neck and back and the tail coverts of the
Sebright cock are nearly like those of the hen of this
breed, and not long and pointed as in the ordinary cock.
When Sebrights are crossed to game bantams (which
have ordinary males), the F', males are hen-feathered.
When these are inbred the two types reappear in the F,
males. One, or probably two, Mendelian factor differ-
ences account for the results.
It has been shown that when the testes are removed
from the Sebright male, he then develops at the next moult
(or at once if some feathers are plucked out) the long
and highly colored feathers of the ordinary male (Fig.
107,b6). It is probable, therefore, that the testes of the
Sebright produce an internal secretion that inhibits in the
male the full development of certain feathers. This makes
him like the hen, and in this connection it is interesting to
note that when the ovary of a hen of an ordinary breed is
removed she also develops the full plumage of the cock, as
Goodale has clearly demonstrated. Whether the testes of
a male are of the sort to develop this inhibiting substance,
depends on the presence in the cells of the testes of certain
genetic factors. These factors are present, presumably,
in all the cells of the body, but if they are, their activity
is ineffective in the absence of secretions produced by the
testes, as is shown by the castrated Sebright becoming
cock-feathered. Whether this substance belongs in the
heterogeneous group of substances called hormones—
defined by the kind of action they produce rather than by
any chemical peculiarity—or to the groups.of enzymes
that have a more or less specific action, cannot be stated.
The foregoing discussion touches upon the question as
to whether there is any evidence that the genes themselves
are to be regarded as enzymes.* In almost all of the
* Inadequate as is our knowledge of the physico-chemical processes that
go on in development, it is enough to indicate that many processes are
at work.
PARTICULATE THEORY OF HEREDITY 245
recent papers (Beijerinck, Riddle, Goldschmidt) that
touch on this question it is argued, from the evidence of the
specific enzymes supposed or demonstrably involved in
the production of some final stage in the chemical reaction
that leads to the character in question, that the gene itself
is the same specific enzyme. The argument shifts back
and forth from unit-character to unit-factor. The reason-
able position to take in this matter is, in my opinion, that
stated by Loeb and Chamberlain (1915), ‘‘The hereditary
factor in this case must consist of material which deter-
mines the formation of a given mass of these enzymes,
since the factors in the chromosomes are too small to carry
the whole mass of the enzymes existing in the embryo
or adult.’’? It should not be forgotten, however, that the
evidence in favor of enzyme action as the most important
developmental process is by no means established, and
even were the evidence for this view adequate, the stages
between such action and the ultimate chemical nature of
the gene may be too great to be cleared at a single bound.
Some of the modern work on the chemical composition of
the nucleus indicates that extremely complex protein com-
pounds may be present in it—even though some of the
split products obtainable from it may be relatively simple.
It seems to me therefore that it is both premature and
highly speculative at present to tie up the genetic evi-
dence concerning the genes with hypotheses concerning
their chemical composition. I urge this, but at the same
time I realize of course that we should endeavor to obtain
as soon as possible better knowledge as to the chemical
nature of the chromatin.
Another question concerning the gene, that has been
raised, is whether it is to be regarded as something having
a definite molecular constitution, or whether the gene is to
be regarded as a quantity of material fluctuating about a
mode—its definiteness representing only a general ten-
dency for the same frequency distribution to recur in
each species. From the nature of the case such a question
246 PHYSICAL BASIS OF HEREDITY
is speculative, and would have little importance were it not
that, by imputing to the advocates of Mendelian heredity
the assumption of absolute fixity to the gene, attempts
have been made to throw the burden of proof that the
genes are ‘‘constant’’ on the advocates of Mendelism.
So far as the genetic evidence is involved, I see at pres-
ent no way of deciding whether the gene has a definite
molecular constitution, or is only something that fluctuates
under the condition of its occurrence about a mode. Inter-
esting as it might be to speculate about these alternatives,
it seems futile to do so at present, but there is one impli-
cation that I should like to examine. If the gene is a
chemical molecule it is not evident how it could change
except by altering its chemical constitution. Its influence,
i.e., the chemical effects it produces, might, however, be
altered by changing other substances with which the mate-
rial it produces reacts. This is the idea involved in the
theory of ‘‘modifying genes.’’
But if the gene is a fluctuating amount of something it
might seem that any ‘‘fluctuation’’ that is present at one
time might be perpetuated by selection, and that a further
fluctuation in the same direction might be utilized for a
further advance, etc. It may be pointed out that this
picture of the process is quite fanciful, and its success
would depend largely on a denial of the premise as to the
nature of the gene, viz., that it is of a fluctuating amount.
Johannsen’s facts contradict an interpretation of the
fluctuations of the character being due to a new modal
position of the gene standing for that character. And his
facts furnish the only crucial evidence we have at present.
Fie. 107.—A. Adult hen-feathered Campine male. B. Adult male of same race that had been
castrated while still a young bird. When it became older it developed cock-feathering. {!t resembles
the male of another race of Campines in which the male is normally cock-feathered. C. Adult hen-
feathered Sebright male. D. Adult male Sebright, that had been castrated while still a young bird, It
developed cock-feathering when it became older.
CHAPTER XX
MUTATION
Concerninc the origin of the germinal differences that
give rise to mutant characters very little is known at pres-
ent except, (1) that they appear infrequently, (2) that the
change is definite from the beginning, (3) that some of
the changes at least are recurrent, and (4) that the differ-
ence between the old character and the new one is small
in some cases and greater in others. I do not think that
any of the work purporting to produce specific mutational
changes has succeeded in establishing its claims, at least
in the sense that we can pretend at present to control the
appearance of specific mutant changes, and until this is
done we can not hope to find out very much as to the
nature of these changes. Our study of the germ-plasm
is largely confined, therefore, for the present, to a study
of transmission of the genes, to the kinds of effects they
produce on the organism, and to the special relations of
the genes in the chromosomes where they are located.
Concerning the frequency of mutation there is a slowly
increasing body of evidence showing in some animals
and plants how often or how rarely changes of this kind
take place. The impression prevails that mutation is less
rare in some species than in others, and while I am inclined
to think that this may be true, not much value can be
ascribed to such impressions; for it is not improbable that
the frequency with which mutations are found is often
directly in proportion to the number of individuals exam-
ined and to familiarity with the type in question, so
that the smaller changes are not overlooked. The dis-
covery of new mutant types in almost every plant and
animal that has been carefully examined indicates at least
the very general occurrence of definite mutations, and the
247
248 PHYSICAL BASIS OF HEREDITY
great variety of types shown by nearly all of our domesti-
cated animals and plants—varieties that follow Mendel’s
law—appears to give further support to the view that the
process of mutation is widespread.
One of the most interesting phenomena connected with
mutation is the recurrence of the same change. It has
long been recognized that certain ‘‘sports’’ such as albi-
nos and melanic forms are found again and again in
nature. In insects there are many records of the sporadic
appearance of the same type, such as the light form (lacti-
color) of the moth Abraxas. It is true that not all such
appearances are to be accepted offhand as the first appear-
ance of the mutative change, since when these are reces-
sive it is probable in most cases * that the actual mutation
occurred several generations before the mutated genes
came together to produce the mutant character. But
granting this, it is at least probable that the same type
has appeared in many cases independently. The only
evidence that can be relied upon in such cases is from
pedigreed cultures, followed up by evidence that the
mutants that look alike are really due to mutations in the
same locus. Fortunately there is actual evidence, both
for plants and for animals, that can be appealed to to show
that the same mutations recur.
The most extensive evidence is from Drosophila
melanogaster. One of the first mutants that appeared,
viz., White eyes, has appeared anew in our cultures about
three times, in cultures known to be free from it before and
not contaminated. The same mutant has been found by
several other observers. The eye-color vermilion has
appeared at least six times; the wing character called
rudimentary, five times; cut wing has been found four
times; truncate wing has frequently appeared, but has
not necessarily been always produced by the same change.
Certain characters such as notch wings, that have
* Recessive mutations in the X-chromosomes of the XX-XY type may
appear in the male in the next generation.
&
MUTATION ~ 249
appeared quite often, represent, it seems, a peculiar
change whose relation to the changes that stand behind
other mutant characters is not yet worked out.
In plants the best evidence is that reported by Emerson
for Indian corn. Emerson has shown that when a race
of corn (Zea mais) having red cobs and red seeds is
crossed to a race having white cobs and white seeds only, -
the two original combinations appear in the second (F,)
generation giving plants with red cobs and red seeds and
plants with white cobs and white seeds. Hither a single
factor determines that both cob and seed are red in one
case and white in the other, or if the color of each part
is due to a separate factor these factors are completely
linked. Now striped seeds with white cobs sometimes
mutate to red seeds and red cobs. The new combination
(red and red) acts as a unit toward the other known com-
binations. Therefore a single factor must have changed,
for, if not, mutation must occur in two (or more) closely
linked factors, 7.e., for seed and cob color at the same time,
which is highly improbable.
In forms propagating by sexual methods it cannot
be told whether mutation has occurred in one locus or in
both homologous loci at the same time, because in the egg
one of each pair of genes is lost in the polar body, and
irrespective of whether one or two mutated genes were
present only one member of the pair is left in the ripe egg;
and in the sperm the chance of any one sperm reaching
the egg is so small that it is unlikely that the difference
between one sperm or two sperms having the mutated
locus could be detected. Itis true that of the twelve domi-
nant mutants that have appeared in Drosophila each
appeared at first in a single individual—never two—which
might appear to favor the single locus view, but this evi-
dence is too meagre to be significant. Mutants from reces-
sive genes usually come to light in about a quarter of the
offspring of a given pair. This means that both parents
were heterozygous for the mutant gene, but this gene
250 PHYSICAL BASIS OF HEREDITY
must have arisen at least one generation earlier, and
have been carried over into the two heterozygous indi-
viduals in question.
It would be a point of capital importance if it could be
determined beyond doubt that at times recessive mutant
genes change back to the original (wild type) gene, or
even if a recessive gene could mutate to a dominant one.
The appearance of the wild type in a pure culture of a
mutant race can be accepted as good evidence of such a
change only when every possibility of contamination by
the wild type is excluded, and this is difficult to regulate.
In our cultures we have come across such cases, but have
not ventured to exploit them, since wild-type flies are
always present in the laboratory, and hence the discovered
form may have arisen through accidental contamination.
Thus even when a red-eyed yellow fly appeared in the
white-eyed yellow stock there is the barest chance that a
yellow red-eyed fly, or an egg of such a fly, had somehow
gotten into the stock. Certainty can be attained only when
a stock, pure for several mutant characters, reverts to the
normal in one of these characters, and not in the others.
Only one case of this kind that is above suspicion has been
as yet recorded. This is a mutant stock in which, as May
has recorded, reversion to the wild type occurs with such
frequency that there can be no chance of error. The stock
in question, bar eye, is a dominant mutant and the rever-
sion therefore is to the recessive wild type of eye (round
eye). The change back to normal is complete, since such
individuals give only normal offspring. When such a
mutant chromosome comes from the mother and goes into
a son he has normal (wild type) eyes; when it comes from
the father, and goes to a daughter, she is heterozygous
for bar eye. Baur has recently recorded the appearance
of recessive (7?) mutants from self-fertilized plants (snap-
dragon) that bred true at once. Punnett has described a
similar case (1919). The result can be accounted for, if a
mutation occurred in only a single chromosome far enough
MUTATION © 251
back in the germ-tract to give rise, after reduction, both
to pollen and to ovules, each one carrying the mutated
genes. Such an interpretation is supported by the evi-
dence from Drosophila, where, although mutations are
much more numerous, no such cases have been observed,
and none such would be expected if mutation occurs in a
single chromosome at a time, since here the germ-cells
come from separate individuals.
Probably the most important evidence bearing on the
nature of the genes is that derived from multiple allelo-
morphs. Now that the proof has been furnished that the
phenomena connected with these cases are not due to nests
of closely linked genes, we can properly appeal to these as
crucial cases. As already explained, in ever-increasing
numbers of animals and plants, series of genes have been
found in each of which mutant characters with the same
normal allelomorph have been found. These mutant char-
acters of each series are also allelomorphs of one another
—only two ever existing in the same individual. Ob-
viously, not all such mutants can be due to the absence
of a factor present in the germ-plasm of the wild type,
since only one kind of absence is thinkable. If to save the
situation for the theory of presence and absence it be
assumed that only a part of the original gene is absent,
and a different part in each case, then nothing is gained by
the admission; and while this may be true it is equally
possible that the genes change in other ways. It is not
essential that we should specify the nature of the change,
but simpler to look upon the mutant gene as due to
some kind of change or changes that have taken place
in the original germ-plasm at a specific locus—there is
nothing known at present to furnish even a clue as to the
nature of this change.
The demonstration that multiple allelomorphs are
modifications of the same locus in the chromosome, rather
than cases of closely linked genes, can come only where
their origin is known, and at present this holds only in
202 PHYSICAL BASIS OF HEREDITY
the case (just stated) for Indian corn and for the fruit
fly. If each member of such a series of allelomorphs has
arisen historically from the preceding one in the series,
by a mutation in a locus closely associated with the locus
responsible for the first, they would be expected to give
the wild type when crossed; and as the proof of their
allelomorphism turns on the failure of members of the
>
rank
eeeaee
APand kr
ee0cee
a b c a e
Fia. 108.—Diagram illustrating mutation in a nest of genes so closely linked that no
crossing over takes place.
series to show the atavistic behavior on crossing, it is
necessary, as stated, to know how they arose. This may
be made clear by the following illustration:
Let the five circles of Fig. 108, A represent a nest of
closely linked genes. If a recessive mutation occurs in
the first one (line B,a) and another in the second gene
(line B, b), the two mutants a and 0 if crossed should give
the atavistic type, since a brings in the normal allelo-
morph. (B) of b, and b that (4) of a. If a third mutation
should occur in the third gene it, too, will give the atavistic
MUTATION 253
type if crossed to a or to b. Similarly for a mutation in
the fourth and in the fifth normal gene. Now this is
exactly what does not take place when members of an
allelomorphic series are crossed—they do not give the
wild type, but one of the other mutant types or an inter-
mediate character. Evidently independent mutation in a
nest of linked normal genes will not explain the results
if the new genes arise directly each from a different nor-
mal allelomorph.
But suppose, as shown in Fig. 4 (line C) after a muta-
tion had occurred in the first gene a new mutant, b, arose
from a new gene, and from b a mutation arose in a third
gene c, and ¢ similarly gave rise to d; then a crossed to b
will give a (or something intermediate if the heterozygote
is an intermediate type). Likewise c crossed to 6 will
_ give b, or c crossed to a will give a, etc. If mutant allelo-
morphic genes in a series such as C, a, b, c, d, e, arise as
successive steps, t.e., Ca to Cb and Cb to Ce, ete., then
the hypothesis of closely linked genes would seem to be a
possible interpretation of the data, but if they do not
arise in this way, but by independent mutations from the
wild type (or even from each other, but not seriatim), then
they must be due to mutations in the same gene: for, to
assume that they are not, requires that, when the second
mutation took place both gene a and gene b mutated at
the same time, and that when ¢ appeared three genes
mutated, when gene d appeared four; when gene e five
genes mutated at once, four of them being mutant genes
that have already arisen independently. Such an inter-
pretation is excluded, since it is inconceivable, even in a
readily mutating form like Drosophila, that five muta-
tions could have occurred at the same time in distinct but
neighboring loci. As has been stated, the evidence from
Drosophila shows positively that multiple allelomorphs
arise at random.
Only two members of a series of multiple allelomorphs
can be present in any one individual, and in the case of
254 PHYSICAL BASIS OF HEREDITY
genes carried by the sex-chromosome only one can exist
at a time in the sex that has only one of these chromosomes.
In the individual with two mutant allelomorphs one of
them replaces the normal allelomorph of the ordinary
Mendelian pair. The two mutant allelomorphs behave
towards each other in the same way as does the normal
towards its mutant allelomorphs. It is doubtful whether
we can conclude anything more from this relation of Men-
delian pairs than we knew before,! although there is at
least a sentimental satisfaction in knowing that both nor-
mal allelomorphs can be replaced by mutant ones without
altering the working of the machinery.
The linkage relation of each member of a series of
multiple allelomorphs to all other genes of its chromo-
some is, of course, the same. While the theory of identical
loci requires this as a primary condition it is not legiti-
mate to use this evidence as a proof of the identity of the
loci, because it is not possible to work with sufficient pre-
cision in locating genes by their relation to other linked
genes to distinguish between identical loci and close-
linked: genes.
The question of lethal genes has attracted in recent
years increasing attention, both on account of their fre-
quency and because of a curious complication they may
produce in hiding the effects of other genes also present.
In Drosophila we have records of more than 20 sex-linked
lethals, and about 15 not sex-linked, and scattering records
of many others. Gametic lethal genes are those that
destroy eggs or pollen cells that contain such genes.
Zygotic lethal genes affect the embryo, the larva, or the
adult, so that it dies. In the case of the garden plant
known as double ‘‘stocks,’’ the genetic evidence obtained
by Miss Saunders indicates that certain kinds of pollen
are not produced, and presumably die because of a con-
tained factor. The same factor does not kill the ovules,
The substitution by crossing over really furnishes as good a demon-
stration of this point.
MUTATION 255
which may therefore transmit the recessive lethal gene to
half the progeny. How far the frequent occurrence of im-
perfect pollen grains in many species of plants is due to
such factors is still uncertain.
Belling found that while the Florida velvet bean
produces normal pollen grains and ovules, and the Lyon
bean, another bean of the same genus, also produces nor-
mal gametes, the F, hybrid contains 50 per cent. abortive
pollen grains, and possibly about 50 per cent. of the ovules
are abortive. In the second generation (F',.) half of the
pollen grains of half of the plants are abortive. The other
half of the plants have normal pollen grains. This is the
result expected if there are present in one of the species
the factors 4AAbb, and in the other species the factors
aaBB, the viable gametes in the F, generation being
those containing Ab, Ba, and the two gametes that die
being AB, ab.
Other observers have made records of abortive pollen
in hybrids, but without knowing the condition of the
pollen in the parents the interpretation of the results is
doubtful, for, as Jeffrey has emphasized, abortive pollen
is a characteristic of many wild species. There is one
fact of capital importance recorded by several botanists,
viz., that the degeneration of the germ-cells only takes
place after the tetrad has been produced, and only in some
of the cells of each tetrad. In other words, the lethal
effect is not observed until the chromosomes have under-
gone reduction. It is obvious that if there is present a
recessive lethal for the germ-cells (or for any cells, in
fact), it causes no injury in the presence of its normal
allelomorph, but kills when the counter-effect of its part-
ner is removed.
Tischler found in a hybrid currant that tetrad forma-
tion was normal, and that the shrinking of the pollen
grains occurred afterwards. Geerts found that one-half
of the pollen grains of @nothera Lamarckiana degen-
erate, and that half of the embryo sacs abort in the tetrad
256 PHYSICAL BASIS OF HEREDITY
stage. Other related (wild) species and genera of the
evening primrose have also been found to have some
abortive pollen and ovules.
Complete or nearly complete abortion has been seen
in other hybrids; viz., by Rosenberg in the sundew, by
Osawa in the Satsuma orange, by Goodspeed and others
in the hybrid tobacco (N. tabacum by N. sylvestris), by
- Jesenko in the wheat-rye hybrid, and by Sutton in the
hybrid between the Palestine pea (Piswm humile) and the
edible pea. These cases may be in part the same phe-
nomenon and in part a different one connected with fail-
ure of the chromosome to conjugate or to be properly
distributed during the maturation divisions.
The ‘‘yellow mouse case’’ is an example of a zygotic
lethal effect. The gene that produces the dominant yellow
color is lethal in double dose, so that all homozygous yel-
low mice die, as Cuénot first discovered, and as has been
more positively demonstrated by the work of Castle and
Little. There is some evidence indicating that these homo-
zygotes die as young embryos. Little has also shown that
black-eyed white mice carry a lethal, that acts in the same
way. In Drosophila there is a sex-linked recessive lethal
factor that causes the development of tumors in the larva,
destroying every male larva that contains the sex-chromo-
some carrying this gene. This effect, discovered by
Bridges, has been the basis for an extensive series of
experiments by Miss Stark. The gene is present in the
X-chromosomes; it follows the rules for all sex-linked
genes in its inheritance. The females of the stock are of
two kinds: One has the lethal in one sex-chromosome, and
its normal, dominant allelomorph in the other. Such a
female has survived because the effect of the lethal gene
is counteracted by the effect of its normal allelomorph.
Half of her sons get the affected chromosome. All such
sons develop the tumor—one or more melanitic growths
that appear in the imaginal discs or in other parts of the
larva. The other sons get the other chromosome with the
MUTATION 257
normal allelomorph. They never produce a tumor and
never transmit the disease. The same mother that gave
these two kinds of sons—having been fertilized by a nor-
mal male, since no affected males exist—produces also two
kinds of daughters, one containing the gene for the tumor
(and its normal allelomorph), the other having two nor-
mal genes. The former transmit the disease as just
explained, the latter daughters are perfectly normal and
do not transmit the disease.
Other lethal genes kill the pup, a few of them even
allow the fly occasionally to come through, but such flies
rarely propagate. Certain races of Drosophila have ster-
ile or nearly sterile females, other races sterile males.
The sterility is here lethal in so far as it affects the germ-
cells. Some effects on other characters are also generally
to be seen.
The presence of a lethal gene near to, i.e., linked to,
another mutant gene may affect the kinds of individ-
uals that appear because owing to the linkage the other
mutant character fails to appear, except when crossing
over takes place. Some examples of this relation may be
given. There is a mutant race called beaded (Fig. 109)
in which the margin of the wing is irregularly broken,
giving the appearance of a beaded edge. The gene for
beaded is dominant, and lethal when homozygous.
As in the case of the yellow mouse, only the hybrid
(heterozygous) combination exists, and consequently
when two beaded flies mate they produce two beaded to
one normal fly, as shown in Fig. 110. Here the first pair
of vertical lines stand for the pair of third chromosomes
present in the egg before its reduction. The two genes
here involved, that for beaded and its allelomorph for
normal, are indicated at the lower end of the vertical lines.
The two corresponding chromosomes in the male are
represented to the right of the last. After the ripening of
the germ-cells each egg and each sperm carries one or
the other of these chromosomes. Chance meetings of egg
17
258 PHYSICAL BASIS OF HEREDITY
and sperm are indicated in the figure by the arrow-scheme
below, which gives the combinations (classes) included in
the four squares. The double dominant BB is the class
that does not come through. The result is two beaded
(heterozygous) to one normal fly.
The beaded stock remained in this condition for a long
time; although selected in every generation for beaded, it
did not improve, but continued to throw 33 per cent. of
normal flies. Then it changed and bred nearly true.
Eggs Sperm Ra
B
B
Br +N Br FN p a
i> 2 Beaded : 1 Normal,
B Ni.
Fie. 110.—Diagram showing the relation of the chromosomes (represented by the
vertical rods) in a cross of ‘‘beaded”’ by ‘‘beaded.’”’ Flies homozygous for beaded die as
indicated by the cross-hatched square.
The change must have been due to the appearance of
another lethal factor (now called lethal three, here 1,) in
Fig. 111). Such a gene was found in the race when
studied later by Muller.
The lethal gene that appeared in the beaded stock was
also in the third chromosome, and in the chromosome that
is the mate of the one carrying the gene for beaded, 1.e.,
in the normal third chromosome of the beaded stock. The
lethal gene lies so near to the level of the beaded-normal
Fra. 109.—Two flies (Drosophila) with beaded wings.
MUTATION 259
pair of genes that almost no crossing over takes place
between the levels occupied by the two pairs. These rela-
tions are illustrated in the next diagram, Fig. 111. Here
again the two pairs of vertical lines to the left represent
the two third-chromosome pairs in the female and to the
right in the male. The location of the two pairs of genes
involved, N-l, and B-N, are indicated. These combina-
tions give the four classes in the squares of which two
classes die, viz., NNBB (pure for beaded) and 1,1,NN
Eges Sperm
ia NB NB
w+ +24 nt 41,
Bt tn BT TN All Beaded
ial
NB LX
Fie. 111.—Diagram to show how the appearance of a lethal near beaded causes
the stock to produce only beaded except for the small number of crossovers, as shown
by the next diagram.
(pure for lethal three). The result is that only beaded
flies come through, and since all these are heterozygous
both for B and 1,, the process is self-perpetuating.
If the preceding account represented all of the facts in
the case, the stock of beaded should have bred perfectly
true, but it has been shown in Drosophila that crossing
over between the members of the pairs of genes takes
place in the female. Hence we should expect a complica-
tion due to crossing over here unless the level of the two
pairs of genes was so nearly the same as to preclude this
possibility. In fact, in addition to the beaded flies the
260 PHYSICAL BASIS OF HEREDITY
stock in this condition alone should give 10 per cent. of
crossing over,i.e.,it should still produce a small percentage
of normal flies. It so happened, however, that there was
present in the stock a third gene that lowers the amount
of crossing over in the female to such an extent that, for
the two ‘‘distances’’ here involved, practically none takes
place. When it does, a normal fly appears, but this is
so seldom that such an occurrence, if it happened in a
domesticated form of which the wild type was unknown,
Crossover Sperm
Eggs
NN NWN
NB 1N
Zoos
| PA
N+ fi, we 43
nt +p st ty 1Bead. 1Norm.0.5%
[><]
Fic. 112.—Diagram showing the results of crossing over in a stock containing both beaded
and lethal, as shown in Fig. 111.
would be set down as a mutation like that shown by the
evening primrose.
The third factor that entered into the result is not
unique, for Sturtevant has shown that crossover factors
are not uncommon in Drosophila. The analysis that Mul-
ler has given for beaded, while theoretical, is backed up
by the same kind of genetic evidence that is accepted in
all Mendelian work. It makes an assumption but one that
can be demonstrated by any one who will make the neces-
sary tests. It is also possible to produce at will other bal-
anced lethal stocks that will ‘‘mutate’’ in the sense that
MUTATION 261
they will throw off a small predictable number of a
““mutant’’ type—a type that we can introduce into the
stock for the express purpose of recovering it by such an
apparent mutation process.
For example, dichete is a third chromosome dominant
wing-and-bristle character and, like beaded, a recessive
lethal. Sturtevant bred flies with the gene for dichete
in one of the third chromosomes and with a gene for the
recessive eye-color, peach, in the other for several genera-
Won crossover eggs
(95% of total) Sperm ra
DNN
D+ ln D+ +} EG Nlp
r de FA Dichete
NT TP NT Tp
Dichete |
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