THE MECHANISM OF MENDELIAN HEREDITY I n m i? 0.0-m 1.5 J -YELLOW WHITE 0.0:: STAR 0.0-- ROUGHOID 0.0,i 0.6/ =.BENT 0.9 EYELESS 5.5- -ECHINUS 75 - - RUBY 9.0:: truncate: •1 13.7 - - CROSS VNL’SS 14 .0 - STREAK 20.0 : 7 CUT 25.8 - SEPIA -HAIRY 27.5 - -TAN 29.0 : - DACHS 33.0 - - VERMILION 3 3.0- - SKI 36.1 - =- MINIATURE 38.5 - -DIC HAETE : SABLE 42.0 : r SCARLET 43.0 - /” 44.4 xgarnet 46.5 - - BLACK 45.5 -1 i-PINK. 52.4- - PURPLE 54.0 : = SPINELESS 56.5n; 54.5 • - BITHORAX 57.0"' ""BAR 59.0 : : GLASS 6 3.5 - -DELTA 65.0 - CLEFT 650 - - VESTIGIAL 65.5 - -HAIRLESS 68.0 - BOBBED 70.0 : LOBE 67.5 - rEBONY /2.0 -WHITE-OCELLI 73.5- -CURVED - ROUGH 86.5 88.0 - HUMPY - 95.4 CLARET 975 \ -- ARC / CD Cn : = M 1N UT E 98.5 - PLEXUS 101.0 - -MINUTE-G 103.0 r BROWN 1050 -r SPECK (Frontispiece) THE MECHANISM OF MENDELIAN HEREDITY BY T. H. MORGAN PROFESSOR OF EXPERIMENTAL ZOOLOGY COLUMBIA UNIVERSITY A. H. STURTEVANT RESEARCH ASSISTANT, CARNEGIE INSTITUTION H. J. MULLER ASSOCIATE PROFESSOR OF ZOOLOGY UNIVERSITY OF TEXAS C. B. BRIDGES RESEARCH ASSISTANT, CARNEGIE INSTITUTION REVISED EDITION NEW YORK HENRY HOLT AND COMPANY Copyright, 1915, 1923, BY HENRY HOLT AND COMPANY PRINTED IN U. S. A. EDMUND BEECHER WILSON Go PREFACE From ancient times heredity has been looked upon as one of the central problems of biological philoso- phy. It is true that this interest was largely specu- lative rather than empirical. But since Mendel’s discovery of the fundamental law of heredity in 1865, or rather since its re-discovery in 1900, a curious situation has begun to develop. The students of heredity calling themselves geneticists have begun to draw away from the traditional fields of zoology and botany, and have concentrated their attention on the study of Mendel’s principles and their later developments. The results of these investigators appear largely in special j ournals. Their terminology is often regarded by other zoologists as something barbarous,—outside the ordinary routine of their pro- fession. The tendency is to regard genetics as a sub- ject for specialists instead of an all-important theme of zoology and botany. No doubt this is but a passing phase; for biologists can little afford to hand over to a special group of investigators a part of their field that is and always will be of vital import. It would be as unfortunate for all biologists to remain ignorant of the modern advances in the study of heredity as it would be for the geneticists to remain unconcerned VII VIII PREFACE as to the -value for their own work of many special fields of biological inquiry. What is fundamental in zoology and botany is not so extensive, or so in- trinsically difficult, that a man equipped for his profession should not be able to compass it. In the following pages we have attempted to sepa- rate those questions that seem to us significant from that which is special or merely technical. We have, of course, put our own interpretation on the facts, and while this may not be agreed to on all sides, yet we believe that in what is essential we have not departed from the point of view that is held by many of our co-workers at the present time. Exception may perhaps be taken to the emphasis we have laid on the chromosomes as the material basis of in- heritance. Whether we are right here, the future— probably a very near future—will decide. But it should not pass unnoticed that even if the chromo- some theory be denied, there is no result dealt with in the following pages that may not be treated inde- pendently of the chromosomes; for, we have made no assumption concerning heredity that cannot also be made abstractly without the chromosomes as bearers of the postulated hereditary factors. Why then, we are often asked, do you drag in the chro- mosomes? Our answer is that since the chromo- somes furnish exactly the kind of mechanism that the Mendelian laws call for; and since there is an ever-increasing body of information that points clearly to the chromosomes as the bearers of the PREFACE IX Mendelian factors, it would be folly to close one’s eyes to so patent a relation. Moreover, as biologists, we are interested in heredity not primarily as a mathe- matical formulation but rather as a problem concern- ing the cell, the egg, and the sperm. T. H. M. PREFACE TO SECOND EDITION We have tried to bring the book up to date not only by adding here and there throughout the text the latest results'on the subject, but also by adding two entirely new chapters, and new maps of the best known mutant factors. Much new material has been added to the chapter on sex; the chapter on selection has been largely rewritten. The new chapters are one on heredity in Protozoa, and one on mutation in the evening primrose. In the latter field, the latest results of de Vries and others on Oenothera, and the work on balanced lethals, bid fair to bring the earlier discoveries of de \ries into line with more recent work in the whole field of mutation and inheritance. In place of the “Appendix” we have prepared a small manual for laboratory use (Henry Holt and Co., Publishers) that gives directions for carrying out genetic experiments with the pomace fly. These experiments have been picked out as the ones most PREFACE suitable for student work, and also because they serve to illustrate, in a practical way, most of the fundamental principles of heredity. In addition, the newest culture methods for breeding Drosophila are given, as well as other methods of handling this material. X CONTENTS CHAPTER I MENDELIAN INHERITANCE AND THE CHROMOSOMES PAGE Introduction. The Groups of Linked Factors and the Chromo- somes 1 The Inheritance of One Pair of Factors 8 The Inheritance of Two or more Pairs of Factors ...... 20 CHAPTER II TYPES OF MENDELIAN HEREDITY Dominance and Recessiveness 27 Manifold Effects of Single Factors 32 Similar Effects Produced by Different Factors 36 Modification of the Effects of Factors 38 I. By Environmental Influences 38 II. By Developmental Influences . 42 III. By the Influence of Other Factors 45 IV. Conclusion . 46 CHAPTER III LINKAGE Examples Illustrating “Coupling” 48 Examples Illustrating “Repulsion” 51 Examples of Different Frequencies of Crossing Over 52 The Mechanism of Crossing Over 59 Double Crossing Over 62 The Principle of Interference 64 The Linear Arrangement of Factors shown by Linkage Relations . 64 Linkage in Other Animals and in Plants . . . . 70 The Reduplication Hypothesis 74 XI XII CONTENTS CHAPTER IV SEX INHERITANCE PAGE The Drosophila or XX-XY Type 78 The Abraxas or WZ-ZZ Type 83 What are Sex Factors? 90 Hermaphroditism and Sex 94 Parthenogenesis and Sex 97 The Sex of Individuals Produced by Artificial Parthenogenesis . 106 Sex and Secondary Sexual Characters 106 Parasitic Castration and Secondary Sexual Characters 114 Gynandromorphs and Sex 116 Intersexes and Sex 124 Triploid Intersexes in Drosophila Melanogaster 130 Sex and Sex-determining Gametes 132 Sex-ratios 134 / CHAPTER V THE CHROMOSOMES AS BEARERS OF HEREDITARY MATERIAL The Evidence from Embryology 141 The Individuality of the Chromosomes 151 The Chromosomes during the Maturation of the Germ Cells . . . 155 Crossing Over 164 Other Theories of Crossing-over 168 Attachment of Sex-chromosomes to Autosomes 172 Tetraploidy 174 CHAPTER VI CYTOPLASMIC INHERITANCE The Self-perpetuating Bodies in the Cytoplasm 182 CHAPTER VII THE CORRESPONDENCE BETWEEN THE DISTRIBUTION OF THE CHROMOSOMES AND OF THE GENETIC FACTORS Parallelism between the Distribution of Chromosomes and of Factors 187 1. In Cases of Normal Distribution 187 CONTENTS XIII 2. In Crosses between Species 188 3. In Mutant Races 193 4. In Tetraploid Races 194 Identity of Distribution of the X-chromosomes and of Sex-linked Factors 195 v/ 1. In Ordinary Crosses 195 2. In Cases of Non-disjunction 196 CHAPTER VIII MULTIPLE ALLELOMORPHS Definition of Multiple Allelomorphs 202 Examples of Multiple Allelomorphs 202 */ The Alternative Interpretations of Identical Loci and Complete Linkage 204 CHAPTER IX MULTIPLE FACTORS The Meaning of the Term “Multiple Factors” 219 Examples of Multiple Factor 219 Selection and Multiple Factors 248 CHAPTER X THE FACTORIAL HYPOTHESIS On the Relation between Factors and Characters 262 1. The Organism-as-a-Whole Objection 265 2. The Invariability of the Factor and the Variability in the Character 266 3. So-called Contamination of Allelomorphs 268 4. Fractionation 268 5. The Presence and Absence Hypothesis 270 Weismann’s Prseformation Hypothesis and the Factorial Theory . 277 CHAPTER XI VARIATION IN THE PROTOZOA Fission Lines 2g2 Change in Number of Nuclei 291 XIV CONTENTS Results of Selection 295 Sexual Reproduction 299 CHAPTER XII OENOTHERA AND THE MUTATION THEORY The Mutants of Oenothera Lamarckiana 308 Chromosome Aberrations of Oenothera 310 Gametic and Zygotic Lethals 312 Pseudo-mutations by Crossing-over 316 Bibliography 321 Index 353 THE MECHANISM OF MENDELIAN HEREDITY CHAPTER I MENDELIAN INHERITANCE AND THE CHROMOSOMES Mendel’s fundamental law of segregation was announced in 1865. It is very simple. The units contributed by each parent separate in the germ cells of the offspring without having had any influence on each other. For example, in a cross between yellow- seeded and green-seeded peas, one parent con- tributes to the offspring a unit for yellow and the other parent contributes a unit for green. These units separate in the ripening of the germ cells of the offspring so that half of the germ cells are yellow producing and half are green producing. This sepa- ration occurs both in the eggs and in the pollen. Mendel did not know of any mechanism by which such a process could take place. In fact, in 1865 very little was known about the ripening of the germ cells. But in 1900, when Mendel’s long-forgotten discovery was brought to light once more, a mechan- ism had been discovered that fulfils exactly the Mendelian requirements of pairing and separation. The sperm of every species of animal or plant 2 MENDELIAN SEGREGATION carries a definite number of bodies called chromo- somes. The egg carries the same number. Conse- quently, when the sperm unites with the egg, the fertilized egg will contain the double number of chromosomes. For each chromosome contributed by the sperm there is a corresponding chromosome con- tributed by the egg, i.e., there are two chromosomes of each kind, which constitute a pair (Fig. 1, a). When the fertilized egg divides, every chromo- some splits into two chromosomes, and these two daughter chromosomes then move apart, going to opposite poles of the dividing cell (Fig. 1, c). Thus each daughter cell (Fig. 1, d) receives one of the daughter chromosomes formed from each original chromosome. The same process occurs in all cell divisions, so that all the cells of the animal or plant come to contain the double set of chromosomes. The germ cells also have at first the double set of chromosomes,but when they are ready to go through the last stages of their transformation into sperm or eggs the chromosomes unite in pairs (Fig. 1, e). Then follows a different kind of division (Fig. 1,/) at which the chromosomes do not split but the members of each pair of chromosomes separate and each member goes into one of the daughter cells (Fig. 1, g, h). As a result each mature germ cell receives one or the other member of every pair of chromosomes and the number is reduced to half. Thus the behavior of the chromosomes parallels the behavior of the Mendelian units, for in the germ cells each unit derived from the father separates from the MENDELIAN SEGREGATION 3 corresponding unit derived from the mother. These units will henceforth be spoken of as factors; the two factors of a pair are called allelomorphs of each Fig. 1.—In the upper line, four stages in the division of the egg (or of a body cell) are represented. Every chromosome divides when the cell divides. In the lower line the “reduction division” of a germ cell, after the chromosomes have united in pairs, is represented. The mem- bers of each of the four pairs of chromosomes separate from each other at this division. other. Their separation in the germ cells is called segregation. The possibility of explaining Mendelian phenomena 4 MENDELIAN SEGREGATION by means of the manceuvers of the chromosomes seems to have occurred to more than one person, but W. Sutton (1902) first presented the idea in the form in which we recognize it today. More- over, he not only called attention to the fact above mentioned, that both chromosomes and hereditary factors undergo segregation, but showed that if the pairs assort independently, Mendel’s second law (“assortment”) is fulfilled. Mendel had found that when the inheritance of more than one pair of factors is followed, the different pairs of factors sort out independently of one another. Thus in a cross of a pea having both green seeds and tall stature with a pea having yellow seeds and short stature, the fact that a germ cell receives a particular member of one pair (ie.g., yellow) does not determine which member of the other pair it receives; it is as likely to receive the tall as the short. Sutton pointed out that in the same way the segregation of one pair of chromosomes is probably independent of the segregation of the other pairs. It was obvious from the beginning, however, that there was one essential requirement of the chromo- some view, namely, that all the factors carried by the same chromosome should tend to remain together. Therefore, since the number of inheritable characters may be large in comparison with the number of pairs of chromosomes, we should expect actually to find not only the independent behavior of pairs, but also cases in which characters are linked together in groups in their inheritance. Even in species where a limited MENDELIAN SEGREGATION 5 number of Mendelian units are known, we should still expect to find some of them in groups. In 1906 Bateson and Punnett made the discovery of linkage, which they called gametic coupling. They found that when a sweet pea with factors for purple flowers and long pollen grains was crossed to a pea with factors for red flowers and round pollen grains, the two factors that came from the same parent tended to be inherited together. Here was the first case that gave the sort of result that was to be ex- pected if factors were in chromosomes, although this relation was not pointed out at the time. In the same year, however, Lock called attention to the possible relation between the chromosome hypothesis and linkage. In other groups a few cases of coupling became known, but nowhere had the evidence been sufficiently ample or sufficiently studied to show how frequently coupling occurs. Since 1910, however, in the fruit fly, Drosophila melanogaster, a large number of new characters have appeared by mutation, and so rapidly does the animal reproduce that in a relatively short time the inheritance of more than a hundred char- acters has been studied. It became evident very soon that these characters are inherited in groups. There is one great group of characters that are sex linked. There are two other groups of characters slightly greater in number. Finally a character appeared that did not belong to any of the other groups, and a year later still another character appeared that was linked to the last one but was independent of all the 6 MENDELIAN SEGREGATION other groups. Hence in Drosophila there are four groups of characters, a partial list of which follows: GROUP I tan, t tiny-br’, tb nick, vgn oblique dwarf, dw dwarf-b Abnormal, A tinged, w( olive ebony, e Bar, B vermilion, v pads ebony4, e4 Bar-def’y Verm.-defy Pale-n, Pii Extended, De bifid, bi Verm.-dup. patched giant, gt blood, w6 white, w pinkish glass, gl bordered, bd yellow, y Plus-mo d.-D Hairless, H broad, br buff, wy GROUP II pink-wing, pw purple, pr hairy, h lntensifier-Bd cherry, wc abrupt purploid, pd Intensifier-s cleft, cf amethyst reduced, rd lntensifier-T club, cl antlered, vg® roof kidney, k crooked, fwc apterous, ap safranin, sf lethals (9) crossv’less, cv arc, a aristaless, al scraggly, rd3 mahogany cut, ct Ski-n, Si maroon, ma cut3, ct3 balloon, ba sienna, pr3 Minute, M cut6, ct6 black, b Snub, T3 Minute-dm depressed blistered, bs square, Tfl« Minute-f double brown, bw speck, sp Minute-g dusky, dy chubby Star, S olive-m dusky2, dy2 cinnabar, cn strap, vg3 Pale-m, Pm echinus, ec Confluent, Cf. straw, sw peach, pp ecru, wec Cream-n Streak, Sk pink, p eosin, we cream-b telegraph, tg Pointed-wing facet, fa cream-c telescope, ts Roof-c forked, f ClIL translucent, tl rotated-abd. furrowed, fw ClIR trefoil, tf rough, ro fused, fu Cus Truncate, T roughoid, ru garnet, g Curly, Cy vortex, T“ safranin-b garnet2, g2 curved, c yellowish scarlet, st ivory, w* lemon dachs, d Dachs-def’y GROUP III sepia, se ski-m, si lethals (50) dachsoid ascute, as smudge lozenge, lz dachsous band, bn sooty, e3 lozenge2, lz2 dash Beaded, Bd spineless, ss miniature, m Detached benign-tumor spread, sd Notch, N (18) expanded, ex bithorax, bx tilt, tt prune, pn roughish, rh flipper, fp bithorax-b tumor, tu fringed, fr cardinal, cd Two-bristles ruby, rb gap-vein claret, ca varnished, vr ruby2, rb2 Gull, G cream-m vortex-m rudimentary, r rud’y7, r7 sable, s humpy, hy compressed warped, wp jaunty, j curled, cu white-ocelli, wo lethals (9) Cm with Sable-dup. scute, sc Lobe, L Lobe 2, L 2 Cm, n Chip GROUP IV short, br3 Minute-b Deformed, Df bent, bt singed, sn Minute-dn Delta, A bent2, bt2 small-eye, sy Minute-e Dichsete, D eyeless, ey small-w’g, si morula, mr dilute eyeless 2, ey 2 spot, y3 narrow, nw divergent, dv shaven, sv MENDELIAN SEGREGATION 7 The four pairs of chromosomes of the female of Drosophila are shown in Fig. 2 (to the left). There are three pairs of large chromosomes and one pair of small chromosomes. One of the four pairs is the pair of sex or X chromosomes. In the male, Fig. 2 (to the right), there are likewise three pairs of large chromosomes and a smaller pair. The two sex chro- mosomes in the male have been found to be dis- Fig. 2.— Diagram of female and of male group (duplex) of chromosomes of Drosophila melanogaster showing the four pairs of chromosomes. The hook on the Y chromosome is characteristic. The members of each pair are usually found together, as here. tinguishable from each other in shape. This distinc- tion was first observed in the oogonial figures of the XXY females that had arisen through non-dis- junction. Satisfactory figures of the spermatogonial groups were much more difficult to obtain, but these also showed that the Y was J-shaped and somewhat longer than the X. In length the chromosomes are in the ratio: X = 100: Y = 112: II = 159: III = 159: IV = 12. Stevens’ work had seemed to show 8 MENDELIAN SEGREGATION that the X chromosome is attached to another chromosome and that there is no Y chromosome. In the earlier papers on Drosophila this relation of the chromosomes was assumed to be correct and the female was represented as XX and the male as XO. In Drosophila, then, there is a numerical corre- spondence between the number of hereditary groups and the number of the chromosomes. Moreover, the size relations of the groups and of the chromosomes correspond. The method of inheritance of the factors carried by these chromosomes will now be considered more in detail. The Inheritance of one Pair of Factors The inheritance of a single pair of characters may be illustrated by the following examples from Droso- phila, one from each of the four groups. The mutant stock called vestigial is so char- acterized because it has only small vestiges of the wings. If a fly with vestigial wings is mated to the wild type with long wings (Fig. 3, Pi), the offspring will have long wings (Fig. 3, Fi). If these hybrid flies of the first generation (the first filial generation, or Fi) are mated to each other, their offspring (or F2) will be of two sorts: some will have long wings and others will have vestigial wings. There will be three times as many flies with long wings as flies with vestigial wings. This is the Mendelian ratio of 3:1 that appears when a single pair of characters is involved. MENDELIAN SEGREGATION 9 Fig. 3.—Vestigial winged by long winged (wild-type) fly. The second chromosome that carries the recessive factor for vestigial is here rep- resented by the oval containing the letter v. The “normal” second chromosome contains here the capital letter V. 10 MENDELIAN SEGREGATION If the factors for vestigial wings are carried by a pair of chromosomes (the chromosomes carrying v in Fig. 3) then at the ripening of the germ cells (eggs and sperm) such a pair of chromosomes will come together and at reduction separate; so that each germ cell will have one such chromosome and not the other. (See Fig. 1, e-h.) If such a germ cell fertilizes an egg of the wild fly that contains a similar group of chromosomes, ex- Fig. 4.—(A.) Fertilization of egg by sperm. (B.) Zygote formed by union of egg and sperm. (C.) Diploid nucleus. cept that the corresponding chromosome carries the factor for long wings (Fig. 4, A), the result will be to produce a fertilized egg (Fig. 4, C) in which one mem- ber of the pair of chromosomes in question comes from the mother and carries the factor for long, and the other comes from the father and carries the factor for vestigial wing. Since this egg with both factors present produces a fly with long wings, the vestigial character is said to be recessive to the long; or conversely the long is said to be dominant to the vestigial character. When the eggs and the sperm of hybrid flies of this origin come to maturity, the homologous chromo- MENDELIAN SEGREGATION 11 somes conjugate in pairs, as shown diagrammatically in Fig. 5, b: The chromosomes then separate (Fig. 5, c and d) at the time of division of the cell, and one of the resulting daughter cells gets the chromosome bearing the vestigial, and the other daughter cell gets the homologous chromosome, bearing the long factor. Hence, there will be two kinds of eggs in the female and two kinds of spermatozoa in the male. When two such hybrid flies mate with each other, any Fig. 5.—Diagram to illustrate in a heterozygous individual the con jugation and segregation of the chromosomes during “reduction.” sperm may meet and fertilize any egg. The possible combinations that result, and the frequency with which they occur, are shown in the next diagram (Fig. 6, and also in Fig. 3.) As shown in this diagram, a sperm bearing the fac- tor for long fertilizing an egg bearing the same factor produces a fly pure (homozygous) for long wings; a sperm bearing the factor for long fertilizing an egg bearing the factor for vestigial wings produces a hy- brid fly (heterozygous) that has long wings, since, as above, the long “dominates” the vestigial character. 12 MENDELIAN SEGREGATION Similarly, a sperm bearing the factor for vestigial fertilizing an egg bearing the factor for long produces a hybrid, or heterozygote, with long wings; a sperm bearing the factor for vestigial fertilizing an egg Fig. 6.—Diagram to illustrate how by the random meeting of two kinds of sperm and two kinds of eggs the typical 3:1 ratio results. bearing the same factor produces a homozygote, having the recessive character vestigial wings. Since the sperm and the eggs meet at random there should be 1 long W, to 2 long Vv, to 1 vestigial vv; or, putting together all flies with long wings, 3 long to 1 vestigial. Three to one is the character- MENDELIAN SEGREGATION 13 istic Mendelian ratio when one pair of characters is involved. In a third-chromosome stock, ebony, the body and wings are very dark in contrast to the wild fly whose color is “gray.” Gray is used to designate the color of the wild fly, whose wings are gray, but whose body is yellowish with black bands on the abdomen. If ebony is crossed to gray the offspring (Fi) are gray but are somewhat darker than the ordinary wild flies. When these hybrids are inbred they give (F2) 1 gray, to 2 intermediates, to 1 ebony. The group of inter- mediates in the second generation (F2) can not be separated accurately from the pure gray type. If they are counted as gray, the result is three grays to one ebony. Since ebony and gray assort independently of long and vestigial, as will be shown later, the factor for ebony must be supposed to be carried by a chromo- some of a different pair from the one that carries vestigial. Since this chromosome behaves in the same way as does the one that bears the vestigial factor, the scheme used for vestigial will apply here also. Another mutant stock is characterized by small eyes, and since in the extreme form it may lack one or both eyes entirely (Fig. 7), the name “eyeless” has been given to this mutant. When this stock is bred to wild flies the offspring have normal eyes. These inbred give three normal to one eyeless fly. As shown in the table on page 6, this character belongs in still another, the fourth, group, and its 14 MENDELIAN SEGREGATION mode of inheritance is explicable on the supposition that it lies in the fourth pair of chromosomes. For an adequate understanding of the inheritance Fig. 7.—Normal eyes of Drosophila a, a'. Eyeless b-d; b, b1 top and side view of head of fly without eyes ;c,c' right and left eyes of another fly; d, small eye on right side, none on left. of factors in the first group it will be necessary to consider the distribution of the sex chromosomes (Fig. 8). In the female of Drosophila there are two X chromosomes (XX). After the conjugation and MENDELIAN SEGREGATION 15 separation of the X chromosomes in the female there is one X chromosome left in each egg. In the male there is one X chromosome and another chromosome, its mate, called the Y chromosome. Hence in the male there are two classes of spermatozoa: one containing X, the other Y. If a Y-bearing spermatozoon should Fig. 8.—Diagram to show the history of the sex chromosomes from one generation to the next. fertilize an egg the result will be an XY individual, or male. It is evident that the Y chromosome is found only in the males, while an X chromosome passes not only from female to female, but also from female to male and from male to female. As will be shown now, certain factors follow* the distribution of the X chromosomes and are there- 16 MENDELIAN SEGREGATION fore supposed to be contained in them. These factors are said to be sex linked. The inheritance of white eyes may serve as an illustration for the entire group of sex linked char- acters. If a white-eyed male is bred to a red-eyed female (wild type) (Fig. 9), the sons and daughters (Fi) have red eyes. If these are inbred the offspring (F2) are three reds to one white, but the white-eyed flies are all males. If we trace the history of the sex chromosomes we can see how this happens. In the red-eyed mother, each egg contains an X chromosome bearing a factor for red eyes. In the white-eyed father, half of the spermatozoa contain an X chromosome which carries a factor for white eyes, while the other half contain a Y chromosome which carries no factors (Fig. 9). Any egg fertilized by an X-bearing spermatozoon of the white-eyed father will produce a female that has one red-producing X chro- mosome and one white-producing X chromosome (Fig. 9). Her eyes are red, because red dominates white. Any egg fertilized by a Y-bearing spermato- zoon of the white-eyed father will produce a son (Fig. 9) that has red eyes, because his X chromo- some brings in the red factor from the mother, while the Y chromosome does not bring in any dominant factor. At the ripening of the germ cells in the Fi female the number of chromosomes is reduced to half. There result two kinds of eggs, half with the red-bearing and half with the white-bearing X (Fig. 9). Similarly in the male there will be two classes of sperm, half with the red-bearing X chromosome, MENDELIAN SEGREGATION 17 Fig. 9.—Red-eved female by white-eyed male (D. melanogaster). This is the reciprocal of the cross shown in Fig. 10. 18 MENDELIAN SEGREGATION half with the indifferent Y chromosome. Random meeting of eggs and sperm will give the result shown in the lower line of the diagram. There will be a 3 :1 ratio, as in other Mendelian crosses, but the white individuals in F2 will be males. The factor for red in the F i male will always stay in the X chromosome, so that all the female-producing spermatozoa will carry red, and consequently all F2 females will be red. The males will have red eyes if they receive the red- bearing chromosome from their mother and white eyes if they receive the white-bearing chromosome from their mother. The reciprocal cross is made by mating a white- eyed female to a red-eyed male (Fig. 10). The daughters will have red eyes and the sons white eyes. If these are inbred their offspring will be red and white in equal numbers, and not the usual three reds to one white. The explanation of this new ratio is at once apparent as soon as the history of the sex chromosomes is studied. The two X chromosomes in the white-eyed mother carry the factor for white eyes. After ripening, each egg carries one white-bearing X chromosome. The single X chromosome of the female-producing sper- matozoon of the red-eyed father carries the factor for red eyes; the male-producing spermatozoa carry the Y chromosome which, as stated above, is indifferent. Any egg fertilized by a spermatozoon containing the red-bearing X chromosome will produce a red daugh- ter, because red dominates white. Conversely, any egg fertilized by the Y-bearing male-producing sper- MENDELIAN SEGREGATION 19 Fig. 10.—White-eyed female by red-eyed male (D. melanogaster). The factors for these characters are carried by the X chromosomes. In this diagram red is indicated by the symbol W and white by the symbol w. The history of the chromosomes is shown in the middle of the diagram. 20 MENDELIAN SEGREGATION matozoon will produce a white-eyed son, because the only X chromosome that the son contains is derived from his mother, both of whose X chromosomes carry a white-producing factor. When these red-eyed daughters and white-eyed sons are inbred the possible combinations are shown in the lower line of the diagram (Fig. 10). There will be two kinds of eggs, one containing a red- bearing, the other a white-bearing, X chromosome. The female-producing spermatozoa will contain a white-bearing X chromosome; the male-producing spermatozoa will contain a Y chromosome. A red- bearing egg fertilized by a female-producing sper- matozoon will produce a red-eyed female; a white- bearing egg fertilized by a female-producing spermato- zoon will produce a white-eyed female. A red-bear- ing egg fertilized by a male-producing spermatozoon will produce a red-eyed male; a white-bearing egg fertilized by a male-producing spermatozoon will produce a white-eyed male. The resulting ratio is 1 red to 1 white, in both sexes. The distribution of the chromosomes explains how in one cross the Mendelian ratio of 3:1 obtains, and also how in the reciprocal cross there is a 1:1 ratio. The Inheritance of Two or More Independent Pairs of Factors The application of the chromosome hypothesis to crosses between races that differ in two pairs of factors is illustrated by the following example (Fig. MENDELIAN SEGREGATION 21 11). If a vestigial gray fly is mated to a long-winged ebony fly, all the offspring (Fi) will have long wings and gray (or slightly darker) body color. If these hybrids (Fi) are inbred, offspring (F2) will be pro- duced in the ratios: 9 Flies with long wings and gray body color. 3 Flies with vestigial wings and gray body color. 3 Flies with long wings and ebony body color. 1 Fly with vestigial wings and ebony body color. In the diagram (Fig. 11) the two pairs of chro- mosomes that carry the genetic factors in question are represented by short rods. In the vestigial fly recessive factors for vestigial (v) are in the “second” chromosome. This same fly has two “third” chro- mosomes that carry only normal factors, hence a pair of factors normal for ebony (E). In the ebony fly the third chromosomes carry recessive factors for ebony (e), while the second chromosomes carry the normal factors for vestigial (Y). The formulae for the two parents are vvEE and YVee, and their germ cells, respectively, vE and Ye. The Fi fly will have the composition vVEe, and will show neither the vestigial nor the ebony char- acter. It is heterozygous in each pair of factors— i.e. one member of the second pair of chromosomes carries v, the other V; similarly, for the third pair of chromosomes, one member carries the factor e and the other the normal allelomorph E. In the maturation of the germ cells of the hybrid, the members of each pair separate from each other as shown in Fig. 11 in the gametogenesis of Fx. 22 MENDELIAN SEGREGATION Fig. 11.—Diagram illustrating assortment of two mutant characters, vestigial and ebony. MENDELIAN SEGREGATION 23 The two pairs of chromosomes “assort” on the spindle in either one of the two ways shown in the diagram; resulting in four and only four kinds of gametes. Fig. 12.—Diagram to show the 16 possible kinds of permutations of the four kinds of gametes of Fig. 11. Along the top line are four kinds of eggs; along the left side are four kinds of sperm; in the squares are the combinations formed by the meeting of each kind of egg with each kind of sperm, giving 9 long gray; 3 long ebony; 3 vestigial gray; 1 vestigial ebony. The process just described takes place both in the male and in the female. Consequently there will be four kinds of eggs and four kinds of spermatozoa. 24 MENDELIAN SEGREGATION Chance meeting between these will give the results shown in the next diagram (Fig. 12). In the table (Fig. 12) the four kinds of eggs are represented at the head of the four vertical columns, and the four kinds of spermatozoa at the left of each horizontal row. In the squares the combination of each kind of sperm with each kind of egg is repre- sented, giving the ratio of 9 long gray: 3 vestigial gray: 3 long ebony: 1 vestigial ebony. The F2 expectation may, of course, be derived more directly as follows: There will be 3 long to 1 ves- tigial. These longs will be both gray and ebony in the ratio again of 3 to 1; hence 9 long gray to 3 long ebony. Correspondingly, the vestigials will be both gray and ebony, in the ratio of 3 to 1; hence 3 vestigial gray to 1 vestigial ebony. The result is the same as before. If one of two independent pairs of characters is sex linked, the same scheme holds in those cases where the recessive sex linked character enters through the grandfather, but the ratio is different when the re- cessive sex linked character enters through the grandmother (viz., 3 :3 :1 :1), as is to be expected from the mode of inheritance of white eyes taken alone ;x and here, too, the result conforms fully to the chromosome scheme. Three factors can be worked out by means of the 1For example, taking white and red alone the ratio of the F2 is 1:1. But among the reds the ratio of gray to ebony will be 3 :1 and among the whites will be 3 :1. Hence the result 3 red gray, 1 red ebony, 3 white gray, 1 white ebony. MENDELIAN SEGREGATION 25 chromosomes as readily as one or two. It will not be necessary to give the full analysis, for it will be easily understood from the scheme already given. If a fly with vestigial wings is crossed to an ebony, eyeless fly three pairs of factors are involved that lie in different chromosomes. The Fi flies are normal, for there is in the hybrid a normal mate for each of the three recessive factors. The possible recombina- tions are shown in the next diagram, Fig. 13. There Fig. 13.—Diagram to show the segregation of the three pairs of chro- mosomes. Eight combinations are possible, giving 8 kinds of germ cells, with 64 possible re-combinations. are four different positions for the chromosome pairs on the spindle, leading to eight kinds of germ cells. By chance meetings of the eight kinds of sperm with the eight kinds of eggs there will result 8 types as follows: 27 Long, gray, normal eye (wild type). 9 Vestigial, gray, normal eye. 9 Long, ebony, normal eye. 9 Long, gray, eyeless. 3 Vestigial, ebony, normal eye. 3 Vestigial, gray, eyeless. 3 Long, ebony, eyeless. 1 Vestigial, ebony, eyeless. 26 MENDELIAN SEGREGATION The same manner of treatment will work for more than three pairs of chromosomes; the number of kinds of germ cells increases in geometrical ratio. In most animals and plants the number of chromo- somes is higher than in Drosophila, and the number of pairs of factors that may show independent assort- ment is, in consequence, increased. In the snail, Helix hortensis, the half number of the chromosomes is given as 22; in the potato beetle 18; in man, prob- ably, 24; in the mouse 20; in cotton 28; in the four- o’clock 16; in the garden pea 7; in corn 10; in the evening primrose 7; in the nightshade 36; in tobacco 24; in the tomato 12; in wheat 8. If 20 pairs of chromosomes are present there will be over one million possible kinds of germ cells in the Fi hybrid. The number of combinations that two such sets of germ cells may produce through fertilization is enormously greater. From this point of view we can understand the absence of identical individuals in such mixed types as the human race. The chance of identity is still further decreased since in addition there may be very large numbers of factors within each chromosome. CHAPTER II TYPES OF MENDELIAN HEREDITY Experience has shown that Mendelian inheritance applies to all sorts of characters, structural, physio- logical, pathological, and psychological; to characters peculiar to the egg, to the young, and even to old age; to length of life; to fundamental taxonomic characters as well as to “superficial” characters; and to characters intimately concerned in maintaining the life of the individual, as well as to characters which apparently do not influence survival. Some of these different types and their mode of inheritance will be briefly described, but since the general principles in- volved are more important than the kind of character that is affected, the results will be treated under general headings. Dominance and Recessiveness The four-o’clock (Mirabilis jalapa) has a white and a red-flowered variety. If these are crossed the hy- brid is pink in color. The pink hybrid inbred (self- fertilized in this case) gives in the next generation (F2) one red, to two pink, to one white (Fig. 14). Owing to the intermediate color of the hybrid (or heterozygote) it is impossible to say that either color dominates the other. The factor for red and 28 TYPES OF MENDELIAN HEREDITY the factor for white both affect the plant in which they occur. In this and in similar cases the F2 ratio of 1 :2 :1 is obtained, because it is possible to distinguish the pure red and the pure white from the heterozygous plants. Fig. 14.—Diagram to illustrate the cross between a red and a white flowered Mirabilis jalapa (4 o’clock), which produces a pink, intermediate heterozygote. The Andalusian fowl is a similar case. When certain races of black are bred to certain races or kinds of “white” the hybrid is slate “blue” in color. These blue birds, called Andalusians, when inbred, give one black to two blue to one white. Blue is TYPES OF MENDELIAN HEREDITY 29 the heterozygous condition; it is not possible to produce a pure breeding race of Andalusians, for the combination that produced an Andalusian falls apart in the germ cells of the Andalusian birds. The bird is blue because the pigment is not spread evenly over the feather but is restricted to small but black specks. Fig. 15.—Normal (a, a') and bar eye (b, b') of Drosophila; shown in side view, and as seen from above. The Andalusian blue is a mosaic of black and white, and not at all a dilute black. A good example of an intermediate hybrid is found when the mutant fly with bar eye (Fig. 15) is bred to a wild fly. The daughters have bar eyes that are not as narrow as those of the pure bar stock. The range of variation is great, however, for some of the hybrids have eyes that are nearly as round as the normal, and 30 TYPES OF MENDELIAN HEREDITY in others the eye is nearly as narrow a bar as that of pure stock. In the male, which has one factor for bar eye, the eye is as narrow as in the pure (i.e., homozygous) female with two factors. The inter- mediate condition in the female which is hybrid (heterozygous) for this factor is hence not explained by the lesser effect of the single factor, but is probably due to the competing influence of the other allelo- morph. Of course it might be contended that since in the male there is a different chromosome complex (XABCD YABCD) from that in the female (XABCD- XABCD) it is this difference in other factors that causes the heterozygous female to have a wider eye than the male; but this argument is rendered improb- able here, when we recall that in only one out of many cases of sex linked inheritance, in which the hetero- zygous female is intermediate, is the male different from the homozygous female. In other cases the influence of one of the parents of the cross may be so slight as to escape detection on ordinary observation, and may require special measurements for demonstration. When flies with miniature wings (Fig. 16) are mated to wild flies, the daughters have long wings, which Lutz has shown to be a little shorter in proportion to the length of the legs than are the wings of wild females; but the difference is so slight that it could not have been detected without biometrical methods. Finally, we must consider the class of cases in which complete dominance has been described. All the cases given by Mendel in peas were supposed TYPES OF MENDELIAN HEREDITY 31 to fall under this heading: yellow dominates green, round dominates wrinkled, etc. Whether a character is completely dominant or not appears to be a matter of no special significance. In fact the failure of many characters to show complete dominance raises a doubt as to whether there is such Fig. 16.—a, Long wing (wild type) of Drosophila; b, miniature wing, (ia and b are not drawn to scale.) a condition as complete dominance. Some cases ap- proach so nearly to that condition that special tests may be required to show that the hybrid is affected by the recessive factor. For instance, in flies the factor for white eyes seems to produce no effect when white is bred to red. The Fi reds are indis- tinguishable from pure reds. But by weakening the red by adding recessive factors other than white, the influence of white can be demonstrated, as Mor- 32 TYPES OF MENDELIAN HEREDITY gan and Bridges have shown. Therefore although the effect of the white factor can not be detected in the single combination with red, it is reasonable to sup- pose that some effect is really present. Similarly, conditions were found in which the effect of hetero- zygosis for eosin, vermilion, or pink could be demon- strated. While the question is one of only sub- sidiary importance, yet in the separation of classes it is often useful to be able to distinguish the pure from the hybrid form; but whether this can or can not be done in any given case does not affect the funda- mental principle of segregation which is the essential feature of Mendel’s discovery. Manifold Effects of Single Factors It is customary to speak of a particular character as the product of a single factor, as though the factor affected only a particular color, or structure, or part of the organism. But everyone familiar at first hand with Mendelian inheritance knows that the so-called unit character is only the most obvious or most sig- nificant product of the postulated factor. Most students of Mendelian heredity will freely grant that the effects of a factor may be far-reaching and manifold. A few examples may make this plain. In Drosophila there is a mutant stock called “club,” in which the wing pads fail to unfold (Fig. 17) in about 20 per cent, of the flies. In the majority of club flies the wings expand fully, and are like those of the wild fly. Owing to this fact, that not all the TYPES OF MENDELIAN HEREDITY 33 flies even in a pure stock of club show this character, it was difficult to study the inheritance of the supposed factor that sometimes inhibits the unfolding of the wing pads. Nevertheless, it was possible even with this handicap to show that the character depended on a sex linked recessive factor. Later Fig. 17.—Club wing (to left). The absence of the spines on the side of the thorax in “club” is shown in c, and the normal condition is shown in b. the discovery was made that a particular pair of spines always present on the side of the thorax of the wild flies, is absent from the club flies, irrespective of whether the wings do or do not unfold (Fig. 17, c). This constant feature of the mutant made its study quite simple. Another pair of spines, those upon the 34 TYPES OF MENDELIAN HEREDITY rear margin of the scutellum, point constantly in an abnormal direction in club stock. The head of club flies is often flattened, the eyes are smaller, and the thorax and abdomen are somewhat distorted. Here we have an example of a single germinal difference, the factor for club, producing several distinct effects, Fig. 18.—Rudimentary wing (to left), and truncate wing (to right) some of which are constant features of the stock, while others are occasional or variable. Another and similar example is found in the rudi- mentary winged flies (Fig. 18, a). The wing is usually shorter than the abdomen, but may be longer and even approach the normal wing in length and shape. The TYPES OF MENDELIAN HEREDITY 35 last pair of legs are often thicker and shorter. If many larvse are present, or the food conditions poor, the larvse of rudimentary flies can not stand the compe- tition and die off, and in consequence the rudimentary class is smaller than expected. The males are fertile, but the females are almost entirely sterile, although rarely one of them may lay a few eggs and some of these hatch. The infertility is probably due to ab- sence or rareness of mature eggs in the ovaries. There are also other effects than these four men- tioned, all of which are produced by the same factor, and, no doubt, were our knowledge complete, we should find in all mutants many differences in addi- tion to the ones picked out for study and called “unit characters.” DeVries’ definition of mutation en- tirely covers this relation; in fact, it even goes further and implies that a single difference may affect the entire organization. Perhaps this does occur, but practically the number of differences that can be observed between a wild and a mutant stock derived from it, is limited. The attack that is some- times made on the unit character hypothesis fails in its intention the moment it is understood that a single factor (difference) has generally not one but many effects. Most workers in Mendelian heredity are fully conversant with these facts. This attack on the unit character conception is usually made by those not familiar with the actual situation and who take the expression unit character too literally. It may be conceded that the expression has at times been abused even by some of Mendel’s followers. 36 TYPES OF MENDELIAN HEREDITY Similar Effects Produced by Different Factors There are many cases in which characters that are superficially alike are the product of different factors. White color that characterizes so many domesticated races of plants and animals is a case in point. There are two pure breeding races of white flowered sweet peas. When crossed, they produce colored flowers. When the Fi offspring are inbred the F2 generation consists of 9 reds to 7 whites. This 9:7 ratio is a special case of the 9 :3 :3 :1, in which the last three classes are superficially alike. The explanation here is that there are two kinds of recessive whites that have originated independently. On the chromo- some hypothesis one white is due to mutation in one chromosome and the other white to mutation in an- other chromosome. When the races are crossed, each race supplies that chromosome which contains the normal factor of the white of the other race. In the F2 generation any plant that contains at least one of the normal chromosomes of both pairs will not be white. There will be nine such cases. Any plant that contains both of the white-producing chro- mosomes of either pair will be white. There will be seven such cases. There are also two pure races of white fowls that, when crossed, give colored birds. Each white behaves as a recessive to color. For instance, the white silky crossed to a white dorking gives colored birds. These inbred give 9 colored to 7 white birds. TYPES OF MENDELIAN HEREDITY 37 There is a third kind of white race of poultry, namely, white Leghorn, in which white is dominant. Crossed to colored birds the offspring are white (with often a few colored feathers, which indicates that dominance is not complete). In the silkworm also a dominant white and a reces- sive white factor have been found. The genetic results are comparable in all respects to those in the fowl. There are also cases of blacks or melanic types, that have different factorial bases. There are three black races of Drosophila—called sable, black, and ebony—that belong respectively to the first, second, and third groups. These are much alike, but close scrutiny reveals slight differences. Any two crossed together give gray Fi flies. There are three pink eye colors in Drosophila, one whose locus is in the third chromosome (pink), and two sex linked eye colors which are so similar that no certain difference between them can be observed. Not only pigment but also structural characters may parallel each other in a remarkable manner. For example, in Drosophila the mutant stocks “bow” (sex linked) and “arc” (II chromosome) have wings that curve evenly downward over the abdomen. There are also two kinds of flies whose wings turn up sharply near the ends. These stocks are 1 ‘ j aunty ’’ (second chromosome) and “jaunty I,” which is sex linked. Two types, called “fringed” (II chromosome) and “spread” (III chromosome), are characterized by thin textured wdngs held out nearly at right 38 TYPES OF MENDELIAN HEREDITY angles to the body. In the case of rudimentary and truncate (Fig. 18) the wings are so similar that without breeding tests one of them might easily be taken for the other. Finally, “facet” and “rough” both have the ommatidia of the eye disarranged very much in the same way. Modification of the Effects of Factors /. By Environmental Influences It is a commonplace that the environment is es- sential for the development of any trait, and that traits may differ according to the environment in which they develop. In most cases different genetic types produce different results in any ordinary environment. The environment, being common to the two, may therefore in such cases be ignored, or rather taken for granted. There are other cases, however, in which a particular genetic type appears different from another one only in a special environ- ment. Where this environment is not the normal one, its discovery is an essential element of the experiment. One of the best cases is that given by Baur. The red primrose (Primula sinensis rubra) reared at a tem- perature of 30°-35° C. (with moisture and shade) has pure white flowers, but the same plants reared at 15°-20° have red flowers. If the white-bearing plants are brought into a cooler place, the flowers that are already in bloom remain white, but those that de- velop later in the cooler temperature are red. There TYPES OF MENDELIAN HEREDITY 39 is another race of primula (Primula sinensis alba) that always has white flowers, even at 20°. Strictly speaking, we should say, not as we generally do for brevity’s sake, that the difference between the two races is that one has white, the other red flowers, but we should say rather that P. rubra reacts at 20° by producing red, at 30° by forming white flowers; P. alba, on the other hand, reacts both at 20° and at 30° by producing white flowers. The constant dif- ference between these races is not in their color, but in the possibility of producing specific colors at certain temperatures. This is the point of view, of course, that must also be taken for cases in which differences exist in all the usual environments; for, here also, it is the different possibilities of reaction that are inherited. Brevity warrants us in speaking of particular characters as inherited, rather than the specific possibility of reac- tion that gave these characters; but no one need be misled by the shorter expression. Two similar cases of the influence of the environ- ment have been found in Drosophila. There is a mutant stock known as abnormal abdomen in which the normal black bands of the abdomen are broken and irregular or even entirely absent (Fig. 19). In flies reared on moist food the abnormality is extreme; but even in the same culture the flies that continue to hatch become less and less abnormal as the culture becomes more dry and the food scarce, until finally the flies that emerge later can not be told from normal flies. If the culture is kept well fed the change does 40 TYPES OF MENDELIAN HEREDITY not occur, but if the flies are reared on dry food they are normal from the beginning. The character is a sex linked dominant, as shown by the following crosses. When an abnormal male is bred to a normal (wild) female, the daughters are abnormal (if the Fig. 19.—Mutant type called Abnormal Abdomen of Drosophila ampelophila (the wings have been cut off); a is female; b, male; c, female that approaches the normal type. food is moist), but all the sons are normal. If the medium is dry, however, both the daughters and the sons alike are normal. But these normal F i daughters will produce the expected abnormal offspring if the conditions are suitable, and these offspring are just as TYPES OF MENDELIAN HEREDITY 41 abnormal as though the female had herself been abnor- mal. The reciprocal cross, viz., abnormal females by normal males, gives abnormal sons and daughters, if the food is suitable, but normal if the food is dry, etc. In both cases the F2 gives the expectation for a sex-linked dominant factor if the medium is suited to bring out the abnormal character, and the result is entirely ob- scured if the food is dry. Here, at will, we can demon- strate a regular Mendelian ratio by control of the environment, and conversely, we can conceal com- pletely what is taking place by substituting another environment. That the same genetic process is going on in both cases can be demonstrated by suitable tests. A case similar in principle occurs in a mutant stock of Drosophila that produces supernumerary legs. This stock was observed in winter to produce a con- siderable percentage of flies with supernumerary legs, but few or none in summer, especially in warm weather. Miss Hoge, who has studied this stock, finds that when the flies are kept in an ice chest at a temperature about 10° C. a high percentage of flies with supernumerary legs occurs. Sometimes several legs or parts of a leg are doubled, or the doubling may occur twice in the same leg. The general rule that Bateson pointed out for duplicated legs in other insects appears to hold here, viz., the adjacent parts are mirror images of each other. In the cold the duplicate leg gives a regular Mendelian result; but at normal temperature the duplication is a rare event and its mode of inheritance 42 TYPES OF MEN DELIAN HEREDITY obscured. In a hot climate there would be no evi- dence that such a factor was being regularly trans- mitted. But if the type moved into a cold region it would show duplication in many of the legs. II. By Developmental Influences “Age/’ too, is in a sense an environmental condi- tion, which influences the development of characters. Thus a white flower may change to purple as the plant gets older, or the flaxen hair of a child may turn to brown when he becomes a man. But, as in the case of other “environmental” conditions, age may not have the same effect on individuals with different factors; in this way it comes about that animals or plants which differ by certain factors may show a difference in character only at certain ages, or may not show the same difference at all ages. In Droso- phila, flies with the factor for pink eyes are easily distinguishable from those with the factor for purple eyes, when the flies are young, but as they grow older, the eyes of both races assume a dark purplish shade, and become practically indistinguishable from each other. Conversely, old flies with the factor for black are usually easy to separate from those having the normal “gray” factor, but the newly hatched flies, in which the black pigment is not yet fully developed, are separated with greater difficulty. These cases in which a factor-difference has a visible effect only at a certain age are in no fundamental respect different from cases like that of the Drosophila TYPES OF MENDELIAN HEREDITY 43 with reduplicated legs, where a factor difference has a visible effect only under special external circum- stances. A number of cases of Mendelian inheritance are known in which only the larvae, and not the adults, are affected. Tower has described crosses in which the beetle Leptinotarsa signaticollis was crossed with L. undecimlineata (Fig. 20, A, B). In the first stage (C), the larvae of these two beetles are exactly alike, but in the second stage, the larvae of L. undecim- lineata are white and the larvae of L. signaticollis are yellow; and in the third stage the undecimlineata larvae are still white without stripes, while the others have well-developed tergal stripes (B). When these species are crossed under certain external conditions the Fi larvae are yellow and, later, striped. The beetles that come from them are intermediate. Inbred, these beetles give three larvae of the yellow type to one of the white type. There is extensive evidence from cytology, experi- mental embryology, and regeneration, to show that all the different cells of the body receive the same hereditary factors. We must suppose, then, that the Mendelian factors are not sorted out, each to its appropriate cell, so that factors for color go only to pigment cells, factors for wing-shape to cells of the wings, etc., but that differentiation is due to the cumu- lative effect of regional differences in the egg and embryo, reacting with a complex factorial background that is the same in every cell. These regional peculi- arities of different parts of the egg and embryo, may, 44 TYPES OF MENDELIAN HEREDITY like the age of the individual, also be considered as influences external to the hereditary factors which affect the development of characters. And not only Fig. 20.—Leptinotarsa signaticollis (above), and L. undecimlineata (below), with their full grown (B) and second stage (C) larvae to the right of each. (After Tower.) do regional peculiarities influence characters, but special regions are usually required for a given factor difference to manifest itself, just as certain tempera- tures or ages may be necessary. Thus when we TYPES OF MENDELIAN HEREDITY 45 speak of factors for eyes or for legs, we really mean factor-differences which can produce effects only in the eye, the leg, or other regions of the body. In other cases the expression of a factor-difference may not be limited to one region but may produce a different effect in different regions; for example, a gray white-bellied mouse, which differs from the yellow mouse by only a single factor, is lighter than yellow on the under side, but darker on the upper side. 717. By the Influence of Other Factors Analogous also is the fact that certain factor- differences produce a visible effect only when they are in company with a particular complex of other heredi- tary factors. Thus, a fly with the factors for ver- milion eyes can not be distinguished from one with the factors for pink eyes if both contain, in addition, the factors for white eyes, for the factors for white allow no other color to develop. Again, it is obvious that without the factors necessary for the develop- ment of a given character, no factors merely deter- mining special modifications of that character can have any effect. In other cases, the effect of a given factor may not be entirely suppressed, but greatly changed, if certain other factors in the hereditary complex are changed. Thus, in flies which already have the factor for vermilion eyes, the factor for purple eyes produces an eye still lighter than ver- milion, but in flies containing the normal allelomorph of the factor for vermilion, the factor for purple pro- 46 TYPES OF MENDELIAN HEREDITY duces an eye decidedly darker than normal. Such cases of interaction of factors, in which the effect of one factor is altered by the action of another factor, are very numerous. IV. Conclusion It would have been indeed strange if Mendelian factor-differences had not been found that require special conditions—environmental, developmental, or factorial—in order to produce a given effect, or any effect at all. For Mendelian factors may cause or influence all sorts of characters—that is, any or all kinds of developmental or physiological reactions; and many of these reactions are known to be affected by age, temperature, region of the body, and so forth. The facts given above are in no possible sense sub- versive to Mendelian principles. On the contrary they illustrate to great advantage the previously given interpretation of all hereditary characters— namely, that every character is the realized result of the reaction of hereditary factors with each other and with their environment. Failure to understand this viewpoint has led to some futile criticism by the opponents of the modern Mendelian interpretation in terms of unit factors. This criticism is as pointless as it would be to criticize the atomic theory on the ground that oxygen does not, under all conditions, and in all its compounds, give rise to substances with the same properties. The validity of the unit factor conception rests TYPES OF MENDELIAN HEREDITY 47 upon the fact that whenever (as often happens) all other conditions, external and internal, that modify characters remain constant, clear-cut ratios are ob- tained which can be explained only as due to segre- gation, in definite ways, of particular hereditary factors that perpetuate themselves unchanged from generation to generation. The validity of the fac- torial hypothesis may also be proved under circum- stances not so well controlled, however. In cases where, on the factorial hypothesis, a certain factor is expected to be present in an individual, then, even if the individual fails to develop the character commonly taken as indicative of the factor, the actual presence of the factor may be demonstrated by breed- ing tests. For if, in subsequent generations, cir- cumstances—genetic or environmental—are provided, like those in which the character previously appeared, it will again show itself. Flies of the race with ab- normal abdomen, if raised in a dry bottle, appear perfectly normal, but the presence within them of the factor for abnormal may be demonstrated by rear- ing their offspring in a wet bottle. Again, the factor for pink eyes may be carried by a race with white eyes, and although pink does not show in the white- eyed race, its presence there may then be demon- strated by crosses of these flies with flies that are not white. Cases like these could be multiplied over and over again. CHAPTER III LINKAGE If two factors lie in the same member of a chromo- some pair we should expect them always to be found together in successive generations of a cross unless an interchange can take place between such a chromo- some and the homologous chromosome derived from the other parent. Whenever the two factors remain together in the same chromosome there will be formed equal numbers of gametes containing the two factors and of gametes containing the normal allelomorphs of the two factors. But if pieces of homologous chromosomes are interchanged, then some of the gametes will con- tain one of the factors in question, and an equal number will contain the other factor. The process of interchange between chromosomes is called cross- ing over; the tendency of factors to stay together is called linkage. An example may make clearer this process of cross- ing over. The factor for black body color and that for vestigial wings both lie in the second pair of chro- mosomes. If a black vestigial fly is crossed to a wild fly (gray, long wings) (Fig. 21) the offspring are gray with long wings. These Fi flies have one chro- mosome containing both the factor for black and the factor for vestigial, and a homologous chromosome LINKAGE 49 with the normal allelomorphs of these factors. After maturation one or the other of these chromosomes Fig. 21.—Back-cross of Fi d1 (out of vestigial by black) by black vestigial 9. will be left in each egg and each sperm. The gametes will consequently contain the same combinations of 50 LINKAGE factors as were present in Pi unless an interchange has taken place between the two chromosomes. The best way to find out whether such an interchange has taken place is to mate the Fi males and females to the double recessive type, black vestigial, because black and vestigial being recessive factors will not obscure the factors that are carried by the gametes of the Fi to be tested. When the Fi male is back- crossed to a black vestigial female, Fig. 21 (second line), only two classes of offspring are produced. Half of the flies are black vestigial and half are gray long. This must mean that there has been no cross- ing over in the hybrid Fi male; for he produces only two kinds of gametes and these are of the kind that combined to produce him. In other words, the chromosomes received from his parents have re- mained intact. If we test the Fi female, by back-crossing to a black vestigial male, the result is different. If such a female is bred to the double recessive male, black vestigial, four kinds of offspring result, as follows: Non-crossovers Crossovers Black, vestigial Gray, long Black,long Gray, vestigial 41.5 per cent. 41.5 per cent. 8.5 per cent. 8.5 per cent. 83 per cent. 17 per cent. Of these four classes the first two correspond to the combinations which the Fx received from its parents, namely, black vestigial and gray long; the other two are classes that would be expected if crossing over had 51 LINKAGE taken place between black and vestigial in the pair of homologous chromosomes. The numerical results Fig. 22.—Back-cross of Fj female (out of black by vestigial) by black vestigial male. show that this crossing over takes place in about 17 per cent, of cases. In other words, the chances are 52 LINKAGE about five to one that the combination that went in holds together. It is also instructive to repeat the cross in such a way that the two mutant factors, black and vestigial, enter from different sides, i.e., one parent contributes black and the other vestigial. As shown in the next diagram (Fig. 22), each parent carries in its chromo- some one mutant factor and the normal allelomorph of the other. If the Fi males are backcrossed to black vestigial females only two classes result, viz., black long and gray vestigial Fig. 22 (third line). These are the combinations that entered; hence no crossing over has taken place in the Fi males. We see that here the linkage is not due to some affinity between the factors black and vestigial, per se, for in this cross they always enter different gametes as surely as they stayed together before. The reason for this difference in result is that in this cross they came from different parents and must have been in opposite chromosomes, whereas in the previous cross they were in the same chromosome. If we test the Fi females by mating to black ves- tigial males, four classes result, viz., Non-crossovers Crossovers Black, long Gray, vestigial Black, vestigial Gray, long 41.5 per cent. 41.5 per cent. 8.5 per cent. 8.5 per cent. 83 per cent. 17 per cent. Crossing over has taken place in the Fi females, and the numerical results show that this happens in 53 LINKAGE 17 per cent, of cases. Here too we see that now the factors tend to separate, whereas in the case of the other Fi female they tended to stay together, since they lay in the same chromosome. In the present case, when the chromosomes interchange, the factors are brought together, and so the crossover classes are just the opposite in the two cases, as also are the non-crossover classes. Yet there is the same amount of crossing over shown in both crosses, so that the frequency of the double recessives and double domi- nants in the first cross is exactly equal to the fre- quency of the single recessive and single dominants in the last cross. Which classes shall have the high frequency and which the low does not depend on the nature of the factors themselves, therefore, but on which ones come from the same parent, i.e., lay in the same chromosome at first, and which lay in opposite chromosomes. The amount of crossing over is seen to be independent of the way in which the factors enter an individual. Hence it is fair to infer that the process is not peculiar in any way to hybrids, but takes place in the same way and to the same extent in gametogenesis in pure homozygous stocks. This is also indicated by the fact, later to be discussed, that when several different allelomorphs of a factor may occur, all give the same per cent, of crossing over with other factors. Many other combinations, involving a large num- ber of different characters in the second group, have been studied and give consistent results. There is never any crossing over in the male; and, in the fe- 54 LINKAGE male, the amount of crossing over is different for different factor combinations, but, for any given com- bination, it is not altered by the way in which the factors entered the cross, and is, ordinarily,1 constant. Tests like the preceding ones for the second group have been carried out for the third group, and give the same kind of results. There is crossing over in the female and no crossing over in the male. In the fourth group, where only three factors are known, it is found that there is no crossing over between them in the male, and only a very slight amount in the female. In the first group (sex linked characters), a very large amount of data has been collected. Here again there is abundant evidence to show that crossing over takes place in the female, but not in the male. The curious fact also comes to light that no mutations have been discovered in the Y chromosome, nor does it contain any factors dominant to any known mutant or normal factors in its mate, the X chromo- some. Since the linkage of a considerable number of factors in the X chromosome has been studied in detail the evidence from this source best serves to illustrate cases where the linkage is strong, where it is moderate, and where it is weak. The body color called yellow and the eye color white have been used in many experiments. If a yellow white female is mated to a wild male (gray red) (Fig. 23), the daughters are gray with red eyes (like the fathers), but the sons are yellow white like 1 Subject to certain variations which will be noted later. LINKAGE 55 Fig. 23.—Diagram illustrating the inheritance of two pairs of sex- linked characters, viz., yellow white and gray red. In F2 the males and the females are of the same classes. 56 LINKAGE the mother. The explanation of this result is obvious; for the son gets his single X chromosome from his mother, and should therefore have the characters that go with this chromosome. His Y chromosome, derived from the father, does not influence the result at all. The daughters, however, get one X chromo- some from the mother (yellow white) and the other from the father (gray red). The factors for gray and red dominating give gray red daughters. The composition of these Fi females can be tested by breeding to the double recessive male (yellow white) since this does not carry any dominant factors which will obscure what factors are received by the F2 females from their mothers. But the Fi males are themselves yellow white, so that the Fi females may be mated to their brothers. In fact, the out- come is the same, whether a yellow white male from stock or a yellow white Fi brother is bred to the Fi female. The F2 offspring of such crosses give the following classes and ratios: Non-crossovers Crossovers Yellow white Gray red Yellow red Gray white 49.5 per cent. 49.5 per cent. 0.5 per cent. 0.5 per cent 99 per cent. 1 per cent. This F2 result reveals the kinds of eggs produced by the Fi female (since a double recessive father was used). Crossing over takes place between yellow and white in only 1 per cent, of cases. There is no way of testing linkage in the Fi male, which is like a homozygous individual so far as the re- 57 LINKAGE suit is concerned, as his Y chromosome does not contain any factors dominant to yellow and white, even though it came from the gray red male. The reciprocal cross also offers certain points of interest. When a gray red female is mated to a yellow white male both sons and daughters are gray red. The daughters get a gray red chromosome from the mother and these factors dominate the factors derived from the father. The sons (Fi) get their single X chromosome from their mother and show her colors (gray and red). If these gray red Fi females are back crossed to a yellow white male they give the same numerical result that this test gave in the reciprocal cross, viz., four classes of offspring with 1 per cent, of crossing over. The Fi males behave in all crosses exactly as do wild males, which is to be expected, since their single X chromosome is derived from the wild type mother. It will not be necessary to consider in detail the same cross when the two factors enter from different parents; they will now keep apart exactly to the same degree that they kept together before. This is illustrated for the backcross as follows: Non-crossovers Crossovers Yellow red Gray white Yellow white Gray red 49.5 per cent. 49.5 per cent. 0.5 per cent. 0.5 per cent. 99 per cent. 1 per cent. As pointed out in the discussion of the black vestigial cross, this fact is very important, for it serves to 58 LINKAGE show in a most striking way that in the previous ex- periment with yellow and white, these factors hold together so strongly from generation to generation, not because of any innate relation between these characters, but simply because they started together in the same chromosome. In the case of yellow and white just given the linkage between the two factors is very strong in the sense just defined, that is, they tend in a high degree to preserve whichever combination they have. Other factors show a different strength of linkage. For example, if a female with white eyes and miniature wings is bred to a wild male, and then the Fi females (red, long) are backcrossed to white miniature males they will give the following classes of offspring. Non-crossovers Crossovers White miniature Red long White long Red miniature 33.5 per cent. 33.5 per cent. 16-5 per cent. 16.5 per cent. 67 per cent. 33 per cent. The two large classes, white miniature and red long, correspond to the combinations that entered. The two smaller classes are the crossover combinations. Crossing over, therefore, takes place in 33 per cent, of cases. Another combination gives a still greater amount of crossing over: the linkage may be said to be weaker. If a white eyed female is bred to a bar male (bar is a dominant mutation), and if the Fi females (red bar eyed) are bred to the double recessive (white round eyed) sons, the following classes appear: 59 LINKAGE Non-crossovers Crossovers White round Red bar White bar Red round 28 per cent. 28 per cent. 22 per cent. 22 per cent. 56 per cent. 44 per cent. Here a large amount of crossing over appears, about 44 per cent. In fact, so freely do the factors inter- change that without sufficiently large and accurate numbers the linkage might entirely escape detection. The Mechanism of Crossing Over If it be admitted that the Mendelian factors are carried by chromosomes it can not be denied that interchange between homologous chromosomes must occur, for sex linked factors cross over from each other, and yet are known to be in the same pair of chromosomes, since they all follow the X chromo- some in its distribution. The evidence allows for no other interpretation. But why should crossing over take place so rarely between certain factors and so often between others? We can make use here of certain information in regard to the chromosomes that gives a very simple answer to the question. In the early germ cells, before the maturation period begins, the chromosomes appear to be scattered in the nuclei, and the homologous chromosomes in many cases show no tendency to lie together, although in some animals, e.g. in many flies, the members of a pair are often found side by side. In this early period the germ cells divide as do other cells and thereby increase in numbers. But at the termination of this 60 LINKAGE period, the homologous chromosomes unite in pairs. There has been much controversy as to how this union takes place, but in some cases at least, the uniting chromosomes twist around each other as they come together. This is illustrated to the left in Fig. 24. As a consequence, parts of one chromo- Fig. 24.—Diagram to represent crossing over. At the level where the black and the white rod cross in A, they fuse and unite as shown in D. The details of the crossing over are shown in B and C. some will come to lie now on one, now on the other side of the mate. If when the twisted chromosomes separate, the parts on the same side go to the same pole the end result will be that shown to the right in Fig. 24. Each chromosome has interchanged a part with its mate. This process has been called crossing over. It is, of course, also possible that the twisted chromosomes do not break and reunite where LINKAGE 61 they cross, and if they do not then when they begin to separate they simply pull apart irrespective of the side on which they lie. When this occurs each chromosome remains intact and no crossing over takes place. Later some of the evidence on which the above statements rest will be examined more critically. For the present it need only be pointed out that such a crossing over of parts of the chromosomes would supply the necessary mechanism to account for interchange. If the crossing over may occur at any point in a chromosome, then the chance of its occurrence between two given loci will be greater, the greater the distance between those loci. *' If then the Mendelian factors lie along the chromo- somes, the amount of crossing over between any two of them will depend on their distance apart. Should two points lie near together a crossover will only rarely occur between them; if they lie further apart the chance of such a crossover taking place at some point between them will be greater. From this point of view the percentage of crossing over is an expression of the “distance” of the factors from each other. In this way the diagram shown in the frontispiece has been constructed. Not only can all the facts of linkage so far studied be explained on this basis, but, as will now be shown, certain further results can be predicted. This is illustrated in what may be called a three-point experiment, i.e., an experiment in which three pairs of factors are involved. 62 LINKAGE The three factors already studied, namely, white, miniature, and bar, furnish an excellent illustration. If we represent the percentages of crossing over as relative distances along the chromosome the three points will lie as shown in Fig. 25. If crossing over takes place between white and miniature and between miniature and bar, then it Fig. 25.—Diagram to illustrate double crossing over. The white and the black rods (a) twist and cross at two points. Where they cross they are represented as uniting (shown in c). That an interchange of pieces has taken place between W and Br is demonstrated by the factor M having gone over to the other chromosome. might be expected sometimes to take place in both regions at once, as shown in Fig. 25, b. The result here would be to produce two chromosomes like those shown in the lower figure. The combinations of factors which these two chromosomes resulting from double crossing over would contain, are white long bar and red miniature round. Since these two classes LINKAGE 63 of gametes are actually produced, the results of the experiment fulfil the theoretical expectation. There is a corollary of importance to this conclu- sion. When a cross is made that involves only white and bar, the double crossing over, that can be de- tected only when an intermediate point is followed, must still be supposed to take place. Whenever it does take place white bar flies and red round flies result. These will be added to the non-crossover classes since they have the same external character- istics. Hence the apparent non-crossover classes will be increased and the crossovers decreased, so that the sum of the value for W and M (33) and that for M and B (22) is much greater than the value for W and B (44) observed when only these two factors are involved. Here then we have an explanation of why long distances taken as a whole give too little crossing over, as compared with the same distances taken section by section. The lowered percentage is an actual mathematical necessity owing to the occurrence of double crossing over. In the case of double crossing over the two points of crossing over can not be near together unless the chromosomes are tightly twisted. Consequently, when crossing over occurs at any point the region on each side should be protected from further crossing over. That this actually happens may now be dem- onstrated. For example, from vermilion to sable is 10 units, and from sable to bar is 14 units more (as seen in frontispiece). When crossing over occurs between vermilion and sable the region between 64 LINKAGE sable and bar should be somewhat protected from crossing over. The usual amount of crossing over between sable and bar is 14 per cent., but in those cases in which crossing over between vermilion and sable occurs, this value becomes reduced to somewhat less than 4 per cent. In this same fashion a region just to the left of sable is protected, but this protec- tion decreases with the distance from the vermilion sable region. The fact that one crossing over makes less likely another crossing over in a nearby region, or in a sense interferes with a second crossing over nearby, is called interference. It is found that in- terference decreases with increase of distance until, in group I, it vanishes at a distance of about 46; i.e., a crossing over at one point does not affect the chance of crossing over at another point 46 units away. Weinstein finds, however, that at a still greater distance interference reappears, so that there is a modal distance between the two breaks in double crossing over, possibly due to the threads bending in loops that tend to have a certain length. In the chromosome maps the distance taken as a unit is that within which 1 per cent, of crossing over occurs. Thus yellow and white are 1.5 units apart in the frontispiece, since there is 1.5 per cent, of crossing over between them. White and bifid give 5.5 per cent., hence are placed 5.5 units apart, and since yellow and bifid give 7 per cent., bifid must be placed on the other side of white from yellow. The other factors have been plotted similarly, each locus being determined, as far as possible, by LINKAGE 65 the per cent, of crossing over between it and the factor nearest to it. For shorter distances it may be said that the number of units on the map between any two factors (A and C), will equal the per cent, of crossing over that will actually be observed between them in an experiment involving these two pairs of factors, even although their distance on the map may not have been obtained directly from their linkage with each other, their positions having, instead, been determined by their linkage with other factors. On account of double crossing over, however, this would not be expected to hold for the longer distances; and, as has been explained, we do actually find that, if long distances are involved, the distance between A and C determined as on the map, by adding the inter- mediate distances A-B and B-C, is longer than the distance AC as directly determined in an experiment involving only these two pairs of factors. It never- theless remains true that, given the distance between any two factors on the map, the per cent, of crossing over between them can always be calculated from this distance (since the amount of discrepancy due to double crossing over also depends on the distance); this shows that the amount of crossing over between them is an expression of their position in a linear series. This striking fact, that the mathematical relations between the various linkage values conforms to a linear series, is a strong argument that the factors are actually arranged in line in the chromosomes. If the relations between the various linkage values were not determined by some linear relation of the 66 LINKAGE factors but were of a random sort, these relations could not be calculated from a linear map. As a concrete illustration of the way in which a group of factors behaves as a linear series, attention may be called to the manner of distribution of the factors among the germ cells of a female heterozygous for a large number of factors in the same pair of chromosomes. Let us write the factors derived from one parent, i.e., those in one of the chromosomes, on one line (see formula p. 67), in the order which they have on the map (see frontispiece), and the allelo- morphic factors derived from the other parent, i.e., those in the homologous chromosome, in correspond- ing positions on the line below. Then in such a case the mature eggs contain either all of the factors represented on one line and none of those on the other, or they contain all of the factors present in one section of the line, and all of the factors present in the re- maining section of the other line. In other words, the factors obviously stick together in sections ac- cording to their position in the linear series. When double crossing over occurs the line is broken in two places, but even here whole sections remain intact. The above facts may be illustrated by an actual case. The first formula shows the composition of a hybrid female which has received from her mother the mutant factors: yellow, white, abnormal, bifid, vermilion, miniature, sable, rudimentary, and forked, and from her father the normal allelomorphs of these factors, together with the dominant mutant factor, bar. 67 LINKAGE fywabivms rfb'l 1 YWABj VMS RFB' j A number of females of this type have been made up by Muller. The next formula shows the kinds of eggs that were produced by one of these females and the numbers of each kind that were produced. y w a bi vms r f b'-6. Y W A Bi VMS RFB'-8. Non-crossovers: Single crossovers: YWabi vms r f b'-2. YWAB; vms r f b'-2. y w a bi VMS RFB'-2. YWAB; Vms r f b'-l. YWABj VMS r f b'-l. ywaR vms RF B'-1. Double crossover: y w a bj VMS RFb'-l. Counts of over 600 offspring from females of the same type have given similar results. The character- istic method of interchange here demonstrated may perhaps be better realized by contrasting the com- binations just given with the following, which illus- trate types of eggs found not to be produced by such females- y W a Bi V m S r f B' YWabi V m s R f B' It is not supposed, however, that the per cent, of 68 LINKAGE crossing over represents precisely the distance between the factors, for it may be that crossing over is more likely to take place in one region of the chromosome than in another. In that case the distances between factors in this region calculated from the- amount of crossing over between them, would be relatively greater than the actual distance. It is supposed, however, that at least the order of the factors in the diagram represents their real order. Sturtevant has found definite factors which alter the amount of crossing over in the chromosomes, and these factors actually do affect the amount of crossing over differ- ently in the different regions. A map of the chromo- somes based upon the per cent, of crossing over when these factors are present would show different rela- tive distances between the loci than those calculated from the normal linkage values. It is to be noted, however, that even in these diagrams, the order of the factors remains unchanged. One of the factors lies in the second chromosome and lowers the amount of crossing over in certain regions of this chromosome; the other lies in the third and apparently affects only this chromosome, and chiefly the end of this chromosome in which it itself is located. Bridges has found that the percentage of crossing over in the sec- ond chromosome is also lowered with increase in the age of the female, and Plough has found that temper- ature as well may affect the amount of crossing over. This variation in crossing over is in no way preju- dicial to the conception of crossing over above out- lined. Variation in the amount of crossing over has LINKAGE 69 also been found in other forms than Drosophila, but in these cases the determining conditions and their effect on the various linkage values have not as yet been discovered. Linkage in Other Animals and in Plants Since the discovery in 1906 of linkage in sweet peas many cases have been found in animals and in plants. In sweet peas themselves two groups of linked factors are now known, one containing three pairs of factors and the other three or possibly four. In garden peas there are two pairs of linked factors and two other cases that are doubtful; in the primrose there is a group of five pairs of linked factors; in the snap-dragon there is a group of three pairs; in stocks there is a group of three or probably four pairs. In animals, linkage, aside from sex linkage, has been discovered in several forms besides Dro- sophila, viz., in domesticated poultry by Goodale, in pigeons by Cole, in rats and mice by Castle, in the silk-worm moth by Tanaka, and in Apotettix by Nabours. There are, it is true, several other cases in which the evidence leads one to suspect that linkage occurs, but these are too uncertain at present to be included in the list. In all the above cases the linkage is “ partial,” that is, a certain amount of crossing over takes place, at least in one sex. There are a number of cases of sex linkage, which, being only a special case of linkage, undoubtedly belong in the same category, but the amount of cross- 70 LINKAGE ing over between the sex factor and the various sex linked factors can not be calculated, since in the sex that is heterozygous for the sex factor no crossing over has been observed. Sex linkage has been found Fig. 26.—Black Langshan female by Barred Plymouth Rock male. Compare with Fig. 30 (similar cross in Abraxas) for scheme of inheritance, which is the same in both. Substitute Black for lacticolor and Bar for grossulariata. in the moths Abraxas (Figs. 30 and 31) and Lyman- tria, in the fowl (Figs. 26, 27, 28, 29) (six factors), canary, pigeon, Drosophila (Figs. 9 and 10), fish, cat, man, and the plant Lychnis. In all, somewhat more than fifteen species show linkage. This number appears small in comparison with the LINKAGE 71 large number of species in which Mendelian inheri- tance has been discovered; but there are several rea- sons why more cases have not been recorded. In the first place, the number of chromosomes is generally large compared with the number of characters that Fig. 27.—Barred Plymouth Rock female by Langshan male. Compare similar cross in Abraxas for scheme of inheritance. have been studied in such a way that linkage would be noticed. Thus, there is little chance of finding two factors lying in the same chromosome. Sec- ondly, unless this linkage is close, it might easily escape detection, especially when the number of off- 72 LINKAGE spring recorded is small. In such cases the data are usually fitted to the nearest “Mendelian” ratio even Fig. 28.—Photograph of the Pi (1 and 2) and Fx (3 and 4) birds in such a cross as that of Fig. 26. though discrepancies are apparent. Even in species where a number of different characters have been studied these are often recorded in separate tables, LINKAGE 73 which excludes the possibility of detecting any linkage that is present, for obviously linkage cannot Fig. 29.—Photograph of Pi (1 and 2) and Fi (3 and 4) birds in such a cross as that of Fig. 30. The Pi male is a standard figure. be seen unless at least two pairs of factors are studied at the same time. The steady increase in the number 74 LINKAGE of cases of linkage that is occurring at the present time, when the importance of detecting them has become apparent, and the methods for studying them have been worked out, appears to presage the realization of linkage as a general phenomenon. Its occurrence in such widely separated types is also a sign that it is a constant accompaniment of Mende- lian inheritance. The Reduplication Hypothesis Linkage has been interpreted by Bateson and his co-workers on a basis entirely different from that adopted in this book. These investigators do not connect Mendelian factors with the chromosomes in any way, and do not suppose that segregation occurs at the reduction division. In a case of linkage be- tween two pairs of factors, Aa and Bb, the doubly heterozygous individual will have the formula ABab. Bateson supposes that in such an individual segre- gation takes place before the reduction division— perhaps in early cleavage stages, perhaps after the formation of the gonads. Two cell divisions are required for this segregation. In the first, A and a do not divide, but one goes to each daughter cell, i.e.f they segregate. B and b, however, both divide, and each daughter cell receives both B and b. The resulting cells then have the formulae, ABb and aBb, respectively. In other words, A and a have segre- gated, but B and b have not. At the next division B and b segregate, giving four cells, with the combina- LINKAGE 75 tions AB, Ab, aB, and ab, respectively. These cells then proceed to divide, the number of divisions not being the same for each, which results in the produc- tion of more of some kinds of cells than of others. But this multiplication must be assumed to be a symmetrical process, since the observed number of AB gametes equals the number of ab, and similarly Ab equals aB. The whole process just described is known as “reduplication.” The term is applied to the same cases as those included under the name of linkage. When three pairs of factors are involved in the same “reduplication series” Bateson supposed at one time that they are segregated at three successive cell divisions, after which the eight resulting cells divide at unequal rates. Later Trow suggested for such a case that perhaps only two segregating divisions occur at first, producing the cells ABCc, AbCc, aBCc, and abCc, which may then multiply so as to give the proper proportions for the A and B combinations. After this there occurs in every cell a division which segregates C and c. The resulting cells then divide again so as to produce the observed relations be- tween the C pair and the other factors. The nature of the factors themselves in the differ- ent lines of cells resulting from segregation can not be supposed to determine the difference in the number of times that these lines divide, because if an indi- vidual has received AB from one parent and ab from the other, the lines of cells reduplicate in a way just opposite to that in an individual which received Ab 76 LINKAGE from one parent and aB from the other. In one individual the line AB divides a certain number of times more than aB, whereas in the other aB divides just that many times more than AB. In other words, the number of times a line of cells divides must be assumed to be determined in some way by whether or not, in its formation, certain factors separated that had established a relation with each other by being present together in the egg or sperm from which the individual came. To explain this, Bateson and Punnett have suggested that at the time of fertiliza- tion there is established in the egg a “polarity” which determines the planes of the segregating divi- sions. But it seems impossible to imagine how this or any other mechanism could bring about the above result. On attempting to follow out in concrete detail the events which must be assumed to occur in any case of reduplication, we find that, if the above stated relation is to hold, then, on “polarity” or any other hypothesis, the assumption of the most intricate and improbable relations and processes is forced upon us. This interpretation of linkage was originally based largely upon the supposed fact that the “gametic ratios” (ratio of parental combinations to new or crossover combinations in the gametes) fell into the series 1:1:1:1, 3:1:1:3, 7:1:1:7,15:1:1:15,31:1:1 : 31, etc. The supposed connection between this series and reduplication is too involved to explain here, and gametic ratios which do not fall into it are now definitely known. In fact, it seems probable that LINKAGE 77 ratios which do fall into it are no more frequent than would be expected from a chance distribution. Another assumption upon which the reduplication hypothesis is based is the old idea of somatic (pre- reductional) segregation. This hypothesis, once ad- vocated by Roux and Weismann as an explanation of differentiation, is opposed by a large body of experi- mental evidence from the fields of regeneration and experimental embryology, and has been given up by practically all students of developmental mechanics, including Roux himself. Altenburg’s crosses of Primula proved segregation of the linked factors to occur after gonad formation. In flies Plough found heat to affect linkage only if applied after the completion of most, if not all, the gonial divisions that might have “reduplicated” the eggs in question. At first it was doubted whether more than two pairs of factors could show reduplication in the same organism, but when it was experimentally proven that two pairs were not the limit, the scheme was extended. When gametic ratios not falling into the 3, 7, 15 series were found, the theory was modified to permit other ratios. Wflien it was found that the result depended upon the way in which the factors entered the cross, the “polarity” hypothesis was added. Some further extension would be necessary to account for interference. That interference is a wide- spread phenomenon is shown by its occurrence in Altenburg’s Primula crosses, and in those of Ander- son on corn—the only crosses outside of Drosophila giving the exact relations of more than two factors. CHAPTER IV SEX INHERITANCE There are two types of sex inheritance known in those species in which separated sexes exist. In one type, which may be called the Drosophila type (XX- XY type, or, for short, the XY type), the female is homozygous for a sex factor, the male heterozygous; in the other, the Abraxas type (the WZ-ZZ type, or, for short, the WZ type) the female is heterozygous for a sex factor, the male homozygous. Since in both cases the heterozygous individuals must always mate with the homozygous ones there should result in each succeeding generation equal numbers of heterozygous and homozygous individuals, and so the bisexual con- dition is perpetuated as follows: The genetic evidence so far gained has placed in the Drosophila type the following animal forms: Dro- SEX INHERITANCE 79 sophila, man, cat; and the plants, Lychnis and Bry- onia. The cytological evidence refers to the same type the insect groups of bugs, flies, beetles, grass- hoppers; the spiders, certain worms (Ascaris), echino- derms, amphibia and mammals (including man). The genetic evidence has placed in the Abraxas type several moths and butterflies, and several birds; viz., chickens, ducks, and canaries.1 Favorable cytological evidence has been found only in the case of a few moths. In many cases of the Drosophila type, in which the history of the sex chromosomes has been worked out cytologically, it has been found that in the male there is a pair of chromosomes, the two members of which are different in size or shape. These are the “sex chromosomes” and are designated as X and Y. In many species of the Drosophila type the Y is slightly smaller than the X, and in the various other species of this type all gradations in the relative size of the Y are found, between this condition and the condition where Y is completely absent. In some related species, on the other hand, the chromosomes which obviously correspond to X and Y are alike in appearance. It is not, after all, the size difference usually visible in the male, between X and Y, which gives these two chromosomes their significance in sex determination, but rather a difference in the factors they contain. The size difference is an incidental concomitant, or, as it were, a token or label that is 1 Richardson’s work on strawberries suggests that this plant may come under the Abraxas type. 80 SEX INHERITANCE not present in all species. In all these cases the female contains two X chromosomes, the Y chromo- some being confined to the male line. This type of sex determination represents all eggs as alike—each containing one X (after the polar bodies have been extruded), but the sperm is of two kinds, one containing the X and the other Y, or merely no X. The scheme is as follows: It will be seen that all the spermatozoa carrying X produce females, while all those carrying Y or no X produce males. The Y chromosome, when present, descends from father to son. It might seem, therefore, that if the Y carried a sex factor for maleness the scheme would work out as well as if a sex factor were carried by the X chromosome. But in several cases there is no Y in the male, and in certain cases to be described later, due to non-disjunction, there are females that have a formula XXY and yet their sex is not affected in any way on account of the presence of the supernumerary Y. It follows that sex is not determined by the presence or absence of the Y chromosome but by the SEX INHERITANCE 81 number of the X chromosomes that are present. In the cases that follow, where sex determination of the Drosophila type was discovered by a study of sex linked inheritance, as well as in the above cases, where the mechanism was discovered through cytological observations, proof that the male is heterozygous for a Mendelian factor for sex is derived from the fact that he gives rise to two kinds of spermatozoa—male pro- ducing and female producing—in equal numbers. We know this in the cases worked out cytologically because here the spermatozoa carrying X must all produce females, while the other half must produce males; and we know it, in the cases worked out gen- etically, because here only half the spermatozoa from a male with a dominant sex linked character carry the dominant factor, and these all produce females, while the rest produce males. The female must con- tain the same Mendelian sex factor as is present in the female-producing spermatozoa of the male; but the female must be homozygous for this factor, since any egg, if fertilized by a male-producing sperma- tozoon, contributes this factor to the resulting male. Although the only way in which the results of sex linked inheritance of the Drosophila type differ from non-sex linked cases is the one above stated, namely, that a dominant male transmits his dominant sex linked factor only to his daughters, nevertheless it may be well at this point to recall specifically what ratios are produced in consequence, in the various types of crosses. Examples of sex linked inheritance in Drosophila 82 SEX INHERITANCE have already been given; that of white eyes is typical of all the rest. The main facts may be restated here. If a white eyed male is bred to a red eyed female the offspring are red eyed (Fig. 9). If these are inbred all of the F2 daughters are red eyed, but half of the sons are white eyed and half red eyed. In a word, the grandfather transmits his characters visibly to half of his grandsons but to none of his granddaughters. In the reciprocal cross (Fig. 10), a white eyed female bred to a red eyed male produces the criss- cross result of red eyed daughters and white eyed sons. These give white and red eyed males and fe- males in equal numbers. On the assumption that the factor for white eyes is carried by the sex chromo- somes the inheritance of white eyes can be readily understood. It will be observed that a female trans- mits to each of her sons one of her X chromosomes with all the factors contained in it. Her sons will show all of these sex linked characters whether they be dominant or recessive since they receive no other X to dominate those characters and the Y contains no dominant factor. For example, if a stock be made up pure for yellow body color, white eyes, ab- normal abdomen, bifid wings, sable body color, forked spines and bar eyes, and if such a female be bred to a wild male, all of her sons will be yellow, white, abnormal, bifid, sable, forked and bar. The daughters, however, will receive not only this chro- mosome from their mother, but will also receive a chromosome from the wild male (their father) con- SEX INHERITANCE 83 taining the normal allelomorphs of all these factors. In the case of all the factor-pairs, except abnormal and bar, the normal allelomorph dominates. There- fore, the females will appear normal for all characters except abnormal and bar, which are dominant. In the cat, Doncaster and Little describe a sex linked factor affecting the color. In man several characters, such as color blindness, haemophilia, and others less certainly identified have been found to follow the same scheme. A comparison of sex linkage in Abraxas with that in Drosophila shows that the mode of inheritance of sex linked characters is identical in these two cases, but the sex relations are exactly reversed. In the Abraxas type sex linked inheritance takes place in accord with the plan that the female is heterozygous in sex production. If the chromosome that carries this sex differentiator is called Z, and its mate in the female W, the formula for the male would be ZZ and that for the female WZ. The scheme follows: Inheritance in Abraxas is illustrated in the follow- ing diagrams (Figs. 30 and 31), in which the common 84 SEX INHERITANCE wild type A. grossulariata is crossed to the rare mu- tant type A. lacticolor. LACTICOLOR $ GROSSULARIATA EGGS SPERM EGGS or rt SPERM or F| Fig. 30.—Abraxas lacticolor female by A. grossulariata male. The sex chromosomes are represented by the circles in the center of the diagram, and the letters contained in them stand for the factors that each carries. The W chromosome, confined to the female line, is represented without either L or 1; for it, like the Y chromosome in Drosophila, carries no sex linked factors. SEX INHERITANCE 85 In the first cross (Fig. 30), where the lacticolor female is mated to the grossulariata male, the off- GROSSULARIATA ? LACTICOLOR