Sex-Linked Inheritance in Drosophila

Part I. Introductory 5

Mendel's law of segregation 5 Linkage and chromosomes 5

Crossing-over 7

The Y chromosome and non-disjunction 8

Mutation in Drosophila ampelophila 10 Multiple allelomorphs 11 Sex-linked lethals and the sex ratio 14 Influence of the environment on the realization of two sex-linked characters 16 Sexual polymorphism 17 Fertility and sterility in the mutants 18 Balanced inviability 19 How the factors are located in the chromosomes 20 The sex-linked factors of Drosophila 21 Map of chromosome X 22 Nomenclature 24

Part Ii. New Data 25

White 25 Rudimentary 25 Miniature 26 Vermilion 27 Yellow 27 Abnormal abdomen 27 Eosin 28 Bifid 28 Linkage of bifid with yellow, with white, and with vermilion 29 Linkage of cherry, bifid, and vermilion 30 Reduplicated legs 31 Lethal 1 31 Lethal 1a 32 Spot 33 Sable 34 Linkage of yellow and sable 35 Linkage of cherry and sable 37 Linkage of eosin, vermilion, and sable 37 Linkage of miniature and sable 40 Linkage of vermilion, sable, and bar 40 Dot 44 Linkage of vermilion and dot 44 Bow 46 Bow by arc 47 Lemon body-color 48 Linkage of cherry, lemon, and vermilion 48 Lethal 2 49 Cherry 51 A system of quadruple allelomorphs 51 Linkage of cherry and vermilion 51 Compounds of cherry 52 Fused 53 Linkage of eosin and fused 54 Linkage of vermilion, bar, and fused 56 {4} Forked 58 Linkage of vermilion and forked 59 Linkage of cherry and forked 59 Linkage of forked, bar, and fused 60 Linkage of sable, rudimentary, and forked 61 Linkage of rudimentary, forked, and bar 62 Shifted 63 Linkage of shifted and vermilion 63 Linkage of shifted, vermilion, and bar 64 Lethals sa and sb 64 Bar 66 Notch 66 Depressed 67 Linkage of depressed and bar 67 Linkage of cherry, depressed, and vermilion 68 Club 69 Genotypic club 70 Linkage of club and vermilion 70 Linkage of yellow, club, and vermilion 70 Linkage of cherry, club, and vermilion 72 Green 73 Chrome 74 Lethal 3 74 Lethal 3a 75 Lethal 1b 76 Facet 76 Linkage of facet, vermilion, and sable 77 Linkage of eosin, facet, and vermilion 78 Lethal sc 79 Lethal sd 79 Furrowed 80 Additional data for yellow, white, vermilion, and miniature 80 New data contributed by A. H. Sturtevant and H. J. Muller 82 Summary of the previously determined cross-over values 83 Summary of all data upon linkage of gens in chromosome I. 84 BIBLIOGRAPHY. 86

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Part I. Introductory.

MENDEL'S LAW OF SEGREGATION.

Although the ratio of 3 to 1 in which contrasted characters reappear in the second or F2 generation is sometimes referred to as Mendel's Law of Heredity, the really significant discovery of Mendel was not the 3 to 1 ratio, but the segregation of the characters (or rather, of the germinal representatives of the characters) which is the underlying cause of the appearance of the ratio. Mendel saw that the characters with which he worked must be represented in the germ-cells by specific producers (which we may call factors), and that in the fertilization of an individual showing one member of a pair of contrasting characters by an individual showing the other member, the factors for the two characters meet in the hybrid, and that when the hybrid forms germ-cells the factors segregate from each other without having been contaminated one by the other._ In consequence, half the germ-cells contain one member of the pair and the other half the other member. When two such hybrid individuals are bred together the combinations of the pure germ-cells give three classes of offspring, namely, two hybrids to one of each of the pure forms. Since the hybrids usually can not be distinguished from one of the pure forms, the observed ratio is 3 of one kind (the dominant) to 1 of the other kind (the recessive).

There is another discovery that is generally included as a part of Mendel's Law. We may refer to this as the assortment in the germ-cells of the products of the segregation of two or more pairs of factors. If assortment takes place according to chance, then definite F2 ratios result, such as 9:3:3:1 (for two pairs) and 27:9:9:9:3:3:3:1 (for three pairs), etc. Mendel obtained such ratios in peas, and until quite recently it has been generally supposed that free assortment is the rule when several pairs of characters are involved. But, as we shall try to show, the emphasis that has been laid on these ratios has obscured the really important part of Mendel's discovery, namely, segregation_; for with the discovery in 1906 of the fact of linkage the ratios based on free assortment were seen to hold only for combinations of certain pairs of characters, not for other combinations. But the principle of segregation still holds for each pair of characters. Hence segregation remains the cardinal point of Mendelism. Segregation is to-day Mendel's Law.

LINKAGE AND CHROMOSOMES.

It has been found that when certain characters enter a cross together (i. e., from the same parent) their factors tend to pass into the same gamete of the hybrid, with the result that other ratios than the chance ratios described by Mendel are found in the F2 generation. {6} Such cases of linkage have been described in several forms, but nowhere on so extensive a scale as in the pomace fly, Drosophila ampelophila. Here, over a hundred characters that have been investigated as to their linkage relations are found to fall into four groups, the members of each group being linked, in the sense that they tend to be transmitted to the gametes in the same combinations in which they entered from the parents. The members of each group give free assortment with the members of any of the other three groups. A most significant fact in regard to the linkage shown by the Drosophila mutants is that the number of linked groups corresponds to the number of pairs of the chromosomes. If the gens for the Mendelian characters are carried by the chromosomes we should expect to find demonstrated in Drosophila that there are as many groups of characters that are inherited together as there are pairs of chromosomes, provided the chromosomes retain their individuality. The evidence that the chromosomes are structural elements of the cell that perpetuate themselves at every division has continually grown stronger. That factors have the same distribution as the chromosomes is clearly seen in the case of sex-linked characters, where it can be shown that any character of this type appears in those individuals which from the known distribution of the X chromosomes must also contain the chromosome in question. For example, in Drosophila_, as in many other insects, there are two X chromosomes in the cells of the female and one X chromosome in the cells of the male. There is in the male, in addition to the X, also a Y chromosome, which acts as its mate in synapsis and reduction. After reduction each egg carries an X chromosome. In the male there are two classes of sperm, one carrying the X chromosome and the other carrying the Y chromosome. Any egg fertilized by an X sperm produces a female; any egg fertilized by a Y sperm produces a male. The scheme of inheritance is as follows.

+------------------------------+ | | | Eggs X--X | | Sperm X--Y | +------------------------------+ | Daughter XX | | Son XY | | | +------------------------------+

The sons get their single X chromosome from their mother, and should therefore show any character whose gen is carried by such a chromosome. In sex-linked inheritance all sons show the characters of their mother. A male transmits his sex-linked character to his daughters, who show it if dominant and conceal it if recessive. But any daughter will transmit such a character, whether dominant or recessive, to half of her sons. The path of transmission of the gen is the same as the path followed by the X chromosome, received here {7} from the male. Many other combinations show the same relations. In the case of non-disjunction, to be given later, there is direct experimental evidence of such a nature that there can no longer be any doubt that the X chromosomes are the carriers of certain gens that we speak of as sex-linked. This term (sex-linked) is intended to mean that such characters are carried by the X chromosome. It has been objected that this use of the term implies a knowledge of a factor for sex in the X chromosome to which the other factors in that chromosome are linked; but in fact we have as much knowledge in regard to the occurrence of a sex factor or sex factors in the X chromosome as we have for other factors. It is true we do not know whether there is more than one sex-factor, because there is no crossing-over in the male (the heterozygous sex), and crossing-over in the female does not influence the distribution of sex, since like parts are simply interchanged. It follows from this that we are unable as yet to locate the sex factor or factors in the X chromosome. The fact that we can not detect crossing-over under this condition is not an argument against the occurrence of linkage. We are justified, therefore, in speaking of the factors carried by the X chromosome as sex-linked.

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