CONTENTS.

PAGE.
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]
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]

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.