. The physical basis of heredity . e characters carriedby the four classes of gametes in the F^ female, all fourclasses of her gametes are revealed in their numericalproportions. Reciprocally, when a male with yellowwings {y) and white eyes {w) is crossed to a wild-typefemale (gray {YY) red {WW), both sons and daugh-ters are gray red, because both get the dominating genesfor these characters carried by the X-chromosomereceived from the mother. If these F/5 are inbred (), the F2 females are gray red, since each contains anX with the two dominant genes derived from the fatherwhose genes ha
. The physical basis of heredity . e characters carriedby the four classes of gametes in the F^ female, all fourclasses of her gametes are revealed in their numericalproportions. Reciprocally, when a male with yellowwings {y) and white eyes {w) is crossed to a wild-typefemale (gray {YY) red {WW), both sons and daugh-ters are gray red, because both get the dominating genesfor these characters carried by the X-chromosomereceived from the mother. If these F/5 are inbred (), the F2 females are gray red, since each contains anX with the two dominant genes derived from the fatherwhose genes have remained completely linked, as thereis no crossing over in the male. On the other hand,there are four kinds of F2 males: yellow white; gray red;yellow red; gray white; because each male shows the SEX-CHROMOSOMES AND INHERITANCE 173 character of his single X-chromosome, and there are fourkinds of these chromosomes in his mother on account ofcrossing over in the female. The other sex-chromosome,the Y, has no dominating Via. 72.—The V-: results from the reciprocal cross of that shown in Fig. 71. Sex-linked Inhebitance of the Abraxas TypeIn certain moths and birds it has been shown by thegenetic evidence that the female is heterozygous for sex-linked factors. The cytological evidence, as far as it goes,supports this evidence, but for birds the material is so 174 PHYSICAL BASIS OF HEEEDITY difficult to interpret that Guyers conclusions do not seemto me as yet to be on as secure grounds as those of Sellersfor moths. Both descriptions give, however, the bases fora consistent explanation of sex-linked inheritance in thistype (WZ-ZZ). Since we do not know as yet whether the same or dif-ferent sex factors are involved in the Drosophila and inthe Abraxas types, it seems best not to use the same sym- FemaleWZ ZZ ^^^ We^s^Z
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