io6 MENDELISM chap, x 



male, nor can it be carried by the Y chromosome. 

 The Fj flies are all heterozygous and red - eyed 

 because every one of them, whether male or female, 

 received the factor for red in the X chromosome 

 from its mother. All the eggs produced by the F^ 

 female contain one X chromosome, but half of these 

 X chromosomes contain the factor for red, while the 

 other half do not. Of the sperms from the F^ male 

 half contain an X chromosome with the factor for 

 red, while the other half contain only a V chromo- 

 some. When these two series of gametes unite, the 

 result, as shown in Fig. 3 1 , must be that the F^ 

 females are of two classes, viz. those containing 2 

 red X chromosomes, and those containing i red 

 and I white X chromosome — in other words, equal 

 number of homozygous and heterozygous red-eyed 

 females. Since the F^ males received only a Y 

 chromosome from their father, their nature expresses 

 directly the composition of their F^ mother, z.e. half 

 of them contain a red X chromosome, and the other 

 half a white X chromosome. The males of the F^ 

 generation consist of equal numbers of heterozygous 

 red-eyed and of white-eyed flies. The theory is 

 fully in accordance with the fact that, in the F^ 

 generation from the cross between a white -eyed 

 male and a red-eyed female, the reds are to the 

 whites as 3:1, and the whites are all males. 

 Moreover, as is evident from Fig. 32, it is equally 

 in accordance with the results of the reciprocal cross. 

 Since the cells of the male contain but one X 

 chromosome, and since the X chromosome alone 

 can carry the factor for red, every red-eyed male, 

 no matter how bred, must be heterozygous for red. 



