GENERAL ZOOLOGY 





I Y Y 1 Red -eyed 

 I A A ) female 



Red-eyed [ Y V I 



male 



Zygotes from 

 which Pi 

 developed 



Gametes 

 of Pi 



Zygotes 

 of Fi 



Gametes 

 ofFi 



Zygotes 



0fF2 



Red -eyed females 



Red -eyed male White -eyed male 



Fig. 6.17. A cross between a red-eyed female Drosophila and a white-eyed male (cf. 

 Fig. 6.9). The diagram illustrates disjunction of sex chromosomes during the formation of 

 gametes and the possible combinations of these; the X-chromosomes carry genes for eye color. 

 This is the reciprocal of the cross shown in Figure 6.18. X, X-chromosome carrying gene 

 for red eye color; X, X-chromosome carrying gene for white eye color; Y, Y-chromosome 

 which is confined to males and does not carry a gene for eye color. 



ries very few genes and does not leave the male line. Each ovum formed 

 during maturation contains one X-chromosome; one-half of the sperm have 

 an X-chromosome, the other half have a Y-chromosome. In the cross be- 

 tween a homozygous red-eyed female Drosophila and a white-eyed male, each 

 ovum contains an X-chromosome carrying a gene for red eye color, whereas 

 half the sperm have an X-chromosome bearing a gene for white eye color and 

 half have a Y-chromosome which has no gene for eye color (Fig. 6.17). Ran- 

 dom unions of ova and spermatozoa result in red-eyed females and red-eyed 

 males in the F^ generation. The females are red-eyed because the gene for 

 red eye color is dominant to the gene for white eye color. These females are, 

 however, heterozygous for eye color and give rise to two kinds of ova, in one 

 of which the X-chromosome carries a gene for red eye color, and in the other 

 the X-chromosome carries a gene for white eye color. Half the spermatozoa 

 produced by the red-eyed males have an X-chromosome with a gene for red 

 eye color and half have the Y-chromosome. Combinations of the gametes in 



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