HEREDITY AND VARIATION 



a cross between a male and a female of the Fj generation result in red- 

 eyed females, red-eyed males, and white-eyed males. In this kind of inherit- 

 ance the white-eyed characteristic of the male parent does not occur in the F^ 

 generation but reappears in one-half the males of the F2 generation. The 

 reciprocal cross, in which a white-eyed female is mated with a red-eyed male, 

 is shown in Figure 6.18. If the distribution of the X-chromosomes is followed, 

 the reason for the difference between the offspring from these two cros.ses 

 should be clear. It is to be understood that sex linkage is not an exception 

 to Mendelian principles but confirms the theory that chromosomes carry the 

 genes. 



Cytology has furnished knowledge of the behavior of chromosomes and 

 their transmission from one generation to the next. The experimental breeder 

 has been able to explain adequately the results obtained in his breeding ex- 

 periments by the assumption that what is present in the zygote and determines 

 the appearance of a character in an adult organism is carried by the chromo- 

 somes. These hypothetical determiners of inherited characters are known as 



©White -eyed 

 female 



Zygotes from 

 which Pj 

 developed 



Gametes 

 of Pi 



Zygotes 

 ofF, 



Gametes 

 of Pi 



Zygotes 

 ofP, 



Red - eyed 

 female 



White -eyed 

 female 



Red - eyed 

 male 



White -eyed 

 male 



Fig. 6.18. A cross between a white-eyed female Drosophila and a red-eyed male (cf. Fig. 6.10). 

 l"he diagram illustrates disjunction of se.x chromosomes during the formation of gametes and the 

 possible combinations of these; the X-chromosomes carry genes for eye color. This is the recip- 

 rocal of the cross shown in Figure 6. 17, the legend of which describes the symbols. 



193 



