THE MECHANISM OF SEX-DETERMINATION 63 



plasm, there is no reason to suppose that it would 

 be distributed in the same way as are the paternal 

 chromosomes. 



EVIDENCE FROM SEX-LINKED INHERITANCE 



The experimental evidence based on sex-linked in- 

 heritance may be illustrated by the following examples. 



The eyes of the wild fruit-fly, Drosophila ampe- 

 lophila, are red. In my cultures a male appeared that 

 had white eyes. He was mated to a red-eyed female. 

 The offspring were all red-eyed — both males and 

 females (Fig. 35). These were inbred and produced 

 in the next generation red-eyed females, red-eyed males, 

 and white-eyed males (Fig. 35). There were no white- 

 eyed females.. The white-eyed grandfather had trans- 

 mitted white eyes to half of his grandsons but to none 

 of his granddaughters. 



Equally important are the numerical proportions 

 in which the colors appear in the grandchildren. There 

 are as many-females as the two classes of males taken 

 together ; half of the males have red eyes and half 

 have white eyes. The proportions are therefore 50 % 

 red females, 25 % red males, 25 % white males. 



Only white-eyed males had appeared at this time. / 

 It may seem that the eye color is confined to the male * 

 sex. Hence the origin of the term sex-limited inheri- v 

 tance for cases like this. But white-eyed females may 

 be produced easily. If certain of the red-eyed grand-/ 

 daughters are bred to these white-eyed males, both 

 white-eyed females and males, and red-eyed females 

 and males, appear (Fig. 37). The white eye is there- 

 fore not sex-limited but sex-linked. 



