- 



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 j ~- 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- 

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

 be produced easily. If certain of the rted-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^ 



