278 GENETICS 



the case of sex-linked characters, the genes for which 

 are undoubtedly located in the sex-chromosomes, and 

 whose inheritance follows exactly the distribution of 

 these chromosomes. About thirty genes of this kind 

 have been discovered in Drosophila alone. (See Fig. 

 76, the left hand line.) 



Sex-linked inheritance, which means that genes for 

 characters other than sex are associated with a par- 

 ticular sex, i.e., are carried in the same chromosome 

 that bears the sex-determining genes, should not be 

 confused with sex-limited characters, i.e., with secon- 

 dary sexual characters that are found in one sex only 

 but the genes for which may be located in any chro- 

 mosome. 



An example of a dominant sex-linked character is 

 the red eye of Drosophila. The manner of its inherit- 

 ance is as follows. 



If a red-eyed female is mated with a white-eyed 

 male (Fig. 85), the Fj generation are all red-eyed, 

 and when members of the F 1 generation are inbred the 

 F 2 generation shows the expected proportion of three 

 red-eyed individuals to one white-eyed. However, a 

 peculiar result appears inasmuch as all of the white- 

 eyed individuals are males. Thus, one half of the F 2 

 males are white-eyed like their grandfathers while all 

 of the Fj females are red-eyed because the character 

 of white-eyes is covered up when the gene for red is 

 present. The eggs of F A females, however, which 

 eliminate the genes for red eyes in the polar body 



