SEX DETERMINATION AND SEX-LINKED HEREDITY 41 j 



the gene are present. The reason why the character appears normally 

 in males only is that males have only one X-chromosome, a situation 

 which makes it possible for any recessive gene located in the X-chromo- 

 some to express itself. The female, however, has always two X- 

 chromosomes, and miless she inherits the recessive gene from both 

 parents — a condition that would rarely occur in nature — she would 



Flies 



Chromosomes 



iXi 

 X n ? 



XX XI X 



$ $ c? 



Fi 



Gametes 



F2 



d 



Fig. 80. —Sex-linked inheritance of white and red eyes in Drosophila. Parents 

 white-eyed male and red-eyed female; Fi, red-eyed males and females; Fj, red- 

 eyed females and equal numbers of red-eyed and white-eyed males. A black 

 X indicates an X chromosome bearing the gene for red eye, a white X bears white 

 eye. @ indicates that X is wantmg; in recent publications Morgan replaces it 

 by Y. (From Conklin, after Morgan.) 



always have the corresponding dominant character in one X-chromo- 

 some to mask or offset the recessive character in the other X-chromo- 

 some. In man it is also the unfortunate male that falls heir to all of 

 the rather detrimental sex-Unked characters, while the female, though 

 inheriting the character more often than the male, practically never 

 shows the effects of it. 



An interesting variant upon the usual type of sex-linked breeding 

 experiment is the so-called reciprocal cross, starting out with a white- 



