280 



EVOLUTION, GENETICS, AND EUGENICS 



eyed female, derived from such an experiment as that just described, 

 and breeding her to a normal red-eyed male. The F x hybrids will be 

 white-eyed males and red-eyed females, the two eye colors simply 

 changing sexes. This is explained by the fact that females always in- 

 herit an X-chromosome from both father and mother, while males al- 

 ways get their X-chromosome from their mothers. We speak of this 



Flies 



XX XX XY XY Fg 



9 9 d" <? 



Fig. 6q. — Sex-linked inheritance of white and red eyes in Drosophila. Parents 

 white-eyed male and red-eyed female; F z , red-eyed males and females; F 2 , 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. 

 Y is the Y-chromosome. {From Conklin, after Morgan.) 



phenomenon as crisscross inheritance. There are many evidences that, 

 in general, daughters inherit more largely from fathers and sons from 

 mothers, and it is probable that the mechanism of this condition is 

 like that just described. But to continue the reciprocal-cross experi- 

 ment to the F 2 generation, let us breed together the males and females 

 of Fi. The result will be red-eyed males and females in equal numbers 

 (Fig. 70). 



The type of sex linkage which we have just described for Dro- 



