THEORY OF EVOLUTION 73 



is being less used by those students of genetics 

 who are more careful in regard to the implica- 

 tions of their terminology. 



There is a class of cases of inheritance, due to 

 the XY chromosomes, that is called sex linked 

 inheritance. It is shown both by mutant char- 

 acters and characters of wild species. 



For instance, white eye color in Drosoph- 

 ila shows sex linked inheritance. If a white 

 eyed male is mated to a wild red eyed female 

 (fig. 35) all the offspring have r^d eyes. If 

 these are inbred, tliere are three red to one 

 white eyed offspring, but w^hite eyes occur only 

 in the males. The grandfather has transmitted 

 his peculiarity to half of his grandsons, but to 

 none of his granddaughters. 



The reciprocal cross (fig. 36) is also inter- 

 esting. If a white eyed female is bred to a red 

 eyed male, all of the daughters have red eyes 

 and all of the sons have white eyes. We call 

 this criss-cross inheritance. If these offspring 

 are inbred, they produce equal numbers of 

 red ej^ed and white eyed females and equal 

 numbers of red eyed and white eyed males. The 

 ratio is 1: 1: 1: 1, or ignoring sex, 2 reds to 



