280 



GENETICS 



is capable of bearing the gene for red in only half of 

 its germ-cells. The F females, which normally carry 

 two ^-chromosomes, all receive an tf-chromosome from 

 their father and are consequently red-eyed, while the 



White-eyed Female Red-eyed 



XX 



XY 



Gametes 



Gametes 



XX 



XX 



XY 



XY 



FIG. 86. Criss-cross inheritance. The reciprocal cross to that 

 shown in Fig. 85. All individuals with underscored X have 

 red eyes. The male is heterozygous. 



Fj males all receive a single ^-chromosome from their 

 white-eyed mother and are, therefore, themselves white- 

 eyed. 



B. COLOR-BLINDNESS IN MAN 



This criss-cross type of inheritance has long been 

 known in man, color-blindness being perhaps the best 



