470 



THE MAINTENANCE OF SPECIES 



In order to obtain a white-eyed female, it was necessary to mate a 

 wliite-eyed male to a hybrid red-eyed female, which works out as 

 follows : 



In this type of sex-linked inheritance, the paternal character may 

 be transferred directly in 50 per cent of the cases from father to 

 son and from mother to daughter. There is another type of sex- 

 linkage, as exemplified by some kinds of color-blindness in man, in 

 which the inheritance is never direct from father to son and from 

 mother to daughter, but indirect, or zigzag, as from father through 

 daughter to grandson. This is called criss-cross inheritance. Thus, 

 when a female, normal for color-blindness, is mated with a color-blind 

 male, the trait skips a generation before it reappears. 



Parents 



Gametes 



F. children 



J(Y 



color-blind (>^ 



Gametes ■^" 



F2 grandchildren XX 



normal ( 



It will be seen that in addition to regular Mendelian inheritance, 

 which has to do with the genes located in the various autosomes and 

 which results in the typical 3 : 1 ratio when the hybrids are bred 

 together, there are two other types of inheritance, involving the sex 

 chromosomes. One of these is the direct type in which the character 

 may be handed on from father to son or from mother to daughter, and 

 the other is the indirect type of criss-cross inheritance in which the 

 father cannot give the character to his son, but may pass it along to 

 his grandson by way of his daughter. 



In drawing this section to a close, it is worth while to quote the 

 opinion of the eminent English geneticist, C. C. Hurst, who says, 

 "Perhaps there is nothing which has helped the study of genetics 

 more than the existence of sex." It would take us too far afield to 

 follow out the enticing vistas of heredity opened up by the phe- 

 nomenon of sex. Some of the many aspects of heredity which might 



