2l8 GENETICS 



Here the a's are in the X-chromosomes, the A In the 

 Y-chromosome. Since Y goes only to males, the Fi genera- 

 tion will be 



Females Males 



aa aA 



Bobbed bristles Normal bristles 



This situation will continue in later generations: all the 

 males dominant, all the females recessive. 



(b) The dominant gene may occur in some of the X's, 

 as well as in Y. Suppose that the X of the male has the domi- 

 nant gene A, as does also the Y, while the X's of the female 

 have the recessive gene a. Representing the X that contains 

 the dominant gene by A, while the Y that contains it is repre- 

 sented by A, the following is the situation 



Dominant Recessive Dominant 



In such a case, therefore, all the males are dominant in 

 every generation. But in F2, half the females are reces- 

 sive, and recessive females will appear also in later genera- 

 tions. 



(c) If Y contains a recessive gene, while the X's contain 

 the corresponding dominant, the effects of the recessive gene 

 will never be manifested, since both sexes carry the dominant 

 X. Thus Y might contain many recessive genes, none of 

 which would ever be manifested. 



(d) In certain fish, crossing-over of genes from X to Y 



