126 GENETICS 



sence from the female results from the fact that she car- 

 ries two X-chromosomes, and consequently produces the 

 female hormone. 



2. But in certain fish, some of the male secondary sex 

 characters apparently do depend on the presence of a Y of 

 a particular type, for they occur only where that type of Y 

 is present, and they can therefore not be inherited through 

 the mother. That is, the Y-chromosomes of certain individ- 

 uals or breeds are so modified as to produce certain definite 

 characteristics in the individuals that contain them, while 

 individuals that contain another type of Y do not show 

 these characteristics. 



An example of this is found in the small fish Lebistes.^ 

 In a certain race, the males have a black spot at the base 

 of the dorsal fin, while the females are without the spot. 

 In another race of the same species neither the males nor 

 the females have the black spot. Thus the females of the 

 two races are alike in lacking the black spot, while the 

 males differ. Is this difference the result of differences in 

 the Y-chromosomes of the two kinds of males? 



This may be tested by crossing members of the two races. 

 In the spotted race, represent the X and Y-chromosome by 

 capital letters; while in the non-spotted race represent them 

 by the small letters x and y. 



A female of the spotted race, XX, is mated with a male 

 of the non-spotted race, xy, thus: 



Females Males 



Parents XX xy 



Non-spotted 



Fi Xx Xy 



Non-spotted 



This produces in Fi females Xx, and males Xy. Neither 

 contains the Y from the spotted race, and neither is spotted. 



