132 GENETICS 



to cause, when by itself, the wings to be rudimentary, but 

 also contains another X-chromosome that is normal, the 

 wings are not rudimentary, but of the full grown normal 

 type. Or if one X-chromosome tends to produce the reces- 

 sive eosin-colored eye, while the other tends to produce the 

 dominant normal red eye, the individual bearing the two 

 has the normal red eye. 



But what will happen if two X-chromosomes that are 

 so modified as to give different recessive characters are 

 brought together in the same individual? What happens, 

 for example, if in the cells of the same individual there is 

 one X-chromosome so modified as to produce rudimentary 

 wings, and another that is so modified as to produce eosin 

 eyes? This can be brought about by mating a male that has 

 the X defective in one way to a female that has its X's 

 defective in another way. For example, an eosin-eyed 

 mother is crossed with a rudimentary-winged father. Their 

 daughters then contain one X so modified as to produce 

 eosin eyes, the other so modified as to produce rudimentary 

 wings (figure 30). 



Such matings have been made thousands of times, with 

 many different types of defective chromosomes. The gen- 

 eral rule is that in such cases the offspring are quite normal; 

 they have neither of the defects. In the case shown in figure 

 30 the daughters have neither eosin eyes nor rudimentary 

 wings; they are fully normal both as to eyes and wings. 



The same result may occur even when the two chromo- 

 somes produce defects in the same part or function of the 

 body. Cross together in Drosophila two parents, one of 

 which has rudimentary wings, the other having miniature 

 wings, both parents thus strongly defective. The daughters 

 produced, bearing both kinds of defective X-chromosomes, 

 have fully developed normal wings. In the same way, if we 

 cross eosin-eyed individuals with vermilion-eyed (both 

 these colors being the result of defects in the X-chromo- 



