114 EVOLUTION AND GENETICS 



other. The genes for black and normal may be said 

 to have crossed over. 



When such a female is mated to a black-vestigial 

 male, whose spermatozoa contain the chromosome 

 with black and vestigial, four combinations are 

 formed. Remembering that normal dominates black, 

 and long dominates vestigial, it will be seen {fig. 49) 

 that four kinds of offspring are expected. 



A second case of crossing over, here in the X- 

 chromosome, is illustrated in figure 50. 



If a female with white eyes and yellow wings is 

 crossed to a wild male with red eyes and gray wings, 

 the sons are yellow and have white eyes and the 

 daughters are gray and have red eyes. If two Fi 

 flies are mated they will produce the following 

 classes : 



Yellow Gray Yellow Gray 



White Red Red White 



]Vot only have tlie two grandparental combina- 

 tions reappeared, but in addition two new combina- 

 tions, viz., gray-white and yellow-red. The two 

 original combinations far exceed in numbers the 

 new or exchange combinations. If we follow the 

 history of the A^-chromosomes we find that the 

 large?' elasses of grandchildren can be explained if 

 A^-chromosomes are transmitted in their entirety 

 from one generation to the next. 



