CONSTITUTION OF THE CHROMOSOMES 



133 



somes), the offspring that contain both kinds of defec- 

 tive X's have normal red eyes. Similarly, yellow body 

 crossed with tan-colored body gives individuals with normal 

 gray body. 



Why do two diversely defective X-chromosomes, when 



rOS\n 



Rudimenlar/ 



M 



ru 



=^^ 



D 



F 



ru 



N 



orma 



I 



Figure 30. Diagram to illustrate the results of putting together two 

 chromosomes that are defective in different ways. M, mother; F, 

 father ; D, daughter. The mother's X-chromosomes are defective in 

 such a way as to produce eosin-colored eyes instead of red eyes; the 

 father's X is defective in such a way as to produce rudimentary 

 wings. The daughters, receiving an X from each parent, have both 

 their X's defective, though in different ways. But they show neither 

 of the defects, having normal red eyes and normal long wings. 



brought together in the same egg cell, yield offspring that 

 possess neither of the defects, but are fully normal? 



We know that when a normal chromosome is present in 

 the same pair with the defective one, the individuals are 

 normal. This suggests a possible explanation. Perhaps the 

 defect is in only a part of the chromosome, and in a differ- 



