56 



THE THEORY OF THE GENE 



that the sex-linked genes are carried by the X-chromo- 

 somes. 



There is another aberrant condition in the X-chromo- 

 somes that also supports this conclusion. A type of 

 female arose whose genetic behavior could be explained 



1^ 



Fig. 36. 



A three-X female, a, having three X-chromosomes and two of each 



of the other kinds (autosomes), as shown in b and c. 



only on the assumption that her two X-chromosomes had 

 become attached to each other. During the maturation 

 division of her eggs both X's go together, i.e., they both 

 stay in the egg, or both go out together (Fig. 37). A 

 microscopic examination shows in fact that the two X's 

 of these females are stuck together end to end, and it 

 shows also that these females contain a Y-chromo- 

 some that acts, presumably, as a mate of the two attached 



