CHAPTER XVII 



SEX-LINKED AND OTHER KINDS OF LINKED INHERITANCE 



IN DROSOPHILA 



All the facts of sex-linked inheritance in Drosophila liar- 

 monize with Morgan's hypothesis that the genes of sex-linked 

 characters lie in a common cell structure (X-chromosome) 

 which is duplex in females, simplex in males. Accordingly, 

 in a race which breeds true for a sex-linked character, that 

 character may be transmitted by every egg, but by only half 

 the sperms, namely by such as possess an X-chromosome and 

 by virtue of that fact determine as female all zygotes into 

 which they enter. To male zygotes the sperm will not trans- 

 mit sex-linked characters. This hypothesis is supported by 

 some curious facts already alluded to but deserving of fuller 

 consideration in this connection, viz., facts observed in re- 

 ciprocal crosses involving a sex-linked character, as for 

 example white eye in Drosophila. 



TABLE 21 



Reciprocal Crosses of White-Eyed with Red-Eyed Drosophila 



Male Female Male Female 



P White X Red Red X White 



Fi Red Red White Red 



F2 1 Red :1 White Red 1 Red: 1 White 1 Red :1 White 



It has already been stated that a white-eyed male Droso- 

 phila crossed with normal females has only normal children 

 of both sexes, while the white-eyed grandchildren are all of 

 the male sex. In the reciprocal cross, between a white-eyed 

 female and a normal male all the daughters are normal, but 

 the sons are white-eyed, and among the grandchildren white- 

 eyed individuals occur in both sexes. Diagrams will best 

 explain these facts on the basis of Morgan's hypothesis. 

 (See Figs. 116 and 117 and Table 21.) 



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