CHAPTER XXXI 



SEX-LINKED AND OTHER KINDS OF LINKED INHERIT- 

 ANCE IN DROSOPHILA AND OTHER SPECIES 1 



WILLIAM E. CASTLE 



All the facts of sex-linked inheritance in Drosophila harmonize 

 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 bysuch 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 transmit 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 reciprocal crosses involving a sex-linked character, 

 as for example white-eye in Drosophila. 



TABLE I 

 RECIPROCAL CROSSES OF WHITE-EYED WITH RED-EYED DROSOPHILA 



Male Female Male Female 



P White X Red Red X White 



F x Red Red White Red 



F a i Red: i White Red i Red: i White i Red: i White 



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

 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. 87 and 88 and Table I.) 



To state the foregoing facts in another way, it will be observed that 

 the recessive sex-linked character in Drosophila, when introduced in a 

 cross by the male parent, disappears entirely in F x and reappears in F a 



1 From W. E. Castle, Genetics and Eugenics (copyright 1920). Used by 

 special permission of the publishers, The Harvard University Press. 



433 



