CHAPTER XXXI 



SEX-LIXKED .\XD OTHER KIXDS OF LIXKED IXHERIT- 

 .\XCE IX DROSOFHILA .\XD OTHER SPECIES^ 



N\'IIXI.Air E. CASTLE 



-\11 the facts of sex-linked iiiheritance in Drosophila harmonize 

 vdih. ^lorgan's h}-pothe5is that the genes of sex-Unked characters he 

 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 eg^, 

 but by only half the sperms, namely by such as possess an X-chromosome 

 and bv \-irtue of that fact determine as female all zvsotes into which 

 they enter. To male z\-gotes the sperm will not transmit sex-linked 

 characters. This h}-pothesis is supported by some curious facts already 

 alluded to but deserving of fuller consideration in this connection, viz., 

 facts observed in reciprocal crosses invohing a sex-linked character, 

 as for example white-eye in Drosophila. 



It has akeady 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 ■uhite-eyed, and among the 

 grandchildren white-eyed mdi\-iduals occur in both sexes. Diagrams 

 will best explain these facts on the basis of Morgan's h>pothesis. 

 (See Figs. 87 and SS and Table I.) 



To state the foregomg facts in another way, it will be obser\-ed that 

 the recessive sex-Unked character in Drosophila, when mtroduced in a 

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



'From W. E. Castle, Gasdics and Eugenics (copyright 1020). Used by 

 special permission of the publishers, The Hanard University Press. 



433 



