158 GENETICS AND EUGENICS 



but when they He in different X-chromosomes, as for example, 

 in those furnished by the father and mother respectively, 

 then each will go with a different X, when the paired chromo- 

 somes separate from each other, as they do when gametes are 

 formed. 



But we are forced to suppose that occasionally in the eggs 

 of Drosophila a gene may detach itself from one X-chromo- 

 some and pass over into the other, for once in a while we find 

 that two sex-linked characters which were repelling each 

 other have in some way got into the same gamete and are 

 now coupled, and vice versa two which were coupled may later 

 show repulsion. Morgan's hypothesis offers a simple expla- 

 nation of such occurrences. The supposed changing of a gene 

 from one X-chromosome to another, when repulsion gives 

 place to coupling or vice versa, Morgan calls a "crossing- 

 over." It occurs only in female individuals, or more properly 

 in their eggs, for it has not been observed to occur in the 

 sperms of Drosophila. 



