88 THE THEORY OF THE GENE 



to normal eve, and this instance has been cited as evi- 

 dence that reverse mutation may take place. The fre- 

 quency of the return mutation varies in different stocks. 

 It has been estimated to occur about once in 1600 times. 

 It was later discovered by Sturtevant and Morgan that 

 when the reversion occurs, crossing-over takes place in 

 the vicinity of the bar gene, and Sturtevant has obtained 

 crucial evidence in regard to the nature of the changes 

 that there take place. 



^ X 1 1 ^ 1 ^ \r f B fu 



9 X ^^H ^M c^X ' ' ' 



^ + B + 



Fig. 50. 



Diagram of a cross between a female bar-eyed fly, heterozygous in 



forked and fused, and a forked bar fused male. 



The demonstration that crossing-over takes place 

 whenever reversion occurs, was as follows: To the left 

 and very close to bar {Ys unit) there is a gene called 

 forked; to the right and very near bar (21^ units) a gene 

 called fused. A female is made up with bar lying between 

 these two genes in one X-chromosome, and bar with the 

 wild type allelomorphs of forked and fused in the other 

 (Fig. 50). Such a female is bred to a forked bar fused 

 male. The ordinary sons will be either forked bar fused 

 or bar, since each has received either the forked bar 

 fused or the not-forked bar not-fused X-chromosome of 

 the mother. When, as happens rarely, a reversion takes 

 place, i.e., a male appears that has round eyes, it is ob- 

 served that crossing-over has taken place between forked 

 and fused. For example, the reverted male is either fused 

 or else forked ; it is never forked and fused, nor is it ever 

 both not-forked and not-fused. Crossing-over must have 



