90 THE THEORY OF THE GENE 



and in the number of facets. In infra-bar stock, reversion 

 also takes place (Fig. 51b), producing a full round eye 

 closely similar to wild type, and a new type called double- 

 infra-bar (Fig. 49, 10). 



(a) 



(b) 



6 Bar BB Double Bar 



"g Bar Nomno-l 



^ Infra Bo-r B'B' Double Infra B^y- 

 q' InfraBar Novnrva.1 



(0 



B Bar 

 ~ Infra Bar 



B 



r_B_B_ Bar Infra Bar 

 ls)ornnal 



B B IrtfraBar Bar 

 Normal 



Fig. 51. 



Diagram of mutation in bar and infra-bar and bar-infra-bar. 



A female with bar in one chromosome and infra-bar 

 in the other (Fig. 51c) produces, when reversion takes 

 place, a full round eye (wild type) and bar-infra-bar or 

 infra-bar-bar types (Fig. 51c). 



Sturtevant has also utilized these two types, the bar- 

 infra-bar type and the infra-bar-bar, in order to prove 

 that when crossing-over takes place in bar-infra-bar over 

 normal (Fig. 52a), the result gives either forked bar or 

 else infra-bar fused, and when crossing-over takes place 

 in infra-bar-bar over normal (Fig. 52b) the result is 

 either forked infra-bar or bar fused, provided the mutant 

 genes all lie in the same chromosome, as shown in the 

 diagram (Fig. 52a, b). 



