214 MULTIPLE ALLELOMORPHS 



after the stock had been crossed to ebony, with which 

 it is allelomorphic. Here too the mutant forms 

 though both recessive to normal do not give normal 

 gray color when crossed together, but a color inter- 

 mediate between sooty and ebony. In both of these 

 cases the complete linkage view would require that 

 one of the mutant types had originated by a muta- 

 tion in two factors at once. In the case of another set 



l&!S.i -uiiiiHiiMiai]" 



a 





o' ^ - >--. y' 



Fig. 53. — The abdomen of normal a,a , and spot, b,b', males. The other 

 allelomorph is yellow (not shown here). 



of triple allelomorphs known in Drosophila, namely, 

 yellow and spot (Fig. 53) and their normal allelo- 

 morph. The above argument does not apply; for 

 although spot and yellow are both recessive to gray 

 and give yellow when crossed to each other, spot 

 originated in flies already containing the allelomorph 

 for yellow. 



The reasons may now be given that incline us to 

 think that the theory of identical loci is much more 



