MULTIPLE ALLELOMORPHS 



167 



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. There is still another set 



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 to 

 this case, however, for although spot and yellow are 

 both recessive to gray and give yellow when crossed 

 to each other, spot originated in flies containing 

 already the allelomorph for yellow. 



The reasons may now be given that incline us to 

 think that the theory of identical loci is much more 



