218 MULTIPLE ALLELOMORPHS 



is no more subject to mutation than are other factors, 

 i.e., the factor has lost its unusual instability. 



There is no a priori answer possible to the question 

 as to whether a mutation having occurred, a further 

 mutation of the mutated factor is more likely to 

 occur, for it is conceivable that while in one case 

 the new factor might be unstable, in another case it 

 might be even more stable than the original one. 

 In regard to the other question, as to whether a par- 

 ticular locus is more liable to mutate, the work on 

 Drosophila shows that certain loci do mutate more 

 often than do others, and this is shown not only in 

 the recurrence of the same mutation, but also in the 

 occurrence of multiple allelomorphs. 



At present the series of white allelomorphs is: 

 white, eosin, cherry, blood, tinged, buff, ecru, ivory, 

 coral, and apricot. Each of these forms has appeared 

 by direct mutation from the wild- type. Crossed to ; 

 each other, the members of this series give com- i 

 pounds that are intermediate in color between the i 

 two types used as parents. In no case do such I 

 crosses give wild-type progeny, which would be the | 

 result expected if they were closely linked but non- i 

 allelomorphic mutants. The hypothesis of close ' 

 linkage would require here the absurd supposition \ 

 that each type was produced not simply by one ■ 

 change but by as many simultaneous changes as ; 

 there are mutant types in the series. , 



