1218 



BIOLOGICAL EFFECTS OF RADIATION 



mutations from darker to lighter allelomorphs are more frequent than the 

 contrary. Moreover, of all mutations at the locus, that to white is the 

 most frequent. 



At other loci, no such elaborate studies have been carried out as yet. 

 There are available data on the frequency of direct and reverse mutations 

 for a number of loci, which are useful for orientation (Table 5). Origi- 

 nally carried out rather for the purpose of demonstrating that reverse 

 mutations could occur (and hence that the action of radiation is not 

 purely destructive), they nevertheless serve the same purpose as the 

 experiments with the white allelomorphs. Patterson and Muller (141) 

 and Timofeeff-Ressovsky (181, 182) have studied the locus of forked 

 in some detail. More recently, Johnston and Winchester (76) have 

 added to Timofeeff-Ressovsky's earlier data on miscellaneous loci a 

 large body of experiments. From a comparison of their data with those 

 of Moore on direct mutation they conclude that, in general, reverse 

 mutations are much rarer than direct ones and that there seems to be no 

 apparent relation between the two frequencies. 



