RADIATION AND THE STUDY OF MUTATION IN ANIMALS 1215 



sions are indicated: the characteristic frequency of mutation at a given 

 locus is a function of that locus; and the sample of mutants from the 

 X-ray experiments is much like the spontaneous sample. There are, 

 at several loci (yellow; vermihon; and particularly cut) suggestions 

 that the relative frequencies do differ. More data are needed to deter- 

 mine this point, which is of considerable interest particularly in connec- 

 tion with Stadler's extensive studies (166) of mutation rates in maize. 



T.^LE 2. — Frequency of Mutation at Different Loci in the Z-Chromosome 



OF Drosophila melanogaster 



Quantitative data of a sort (Table 2) have been provided for the loci 

 white and forked, by the experiments of Patterson and Muller (141) 

 and Timofeeff-Ressovsky (175, 179, 181 to 183). More recently, the 

 data of Moore (96) for eight other loci have been presented by Johnston 

 and Winchester (76). Different loci apparently have their own char- 

 acteristic mutation rate (Table 2) measured in the same experiment. 

 The differences are more striking than is apparent in the recurrence table. 

 It should be remembered, however, that the values in this table come 

 from the addition of many heterogeneous groups of data. Moreover, 

 certain other experiments of Timofeeff-Ressovsky (179) throw some light 

 on one source of the heterogeneity. 



He has studied the rates of mutation of the normal allelomorphs of 

 white in two different geographical races of Drosophila mela?iog aster, 

 and found in a series of carefully controlled experiments (Table 3) that 

 the mutation frequency for these allelomorphs, which he could otherwise 

 not distinguish from each other, shows a distinct difference. In other 

 words, differently mutable forms of the same gene may exist. The 

 importance of these data for the present discussion is obvious. Not only 

 do genes at different loci have characteristically different mutation rates, 

 but even the same locus may vary. 



