1216 



BIOLOGICAL EFFECTS OF RADIATION 



Table 3. — Comparison of the Mutability of the Normal Allelomorphs of 

 White (TF^ and W^) in Two Geographical Races of Drosophila 



melanogaster 

 Adult cf cf, treated with X-ray dosage of 4800 r (Timofeeff-Ressovsky, 179) 



W^ with "American" 

 modifiers 



W'^ with Russian modi- 

 fiers 



PT^ total 



W^ with Russian modi- 

 fiers 



W^ with American 

 modifiers 



PF« total 



Number 



of 



X-rayed 



genes 



31,000 



28,200 

 59,200 



49,200 



26,100 

 75,300 



Number of mutations 



W 



w 



22 



19 

 41 



13 



6 

 19 



W — w'^ 



9 

 14 



13 



8 

 21 



Total 



27 



28 

 55 



26 



14 

 40 



Total 

 muta- 

 tions 



0.087 



0.100 



. 093 + 



0.012 



0.053 



0.054 



0.053 ± 



0.008 



Per- 

 centages 

 W - IV, 

 of all 

 muta- 

 tions 



81 



68 

 75 ±5 



50 



43 



47 ±7 



W-w 

 among 



all 

 muta- 

 tions 



19 



32 

 25 + 5 



50 



57 

 53+7 



Total mutations ^F■^ - IF« = 0.040 ± 0.013. 



Relative per cent w and w' mutations = 28 ± 8.5 per cent. 



Demerec (13 to 21, 23) has made extensive studies of a group of 

 frequently mutable genes in Drosophila virilis, which may be relevant 

 to this problem. These are a group of recessives which frequently 

 revert to the normal allelomorph. They exist in different forms which 

 are distinguished by differences in their rates of mutation in the different 

 tissues. It is, however, as yet an open question whether or not these 

 genes belong to a special category; and therefore the differences between 

 the various forms cannot be compared safely with those found by 

 Timof^eff-Hessovsky. 



THE STABILITY OF DIFFERENT MEMBERS OF ALLELOMORPHIC SERIES 



In the preceding section it has been shown that mutation rate varies 

 from locus to locus, and even that the same gene may exist in different 

 forms distinguishable only by their mutation rates. For a closer analysis 

 of this variation, it is necessary to consider the data on mutation rates 

 in the different members of a series of multiple allelomorphs. Tliis has 

 been provided largely by Timofeeff-Ressovsky (179, 182, 183) for the 

 white locus in Drosophila melanogaster, where the series is one of the most 

 extensive available. 



One of the necessary prerequisites to such work is the demonstration 

 that recurrences of the different members of an allelomorphic series are 



