298 GENE MUTATIONS CAUSED BY RADIATION 



evident that the gene mutations induced by radiation form no distinc- 

 tive category. Moreover, the results show that the likelihood of occur- 

 rence of mutations at different loci, and of different mutations at a given 

 locus, if not raised quite equally by the application of radiation, must 

 usually be raised at least sub-equally. This is a quite remarkable rela- 

 tionship in view of the fact that the absorbed energy of the radiation 

 "hit" is so inordinately higher — in a considerable proportion of the hits 

 by some two orders of magnitude — than the energy likely to be involved 

 in the course of most spontaneous mutations. 



Evidence was presented by Muller and Altenburg (55) in 1919, and 

 confirmed almost a decade after that [Muller (40, 41)], that temperature 

 influences the production of spontaneous mutations in Drosophila and 

 acts in fact as a limiting factor in controlling their frequency. The 

 warning was given (1928) that the effect of thermal agitation here might 

 have been indirect, as, for instance, by having caused some special kind 

 of chemical change in the food. For this reason it has been especially 

 desirable to have the necessary large-scale work of this kind repeated 

 with other organisms, and also with other stages of the same organism. 

 Yet the decades have passed without this having been done by anyone. 

 However, as the present writer also pointed out in 1928, the agreement 

 in sign and in general magnitude of the effect with what was to be ex- 

 pected of a simple intervention of thermal agitation in the mutation 

 process itself makes it seem likely that this temperature effect is a direct 

 one. 



The above conclusion seemed to be strengthened, after Timofeeff-Res- 

 sovsky (71) had obtained similar data on Drosophila, by considerations 

 presented by Delbriick (11) concerning the relatively large amount of 

 temperature influence, having a Qio of about 5, to be reckoned for re- 

 actions with rates as slow (molecular changes as infrequent) as those 

 here dealt with. The molecular changes in question, here represented 

 by mutations in individual loci, are so infrequent as to result in a half 

 life of some thousands of years for the individual gene, as first shown 

 by Muller and Altenburg (55). It was noteworthy that, in agreement 

 with the calculated Qio of 5 for rates as slow as this, there is in fact an 

 approximately five-fold increase in mutation frequency with a 10° C 

 rise in temperature. The basic assumption in this calculation was that 

 the mutation occurs whenever a certain energy level, which is inferred 

 to be about 1.5 ev, happens to be attained by the mutable material. 

 This supposedly requisite energy level is so high in comparison with the 

 average kinetic energy of the particles in the protoplasmic medium (some 

 50-70 times as high) that it would be attained very infrequently, giving 

 the reaction the low rate found, yet this rate would be raised by a 10° C 



