310 GENE MUTATIONS CAUSED BY RADIATION 



Reference to Novick and Szilard's graphs shows that, at the dose in 

 question (without light treatment), E. coli has a frequency of about 1 

 induced mutation in 5000 to T4 resistance and 1 in 20,000 to Tq resist- 

 ance or Ti resistance, respectively. At this same dose, Drosophila gives 

 visible mutations in the specified individual loci studied at the rate of 

 some 1 in 5000, although actually the rate varies from about twice to 

 half this value, according to the locus.* 



There are too many sources of error in both the above sets of values 

 for the comparison to be valid except as regards the orders of magnitude 

 involved. Moreover, the assumption has not been proved that the same 

 locus is always involved in the independently arising bacterial mutations 

 that give resistance to the same type of phage. Nevertheless, it is of 

 interest that the results on tw^o such different organisms should agree as 

 closely as those here found seem to do. We are reminded at this point 

 of some similar apparent agreements in per-locus rates of spontaneous 

 and of x-ray mutations that have been reported in other comparisons of 

 widely different organisms. At any rate, in view of the wide range of 

 conceivable values of mutation rate, and of values found under other 

 circumstances, it does not appear very plausible to regard the present 

 agreement as a complete coincidence. If it is not, however, it would 

 indicate a similarity in the quantitative features of the mechanism of 

 mutagenesis, as well as in the nature of the genetic material, in the two 

 cases. 



The above considerations appear to favor the conclusion that more 

 often only two and seldom more than three quanta of 2537 A ultraviolet 

 are involved in the production of a gene mutation. At the same time, 

 however, it can be calculated that there is probably somewhat less than 

 one chance in a thousand of actually getting a mutation, even when as 

 many as three of these ultraviolet quanta have been absorbed by one and 

 the same purine or pyrimidine group of the desox^Tibonucleic acid por- 

 tion of a chromosome. Other "accidental" circumstances, therefore, 

 must determine whether the absorbed energy results in a genetically re- 

 producible change. It is to be noted that the chance is also not much 

 over one in a thousand (about }<joo or less, as noted on p. 312), in Dro- 

 sophila, that a mutation will result when an ionization has been produced 

 by x-rays or gamma rays within a chromosome. Yet in a considerable 

 proportion of cases such an ionization involves a much larger energy 

 transfer than would three ultraviolet quanta added together. 



The above energy relations make it seem very likely that when x-rays 



* The work on the production of mutations in Drosophila by ultraviolet, referred 

 to in this section, was supported by a grant from the Public Health Service, with 

 the cooperation of the National Advisory Cancer Council. 



