314 GENE MUTATIONS CAUSED BY RADIATION 



showing the greater effectiveness of neutrons, and their hnear rather 

 than exponential relation to frequency of effect, in producing chromo- 

 some rearrangements. All these results were to be expected in conse- 

 quence of the greater density of ions in the tracks arising from neutron 

 treatment, provided only the mutagenic effects remain more or less local- 

 ized, on a microscopic or ultramicroscopic scale, in the neighborhood of 

 the tracks themselves.* 



However, the above-indicated limitation in the range of transmission 

 of most of the mutagenic effects of high-energy radiation in the material 

 studied is entirely consistent with their transmission over a more minute 

 but still significant distance or even, under other circumstances, over a 

 greater distance. That is, it by no means proves the assumption that 

 every mutagenic ionization, or even the great majority of them, must 

 have occurred within the gene. Thus, as Lea (33) himself calculated in 

 1947, the ions produced by x-rays in water usually travel through what 

 is, in molecular but not in microscopic terms, a quite significant distance 

 before having their charges neutralized. As Fricke and others long ago 

 maintained, and as has been shown with increasing force in recent years, 

 these ions and their reaction products are often very potent chemically. 

 Moreover, w^hether or not these are the usual initiators of the events 

 which result in radiation mutations, there are now, as we shall see, 

 abundant empirical data which demonstrate the importance that inter- 

 mediate chemical reactions of one sort or another have in mutagenesis. 

 It is, however, difficult to estimate at present, in terms of atomic dis- 

 tances, the spatial range of the reactions from which the majority of 

 radiation mutations in, for example, Drosophila, result. More especially, 

 w'e do not yet know to what extent these reactions are ordinarily initiated 

 within, and to what extent outside, the visible bulk of the chromosomes, 

 or, more specifically, of the genetic material itself. Yet the mere exist- 

 ence of the intermediate reactions should make us very wary in assuming 

 that the mutagenic effect of each hit is always localized within the very 

 particle that was itself hit. And we should also bear in mind the theo- 

 retical possibility, to which some results in radiation genetics have ap- 

 peared to lend support, that a single ionization may on occasion, by a 



* Since the above was written Muller and Valencia {Rec. Genetics Soc. Amer., 

 20: 115-116, 1951, and Genetics, in press) have reported evidence that the lesser 

 apparent effectiveness of neutrons in producing lethals in Drosophila is caused by 

 the genetic effects often being too close together to be recorded separately, but not 

 on the same gene. Although, on the one hand, this shows the mutagenic chain of 

 reactions in this material to be highly localized, it also shows that most ionizations 

 within the genetic material fail to be mutagenic, and so it confirms the conclusions 

 arrived at by other methods on the preceding pages. 



