522 RADIATION BIOLOGY 



sterile. Previous work of this kind with X and 7 radiation (J. I. Valencia 

 and MuUer, 1949) had shown that, at certain of these loci, e.g., that of 

 yellow body, white eye, or forked bristles, the great majority (four-fifths 

 or more) of the point mutations so obtained (i.e., those mutations which 

 were not associated with a deficiency or other structural change cyto- 

 logically detectable in the salivary glands) acted as nonlethals. At 

 certain other loci, on the other hand, e.g., that of carnation eye, raspberry 

 eye, or cut wing, lethal effects were in the great majority even among the 

 point mutations. It may be presumed, especially from the similarity of 

 the over-all point-mutation rates at most of the loci of both these groups, 

 that true gene mutation at a locus of the first group does not give a 

 lethal effect, whereas it usually does when it occurs at a locus of the 

 second group. Corresponding with this conclusion is the fact that, in 

 the case of the locus of yellow or of white, even a total loss of the locus 

 allows the retention of viability (Kossikov and Muller, 1935; Panshin, 

 1941). Thus those exceptional point mutations of the first group which 

 were associated with lethality were in all probability either ultraminute 

 deficiencies or inversions, too small to be detected cytologically, or cases 

 of two gene mutations in separate but very closely neighboring loci, one 

 of these mutations having the visible and the other the lethal effect. 



With this background, derived from X- and 7-ray results, the results 

 from neutrons may now be evaluated. For the genes of the second group 

 it was, of course, found that, as with less densely ionizing radiation, the 

 great majority of the point mutations were again lethal. However, with 

 the genes of the first group, those which, with the other radiation, had 

 given a great majority of viable point mutations, it was found that the 

 neutrons gave rise to a considerable proportion (a third or more) of lethal 

 type among those showing no cytological change. These cases then were 

 to be regarded as involving either two breaks or two gene mutations, pro- 

 duced in close proximity. The great excess of such cases from neutrons 

 as compared with X or 7 rays could only mean that the much greater 

 crowding of the atomic changes occasioned by the neutrons had led (as it 

 had also in the production of translocations in Tradescantia) to a much 

 greater crowding of the mutagenic effects. Therefore the mutagenic 

 effects must have occurred in close proximity to the points of origin of 

 those ionizations or activations which had caused them ; in other words, 

 the mutagenic pathway had been extremely short. In fact, it must have 

 been of even smaller than microscopic dimensions, otherwise there would 

 have been visible deficiencies in these cases if breaks had been involved, 

 or two mutant genes, a visible and a lethal, separable without too great 

 difficulty by crossing over, if gene mutations had been involved. Never- 

 theless, although so close together, the mutagenic effects in question 

 were, if breaks, at least two genes apart and, if gene mutations, at least 

 one gene apart. Moreover, in view of the general similarity in the means 



