492 RADIATION BIOLOGY 



both or all sister strands, and radiation of higher specific ionization would 

 (up to a point representing the degree of crowding optimal for efficiency in 

 this respect) be better able to effect the required multiple breakage. 



Another way in which greater local ion density might in some material 

 favor the production of breaks which persisted long enough to be detected 

 is, as was pointed out in connection with a radiation, by the production of 

 two or more nearby breaks in the same strand. This mechanism would 

 not recjuire the presence of more than one strand in a chromosome. How- 

 ever, the apparent loss in efficiency caused by the recording of a multiple 

 break as a single one would have to be compensated for by a double break 

 being more than twice as likely as a single one to result' in the persistence 

 of visible fragmentation of the chromosome. A relation of this kind is 

 indeed a probable one for some types of material, since a small interstitial 

 fragment which was deleted might more readily float away than the large 

 pieces, and the broken ends of the latter, being farther removed from 

 one another than if they had been derived from a really single break, 

 might then be less than half as likely to achieve union with one another. 

 This would furnish an explanation of the finding (Dempster, 1941b; Fano, 

 1942) that neutron treatment of Drosophila spermatozoa is, at low doses, 

 13^-^-2 times as efficient as X-ray treatment in producing dominant lethals. 

 The accessory hypothesis that the ionizations act synergistically in the 

 production of the primary breaks would then be unnecessary, and it 

 would also be unnecessary to assume that the spermatozoan chromo- 

 somes consisted of more than one strand. 



4. PRODUCTION OF MINUTE STRUCTURAL CHANGES 

 BY IONIZING RADIATION 



Evidence that structural changes involving two breaks at points very 

 near each other in the same chromosome are produced in higher frequency 

 than would be expected if breaks and unions occurred in a random way 

 was first obtained from an analysis of nondeficient structural changes 

 having a position effect on a particular group of loci, that of the scute 

 region of Drosophila (Muller, Prokofyeva, and Raffel, 1934, 1935). Ordi- 

 nary two-break deletions (deficiencies) are not used as evidence here 

 because, before the offspring in which they are detected come to maturity, 

 these deletions have been subjected to a natural selection for small size 

 by reason of the low survival of individuals heterozygous for moderate- 

 and large-sized deficiencies. However, it was possible to use minute dele- 

 tions to obtain information of another kind bearing on this matter. This 

 involved a study of their frequency in relation to the dose of radiation, 

 to determine whether it depends on the dose in a simple linear manner, 

 as is true of gene mutations, or varies as the % power of the dose (as 

 had already been found to hold for gross rearrangements in Drosophila). 



