320 GENE MUTATIONS CAUSED BY RADIATION 



which are found near the sites of chromosome structural change are 

 usually position effects of the latter. 



It may be added that this problem appears to arise very little except 

 in Drosophila and other Diptera, and possibly Neurospora. In mammals, 

 for instance, there is evidence that few if any structural changes entail 

 a position effect, for translocations seldom give evidence of lethal or 

 other mutational effects when homozygous. Thus there has been no 

 reason to suppose changes in gene functioning to be in any way con- 

 nected with chromosome l^reakage in organisms in general. Neverthe- 

 less, in all organisms, small deficiencies, caused by two breaks near to- 

 gether with loss of the interstitial piece, can give effects similar to those 

 of drastic gene mutations. These very similarities in phenotypic effect 

 make it the more important to exert ourselves to maintain our theo- 

 retical distinction. And, although there is undoubtedly much in com- 

 mon in the mechanism of production of both types of genetic change by 

 radiation, they must be studied separately, for we cannot now know in 

 what further particulars, besides the somewhat contrasting influence of 

 ultraviolet on the two, they may be found to differ. 



The Need for More Exact Knowledge of Gene-Mutation 



Frequencies 



The above discussion has been concerned mainly with questions of 

 the nature of the process whereby mutations are induced by radiation. 

 Equally important, both from a practical standpoint and from the view- 

 point of evolutionary and even sociological theory, are the questions 

 concerning the effects of the induction of mutations on populations sub- 

 jected to such treatment. Now it is evident that one of the first desider- 

 ata in the assessment of such effects is a knowledge of the actual fre- 

 quency of gene mutations in any given population, both in the absence 

 of treatment and after delivery under known conditions of a specified 

 dose of radiation. 



It might be thought, in view of the positiveness of our knowledge of 

 the linear relation of induced gene-mutation frequency to dose, discussed 

 in an earlier section, that sufficient information of the requisite kind is 

 already at hand for the making of fairly good estimates of the conse- 

 quences of applying a given dose. However, even in Drosophila, the 

 absolute gene-mutation frequency for a given dose is a matter of con- 

 siderable uncertainty — much more so than that of the frequency at one 

 dose relative to that at another dose. Now, as we shall see later, the 

 absolute frequency is a most important datum when the effects of a 

 given dose on the population are to be reckoned. 



