262 



society would do well to insure that as 

 many people as possible, of as diverse 

 racial origins as possible, get an oppor- 

 tunity to show what they can do to 

 advance civilization. It may confi- 

 dently be expected that individuals of 

 various races will have the necessary 

 genetic equipment to make unique 

 contributions. 



I wish to devote the rest of this 

 paper to the effects of high-energy 

 radiation on the genetic properties of 

 man. This is a matter that has been of 

 significance chiefly because of the use 

 of x-rays for diagnostic and therapeu- 

 tic purposes; but with the develop- 

 ment of A-bombs and H-bombs it has 

 become of far more general impor- 

 tance, for it is already true that all of 

 us have been subjected to irradiation 

 from these sources. 



There are two possible types of ra- 

 diation damage to be considered— dam- 

 age to the exposed individual, and 

 damage to the genes in his germ cells. 

 The first will be more or less im- 

 mediate in its manifestation, whereas 

 the latter will have detectable effects 

 only in future generations. This, how- 

 ever, is not the most basic distinction. 

 Irradiation has a gross effect on tissues, 

 resulting in the burns and other symp- 

 toms recognized as direct effects of 

 heavy dosages; there is also an effect 

 on the genes, leading to mutations. 



The former, tissue effect, appears to 

 be substantially absent at low doses, 

 recovery from moderate effects is pos- 

 sible, and doses spaced well apart in 

 time have little or no cumulative ef- 

 fects. It is on the basis of these effects 

 that the "permissible" dose, to which 

 it is supposedly safe to expose indi- 

 viduals, is calculated. But there is rea- 

 son to suppose that gene mutations, in- 

 duced in an exposed individual, also 

 constitute a hazard to that individual 

 —especially in an increase in the prob- 

 ability of the development of malig- 



STURTEVANT 



nant growths, perhaps years after the 

 exposure. There is, in fact, no clearly 

 safe dosage— all high-energy radiation, 

 even of low intensity and brief dura- 

 tion, must be considered as potentially 

 dangerous to the exposed individual. 



Let us now turn to the effects of ir- 

 radiation on the genes in the germ lines 

 of exposed individuals. Here again we 

 are handicapped by the special diffi- 

 culties of dealing with the genetics of 

 man, for the quantitative determina- 

 tion of the genetic effects of irradia- 

 tion requires much more refined tech- 

 niques than are possible with man— a 

 point that becomes obvious when one 

 tries to evaluate the data available con- 

 cerning the survivors of the Hiroshima 

 bomb. There is sufficient evidence that 

 quantitative results obtained with one 

 organism cannot safely be applied to 

 a wholly different kind of organism. 

 However, there are certain general 

 qualitative results that have now been 

 so widely confirmed that we may con- 

 fidently assert that they apply to all 

 higher organisms, including man. 

 These results are: 



1 ) High-energy irradiation produces 

 mutations. 



2) The frequency of induced muta- 

 tions is directly proportiona! to the dos- 

 age of irradiation. There is almost cer- 

 tainly no threshold value below which 

 irradiation is ineffective. 



3 ) The effects of successive exposures 

 are cumulative. 



4) The effects are permanent in the 

 descendants of the affected genes. There 

 is no recovery. 



5) The overwhelming majority of 

 these mutations is deleterious — that is, 

 they seriously affect the efficiency of in- 

 dividuals in later generations in which 

 they come to expression. These deleteri- 

 ous genetic effects may lead to early 

 death or to any of a wide variety of de- 

 fects, often gross ones. 



There is a store of such undesirable 

 genes already present in any popula- 



