324 GENE MUTATIONS CAUSED BY RADIATION 



even though the exposed parents had undergone interbreeding in the 

 ordinary more or less random fashion. This does not mean that this 

 detrimental effect would be large enough to be technically perceptible, 

 however; in fact, even in Drosophila whose parents had been given very 

 high doses of radiation, such as 5000 r, it would be very difficult to 

 demonstrate directly. But the decreased viability, which we are sup- 

 posing to have been caused by the exposure of only one generation, 

 would continue over scores of generations of descendants, until it had 

 faded away by reason of the "genetic deaths" which it had occasioned. 

 For this reason the total damage entailed might be very considerable, 

 even though that in any one generation was too little, scattered, and 

 intermingled with variations of other origin, to be demonstrable by ordi- 

 nary means. 



If now such effects were, as a matter of policy, judged to be too in- 

 significant to justify the measures needed to avoid them, and the same 

 mutagenic influences were in consequence repeated in every generation, 

 the effects, shght in any one generation, would, as it were, pile up layer 

 on layer, towards an equilibrium at which the effect on the viability of 

 each generation had finally become proportional to the artifically raised 

 mutation rate. Thus if, for example, the mutation rate had been raised 

 by an amount equal to 25 per cent of the spontaneous value, as would 

 happen in Drosophila by application of some 25 r per generation to the 

 immature germ cells, then the population, after reaching genetic equilib- 

 rium for this dose, persistently repeated, would in each generation have 

 a death rate, due to genetic causes, which was 25 per cent above that 

 existing in non-treated material. As this genetic death rate in ordinary 

 non-treated Drosophila, due to spontaneous mutation, appears to be at 

 least 10 per cent (that is, 10 per cent of individuals die or fail to repro- 

 duce because of their spontaneous mutations), that in the treated popu- 

 lation would in this case rise to some 123/^ per cent, the difference, com- 

 prising 2.5 deaths per hundred individuals, having been caused by the 

 25-r exposure. 



A figure of this kind has more than academic interest, for it can be 

 shown that, in a population of stable size, the number of "genetic deaths" 

 which are caused in each generation by a mutagenic agent that has been 

 applied repeatedly, for an unlimited succession of generations, is in fact 

 the same as the total number of genetic deaths caused throughout future 

 generations, until the effect has faded away, by the same mutagenic 

 agent, if applied in the same strength, but for only one generation. In 

 our example, this would mean that the 25 r if applied to only one genera- 

 tion would eventually cause two and a half deaths, scattered through- 

 out the future, for every hundred members of the one treated generation. 



