428 MAMMALIAN RADIATION GENETICS 



2. The damage is transmitted to descendants. This is, of course, im- 

 phcit in the term "genetic." Nevertheless, it is sometimes forgotten. 

 Arguments to the effect that we should be phlegmatic about the effects 

 of small doses of radiation, because the total damage is probably no 

 worse than that resulting from various other insults to the organism 

 which man tolerates or even enjoys, ignore the fact that among commonly 

 tolerated insults radiation is the only one known to affect descendant 

 generations. 



3. The damage is hidden for a long time before it becomes manifest. 

 Hereditary effects obviously require at least one generation to express 

 themselves. For the large class of recessive mutations, many genera- 

 tions would, on the average, be required in a large, more or less random- 

 breeding population. This class of mutations is particularly insidious 

 in the sense that, even when a particular recessive mutation is finally 

 revealed, it is usually only a very small fraction of the total effect that 

 has become manifest, the rest still being hidden in individuals heterozy- 

 gous for the mutation, 



4. There is no threshold dose. In other words, genetic changes may 

 be expected at any dose, no matter how small. There is a large body 

 of experimental evidence of various kinds which supports this conten- 

 tion and some actual confirming data from doses as low as 25 r in Dro- 

 sophila [Spencer and Stern (21)]. It is true that Caspari and Stern (2) 

 have presented data which could be interpreted as indicating that the 

 linear relationship between gene mutations and dose does not hold at 

 low doses, but, as the authors themselves point out, other interpreta- 

 tions are possible. It should also be mentioned that, although the dif- 

 ference in mutation rate between controls and experimentals (given 52.5 

 r chronic exposure) is not statistically significant in these data, this 

 should not be allowed to obscure the fact that the rate observed in the 

 experimentals is higher. 



In drawing up safety measures against genetic effects of radiation in 

 human populations, serious attention must be given to point 4. If there 

 is no threshold dose, then a so-called "tolerance" dose cannot be one 

 which produces no genetic effect, but only one which does not add a 

 "serious" increase to the effects that already occur as a result of natural 

 radiation and other causes. The problem of estimating the genetic 

 hazard of radiation, therefore, resolves into two main questions : (i) What 

 is the increase in mutation rate per dose? (ii) How great an increase 

 can be tolerated? 



The first question is relatively simple, although the designing of prac- 

 tical experiments to answer it may not be easy. 



The second question is highly complex. In the first place, it involves 



