434 RADIATION BIOLOGY 



tion taken together, would become converted into a "permissible average 

 dose." Individuals might then be allowed, as far as genetic considera- 

 tions were concerned, to receive more than this permissible average, pro- 

 vided so many others received less that the average amount received by 

 all did not rise above that held to be permissible. It is certainly of major 

 importance, from the viewpoint of humanity in general, to draw some 

 such over-all line. It should be based on such considerations as the 

 critical mutation rate or the amount of genetic load to be regarded as 

 tolerable. On the other hand, every increment in the genetic load must 

 be regarded as in itself objectionable, and to be avoided if possible, even 

 if it does not threaten to wipe out mankind as a whole, and even if it is 

 distributed in such a way that it cannot be recognized as such. 



It is in addition desirable to realize how much damage to descendants 

 the exposure of any one given individual may give rise to, in order that 

 it may be decided whether the benefit or chance of benefit to be derived 

 from one or more proposed exposures of this individual is enough greater 

 than that to be derived from alternative procedures to justify taking the 

 risk of damage in the given case. In order to assess this probable damage, 

 it is necessary to have an estimate of the chance of the production of 

 mutations, and of their inheritance by one or more offspring. To set 

 against this the probable benefit also should be assessed. 



An example will be used to illustrate the theory to be followed in such 

 a case. Let us assume, as was done in the foregoing, that a dose of 80 r 

 received by the immature germ cells of human gonads gives a 0.3, or 1 

 in 3 3^, risk of inducing a detrimental mutation in any of these germ 

 cells. Suppose now that the question has been raised whether a given 

 young woman who has hitherto been unable to ovulate successfully should 

 have her ovaries treated with a dose of some 275 r of X rays, as is some- 

 times done in such cases, in the hope of enabling her to fulfill her strong 

 desire to have children. It will be seen that this dose, being approxi- 

 mately d}i times 80 r, will produce an average of about 1 detrimental or 

 lethal mutation per germ cell. If the woman thereafter succeeds in pro- 

 ducing 3 children these will (in the average of such cases) carry three 

 induced mutations (one each, most probably), in addition to those mutant 

 genes which would have been in them anyway. There is a negligible 

 chance that these children, or their children in turn, would be perceptibly 

 affected in consequence of these induced mutations. However, three 

 "unit loads" would have been created which would, on the average, be 

 passed down to three lines of descendants, until finally three genetic 

 deaths occurred or if, to be conservative, the maximum plausible allow- 

 ance is made for the overlapping of gene effects previously discussed, one 

 case of genetic death. Moreover, there would in the meantime have 

 been a long series of small disabilities (added to those which would have 

 been present anyhow), which were collectively equivalent, assuming this 

 much overlapping, to one total disability. Thus the net result of a series 



