DISCUSSION 331 



Muller: 



In answer to Latar jet's first question, I feel that the relation of mutation rate 

 growth is suggestive in this matter but not proof positive. Perhaps tagging 

 might help. With regard to the second question, I would agree that the rela- 

 tion is not linear, as I had at first thought it to be. 



NiMS : 



The long-range effects discussed by Muller in the latter portion of his paper 

 are most important. He has stressed that the radiation-induced mutations do 

 not differ from the spontaneously occurring ones. Since this is so, it could be 

 assumed that all the gene variabilities exist in the human population and are in 

 a steady state. If this is so, all that radiation does is to increase the rate of 

 mutations without greatly disturbing the steady state. That is to say, there 

 would be no wide departure from existing genie patterns, nor would new dele- 

 terious genes be introduced into the population. 



Muller : 



The answer to this line of reasoning lies in the fact that natural selection 

 throws out the deleterious mutations as fast as they occur in those races that 

 manage to survive. The major mutations are unimportant, as a rule, because 

 of their rarity. They are important only as handles to study genetics. It is 

 the sum of many minor or undetectable changes that gives importance to gene 

 change. The number depends on the spontaneous rate and is proj^ortional to it. 

 If the rate is doubled, the genetic death rate associated with it is also doubled, 

 and twice as heavy a load is placed, so to speak, on the individual who doesn't 

 die. How far the Drosophila data can be applied to mammals is not certain. If 

 the data are applicable, then an increase of 25 per cent in the natural mutation 

 rate is far from negligible. A new equihbrium will be established if the exposure 

 is continued. At the present time, equilibrium, in my opinion, is not established, 

 since the rate of radiation exposure is increasing. About 0.01 per cent of the 

 mutations may be beneficial, but the sacrifice of natural selection means that 

 human beings cannot accept the burden of the harmful mutations for the sake 

 of the very occasional beneficial one. 



It is possible that in another generation the increase in exposure to radiation 

 may, on an average, equal 25 r per person. 



Sparrow: 



In plant material the situation does not appear to be as gloomy as Muller 

 suggests for man. When Tradescantia are grown under continuous gamma ir- 

 radiation, the postmeiotic aberration frequency (micronuclei resulting from 

 meiotic chromosome fragmentation) gradually increases and reaches a maximum 

 by 16 days. Further exposure at a given dose rate causes no further increase. 

 If the plants are removed from the radiation field, the aberration frequency 

 returns to normal in a few weeks. In this case dose rate seems to be more im- 

 portant than total cumulative dose once the minimum period of about 16 days 

 is passed. Sax has obtained similar results in his experiments with Tradescantia. 



