MANNER OF PRODUCTION OF MUTATIONS 583 



total, 0.045 r, is only 0.00056 (3^180 0) of the 80 r required to produce 

 mutations at the rate at which they actually arise, on the supposition 

 that all spontaneous mutations are caused by radiation. The cosmic 

 radiation received at an altitude of 13,000 ft could result in an increased 

 reception of about 0.013 r in four weeks (0.17 r/year), making in that 

 case a total of 0.059 r. There is a similar increase at sea level under 

 some circumstances, as in houses built of material having a higher than 

 usual amount of radioactivity. Yet, even the total of about 0.06 r 

 is only 0.00075 (1 in 1333) of the 80 r required for producing mutations 

 at a rate equal to the spontaneous one. 



However, in the case of man and other long-lived organisms the amount 

 of radiation received per reproductive generation is so much more than 

 that received in Drosophila' s four weeks that it can be calculated to 

 play a probably significant role in the production of the natural muta- 

 tions (MuUer, 1941c, 1952b), especially when consideration is given to 

 Russell's (1952) finding of the much higher mutagenic effectiveness of 

 ionizing radiation in mammals (mice) than in Drosophila. The above 

 calculated 0.045 r in four weeks becomes 17.5 r in thirty years, the 

 approximate span of a reproductive cycle (generation) in man. Taking 

 Russell's figure of 2.6 X 10~^ mutation induced, on the average, in a 

 given locus by 1 r applied to spermatogonia, it is found that the 17.5 r 

 received in thirty years would induce 4.55 X lO^'' mutation, or 1 in 

 220,000, per locus. This figure is to be compared with the estimated 

 average spontaneous mutation frequency in man of 2 X 10~^, or 1 in 

 50,000, per locus. Thus it will be seen that in man the natural radiation 

 may be producing nearly a fourth of the spontaneous mutations. It is 

 probable that, in the case of sequoias and certain other organisms having 

 exceptionally long life spans or in those which (as in the case of lotus 

 seeds in peat bogs or rotifers in frozen soil) may be dormant for very 

 long periods, the mutations caused by radiation may in some instances 

 considerably outnumber those due to other natural causes, inasmuch as 

 the background radiation may in the course of 1000 years amount to 

 about 500 r. 



At an altitude of about 13,000 ft, at which 5 r more (or 22.5 r in all) 

 may be received in thirty years, the share of radiation mutations in the 

 total mutation rate of human beings may, according to the foregoing 

 values, become 28 per cent, but even at 100,000 ft at high latitudes, where 

 the cosmic radiation amounts to 0.009 r/day (Schaefer, 1950, 1951), or 

 3.3 r/year, a total exposure of about eighteen years, giving 60 r, would be 

 needed to result in a production of mutations by radiation equal to that 

 arising in man in thirty years in the total absence of radiation. More- 

 over, even in the presence of dense material so distributed as to give 

 maximum secondary radiation, about five years of exposure at this alti- 

 tude would still be required for this amount of effect. 



