308 GENE MUTATIONS CAUSED BY RADIATION 



the non-mutants would have been. Although it is hard to rule out dif- 

 ferences in susceptibility, there were probably no important differences 

 in the amounts of exposure of the genetic material of the different organ- 

 isms, since they are so minute and were agitated during treatment. It 

 is true that the individual mutation-frequency determinations were sub- 

 ject to a high statistical error, yet the effect of this is minimized by the 

 fact that the results from all the series studied agree very closely in 

 principle. For, when the mutation frequency is represented on a loga- 

 rithmic scale as ordinate and the dose on a logarithmic scale of the same 

 magnitude as abscissa, the lines connecting the datum points are found 

 to have the same slope in the case of all six series (a light-treated and 

 a dark series for each of the three mutant types). Moreover, since the 

 effect of the light follows the same formula throughout (see p. 306), it is 

 possible to allow for it and thus to combine the results of the light and 

 dark treatments into one curve for each of the three types of mutations. 



When this conversion is carried out it is found that the effective dose 

 (that after correction for light treatment) varied, for two of the mutant 

 types, by a factor of about 3, and for the other by a factor of about 5. 

 There are several scattered datum points along each of the three curves, 

 and each curve is found to form, so far as can be judged, a straight line, 

 on the log-log scheme of representation used. This means that the mu- 

 tation frequency, over the whole range of doses employed, rises as a 

 sensibly constant power of the dose. 



The slope of the straight lines, on this log-log curve, having a tangent 

 of about 2.3, is such as to show that for each small increment of dose 

 there was about 2.3 times as great a relative increment of mutation fre- 

 quency. This relation, showing that the mutation frequency rises as 

 the 2.3 power of the dose, would ordinarily be interpreted (although the 

 authors do not discuss this matter) as meaning that in the production 

 of each mutation 2-3 independently produced unit elements of the dose, 

 that is, in this case 2-3 activations (sometimes 2 and sometimes 3), usu- 

 ally take part. Indeed it does not seem possible to avoid this conclusion, 

 except by gratuitously assuming still greater complications, with effects 

 canceling one another sufficiently to return the final, net effect to the 

 2.3-power relationship to dose. In other words, there is evidence for an 

 only slightly higher than two-hit curve. And, since the same curve (al- 

 though with different absolute frequency of effect) was found in the case 

 of each of the three mutants for four or more points over a three- to 

 five-fold dose range, it does not seem likely that it has at these doses 

 been modified appreciably b}^ those influences above discussed, which in 

 other material tend to cause an apparent leveling off or drop at very 

 high doses. A curve of 2-1- "hits" thus seems to represent the process 



