IMPLICATIONS OF THE RESULTS WITH ULTRAVIOLET 305 



accidental groups to be so rare, even at best, that they could not exert 

 a perceptible influence on the results found. This is the more true the 

 higher the number in the group, but it holds even for groups of two. 

 Thus, even if a gene mutation did require two or more ionizations, such 

 a large proportion of the clusters that produced them would at all doses 

 used be "natural" clusters, the frequency of which varied linearly with 

 the dose, just as that of single ionizations does, that we should obtain 

 no evidence on the matter through mere dosage studies. 



Iaiplications of the Results W'ITh Ultraviolet 



Nevertheless it seems unlikely that more than one ionization should 

 be necessary to produce a gene mutation, in view of the mutagenic 

 action which even ultraviolet exerts, and the fact that this compara- 

 tively low-energy agent works through activations which, within any 

 given substance, are distributed at random with regard to one another. 

 Yet it is true that the total amount of energ}^ which must be absorbed 

 for the production of a gene mutation is far higher for ultraviolet than 

 for x-rays or gamma rays. Some work on Drosophila by Meyer, E. and 

 L. Altenburg, Edmondson, and Muller shows that, for the most efficient 

 ultraviolet doses which we have thus far worked with, between 100 and 

 1000 times as much energy must be absorbed by the chromosomes them- 

 selves as is absorbed by them when x-rays enough to give the same mu- 

 tation rate are applied.* 



The precise interpretation that should be placed on the mutation fre- 

 quency-dosage relation found for ultraviolet is subject to much uncer- 

 tainty because of the complexity of that relation. In both fungi [Hol- 

 laender (24, 25), Hollaender, Sansome, Zimmer, and Demerec (27)] and 

 Drosophila [Sell-Beleites and Catsch (65), L. and E. Altenburg, Meyer, 

 and Muller (1)] the curve expressing this relation, with mutation fre- 

 quency as ordinate, after rising for a short space in a more or less linear 



* In the paper as presented at the meeting it was stated that approximately equal 

 amounts of energy were absorbed for the production of a gene mutation whether 

 by ultraviolet or by x-rays. This is, however, true only of the energy absorbed by 

 the cells in question (primordial germ cells) as wholes. As the ultraviolet, unlike 

 the x-radiation, is absorbed selectively by the chromatin, which constitutes less than 

 one-hundredth of the cell material, we must reckon the ultraviolet as correspond- 

 ingly less efficient. (See later discussion on the limitation in the spatial range of 

 mutagenic effectiveness of the energy derived from x-ray absorption.) The treat- 

 ment of the relation of mutation frequency to dosage of ultraviolet as the latter is 

 varied has also been radically revised in the article as now presented. The author 

 wishes to acknowledge his indebtedness to Dr. C. P. Swanson and Dr. N. H. Giles 

 for having called his attention to some facts in this connection, although these in- 

 vestigators are in no way re-sponsible for the interpretations here presented. 



