538 RADIATION BIOLOGY 



power. Here also, the interpretation in terms of individual genes is still 

 uncertain. 



The same stricture applied to the so-called lethal "mutations" (actu- 

 ally, delayed inactivations of unknown basis) produced in the work of 

 McAulay and Ford on the fungus Chaetomium. These inactivating 

 effects, even with X rays, follow approximately the fifth power of the dose 

 (Ford and Kirwan, 1949), whereas with ultraviolet the exponent of the 

 dose to which their frequency is proportionate is approximately the square 

 (McAulay, Plomley, and Ford, 1945). The author would suggest that 

 the lower value of the exponent with this agent is in part a resultant of 

 the concomitant reparative action of the ultraviolet. At any rate, the 

 disagreement of even the X-ray effect with that obtained for individual 

 gene mutations in other work makes the corresponding ultraviolet results 

 of very doubtful significance in relation to mutations in individual genes. 



On the other hand, in Bacterium prodigiosum Kaplan (1948b) found 

 that the frequency of mutations (to forms with retarded colony growth) 

 was proportional to the first power of the dose, i.e., the relation was a 

 simple linear one. Whenever such differences exist, it seems probable 

 that the simplest relation represents the fundamental process involved, 

 in this case, the production of a gene mutation at an individual locus. 

 Moreover, it seems reasonable to infer, by analogy with the simple linear 

 relation to ultraviolet dose even at low doses found by Swanson (1940, 

 1942) for simple chromosome breaks, that ultraviolet-induced gene muta- 

 tions likewise are probably single-hit phenomena, at least in some cases. 

 This conclusion would, however, depend on the condition that, within 

 the limits of the dosage experiment, the effect was independent of the 

 timing of the dose and was carried out at doses low enough to preclude the 

 reparative action of ultraviolet from exerting a significant amount of 

 effect. Although time independence was not proved in Swanson's mate- 

 rial, it has recently been found by Altenburg, Bergendahl, and Altenburg 

 (1952) that the lethals produced in Drosophila pole cells are independent 

 of the time and intensity of the ultraviolet over a nineteenfold range. 

 Thus, despite the existence of the conflicting evidence, it seems likely 

 that both chromosome breaks and gene mutations can in some cases be 

 produced by single ultraviolet-induced activations. 



Until linearity at low doses is established, a valid comparison cannot 

 be made between the amount of absorbed energy required to produce a 

 point mutation by ultraviolet and by ionizing radiation. However, if a 

 fundamentally linear relation is assumed to exist, such a comparison can 

 be made by using the values found in the rising straight-line portion of 

 the ultraviolet curve. In these experiments there is much uncertainty 

 in regard to the amount of ultraviolet actually reaching and absorbed by 

 the chromosomes. Despite this, a calculation made by Muller (1952a) on 

 the basis of results already published, including those obtained in the 



