584 RADIATION BIOLOGY 



This calculation of the amount of effect of high-altitude cosmic radi- 

 ation on the mutation rate fails to take into account the fact that part 

 of this radiation is distributed in the form of densely ionized tracks, some 

 produced by protons and a particles toward the end of their runs and 

 others by heavier nuclei (see Schaefer, 1951, 1952a, b, c). The dense 

 portions of the proton and a tracks would, as noted on pp. 521-524, be less 

 efficient than the same dose of X or 7 rays in causing effectively separate 

 gene mutations, although much more efficient in causing chromosome 

 changes. The heavier nuclei, however, over most of the length of their 

 tracks, would probably be as efficient as X rays in causing gene mutations 

 because the ionizations would be widely dispersed enough for this in the 

 cross-sectional area lying at right angles to the axis of the track, but 

 they would be more efficient than X rays (although not as much so as 

 ordinary proton and a tracks) in causing chromosome changes. Yet, at 

 the same time, a larger proportion of the gene mutations would be lost, 

 by being produced in chromosomes and in cells destined to perish by 

 reason of their aneucentric or aneuploid chromosome changes, than if 

 X rays had been used. Thus even the gene-mutation rate would appear 

 to be somewhat lower than with X rays (Muller, 1952b). 



19. NUCLEAR TRANSMUTATION AND OTHER PHYSICAL INFLUENCES 



AS MUTAGENS 



The mutagenic action of radioactive materials some of which have been 

 absorbed by the cells under investigation has been studied on a number of 

 occasions. It is to be expected that the resultant transmutation (usually 

 accompanied by the fast recoil) of an atom located at a strategic point 

 within a gene would at least on some occasions result in the mutation 

 of the gene. There is often considerable difficulty in proving that such 

 mutations have occurred, however, since ionizing radiation is released in 

 the transmutation process, and this can produce mutations in the manner 

 usual for such radiation, thus obscuring the picture. 



Experiments of Giles and co-workers (Giles, 1947; Giles and Lederberg, 

 1948; Giles and Bolomey, 1949) on Tradescantia and of both Arnason and 

 co-workers (Arnason, Cummings, and Spinks, 1948a, b; Arnason, 1949) 

 and Ehrenberg et al. (1949) on wheat and barley showed that absorbed 

 P^^ does produce abundant chromosome breaks and structural changes, 

 but it was not clear whether the effects were entirely caused by the 

 released /3 rays or in part also by the transmutation of phosphorus atoms 

 located in the chromosomes or in substances outside the chromosomes 

 which had thereby been rendered mutagenic. However, in experiments 

 on Drosophila, Bateman and Sinclair (1950) and R. C. King (1952) found 

 that the mutations in their material w^ere produced at a rate which could 

 be approximately accounted for by the released rays alone. This con- 



