268 Radiation, Evolution, and the Position Effect 



The "direct hit" theory assumes that the electrons given off 

 when a plant or animal is subjected to X-rays or radium hit 

 the genes directly. It is assumed that whenever an electron hits 

 a gene a chemical change is brought about in the gene, as the 

 result of which the gene thereafter produces a phenotypic effect 

 different from the effect produced by the gene before it was hit. 

 If this theory is correct, the greater the number of electrons, 

 the greater the number of hits, and, therefore, the greater the 

 number of mutations. In other words, there should be a linear 

 relationship between the X-ray dosage and the frequency of 

 gene mutation. Such a linear relationship has been found by a 

 number of investigators for the frequency of induced lethal mu- 

 tations in the X chromosome of Drosophila. However, if direct 

 electron hits are the only determining factor in induced gene 

 mutation, all strains of Drosophila melanogaster would be ex- 

 pected to show the same mutation frequency when the same 

 dosage was applied. Since different strains from different regions 

 do not show the same frequency of induced lethal mutations 

 under similar treatment, other factors, probably biochemical, 

 must be taken into consideration, indicating that the direct hit 

 concept is too simple. There is also another piece of evidence 

 against the theory of direct hits. It has been shown that for 

 lower wave lengths of X-rays, mutation frequency is independent 

 of the wave length used. When continuously higher values of 

 the wave length are used, a point should be reached at which 

 the distance between adjacent molecules on the chromosome is 

 about the same as the diameter of the genes. From that point 

 on, mutation frequency should no longer be independent of wave 

 length. Studies on Drosophila, however, have shown that even 

 beyond this point mutation frequency is not dependent upon 

 wave length. 



The other theory maintains that mutations are not caused by 

 direct hits of electrons on genes but by the transfer of energy 

 from neighboring molecules in the gene environment and that 

 these molecules are activated first by the radiation. Such sen- 

 sitized reactions may be the result of the activation of molecules 

 by the passage of a phptoelectron. There is competition among 

 various cell constituents, including the genes, for the energy of 

 these activated molecules. Genes that win this energy become 



