394 RADIATION BIOLOGY 



pioneer suggestions of Boveri (1914) when he proposed the somatic 

 mutation view in its first crude form, provide any material sul)tly and 

 delicately enough differentiated to succeed in carrying out such growth 

 in the face of the competition and opposition of the normal tissues. But 

 mutations and perhaps also, sometimes, minute losses of genes — in any 

 case what are called "point mutations" (a term somewhat broader than 

 "gene mutations" — see p. 389) should afford a sufficient range and 

 specificity of changes to include an occasional alteration having such an 

 effect. As one line of evidence that such is the case, it is to be observed 

 that the point-wise, sporadic manner of origination of malignant develop- 

 ments finds a parallel in the manner of occurrence of mutations. More- 

 over, the fact that the same agent, namely, radiation (either ionizing or 

 ultraviolet) , which produces mutations is also effective in producing these 

 growths adds to the plausibility of the interpretation. A further discus- 

 sion of this matter will be reserved for Sect. 19. 



12. RELATIVE FREQUENCIES OF DIFFERENT TYPES OF CHARACTER 

 CHANGE CAUSED BY RADIATION-INDUCED AND SPONTANEOUS 



MUTATIONS 



Examination of the types of effect produced by gene mutations shows 

 that in Drosophila those radiation mutations which have a visible morpho- 

 logical expression resemble in their general distribution of types the ones 

 w^hich have arisen spontaneously. This does not necessarily mean that 

 the relative freciuencies of mutation for different genes — still less, those 

 of different kinds of mutation (to alleles of different kinds) for the same 

 gene — are identical for radiation mutations and spontaneous ones. 

 Spontaneous mutations are too rare to have allowed a reliable frequency 

 distribution of this kind to have been made for them, except in a few 

 special cases. However, experience has indicated that any type of gene 

 mutation which has been found to arise spontaneously can also, when an 

 intensive search is made, be found after the application of ionizing radia- 

 tion, and probably also after ultraviolet treatment, and that the converse 

 proposition holds likewise. Moreover, the kinds of morphological effects 

 do occur in similar relative freciuencies. There are, for instance, in both 

 the radiation and the spontaneous series in Drosophila, very many muta- 

 tions, in any one of numerous different genes, which give a minute bristle 

 effect, and rather many that give roughened eyes or wings held apart, 

 while on the other hand mutations of so-called "achaete" appearance, 

 which cause an absence of bristles that is largely restricted to the middle 

 of the back, are exceedingly rare in both series. Again, studies by the 

 writer and more especially by Timofeef-Ressovsky (1937) have indicated 

 that the ratio of mutations so detrimental as to be practically certain of 

 killing the individual before its maturity — those designated as lethals — to 



