302 GENE MUTATIONS CAUSED BY RADIATION 



manner, that is, by given substances and/or with attendant conditions 

 of a suitable kind. This is shown clearly by the great variations in the 

 overall frequency of spontaneous gene mutations found in different ex- 

 periments, amounting to 10 times or more [Muller (40, 41)]. Not only 

 can genetic differences have effects as great as this [Demerec (12), Neel 

 (60), Ives and Andrews (30)], but also differences in cellular conditions, 

 depending on age and stage of development [Muller (48)]. The action 

 of ''temperature shocks" appears to come in the same category. Finally, 

 as has been known since the work of Auerbach and Robson [see sum- 

 mary by Auerbach (2)], chemical influences of specific types are potent 

 causes of mutation, a finding long to have been expected in view of the 

 above facts and considerations. 



The question, w'hich the present author raised in 1928 (41, p. 345), as to 

 whether gene mutation occurs by the chemical change of a pre-existing 

 gene or by a misstep in the synthesis of the daughter gene by its mother 

 gene, is one which, for spontaneous mutations, seldom admits of an 

 answer in any given case. However, the intense concentration of spon- 

 taneous gene-mutational occurrences in Drosophila into those stages of 

 the germ cycle in w^hich gene duplication (as evidenced by mitosis) is 

 going on more actively [Muller (48)], and earlier findings of a similar 

 nature in bacteria by Zamenhof (80) and later by others, strongly sug- 

 gest that in these cases spontaneous mutations usually take place by 

 misconstruction of the daughter gene. On the other hand, the accumula- 

 tion of spontaneous mutations in mature, dormant spermatozoa [Muller 

 (48)] indicates that in them it is the old gene which is being transformed. 

 The proof of this will not be complete, however, until it is shown that 

 the mutant offspring from the aged sperm are mutant throughout their 

 bodies. To obtain evidence on this matter for spontaneous mutations, 

 very large-scale work involving the study of certian particular types of 

 mutations, which affect the whole external surface of the body visibly, 

 is required. Yet, however that may be, the answer to the question is 

 already clear so far as the great majority of gene mutations induced by 

 radiation of spermatozoa is concerned: that is, in this case, the pre- 

 existing gene undoubtedly becomes altered in its composition. For, 

 wherever evidence concerning the point at issue is available, for radi- 

 ation mutations induced in spermatozoa, it is found that the whole body 

 is usually involved. 



In view of this apparent difference in the usual method of their origi- 

 nation, the spontaneous mutations studied having more often arisen 

 through the misconstruction of the daughter gene and the radiation mu- 

 tations studied having more frequently involved changes in the com- 

 position of the already finished gene, the previously noted similarity in 

 type of product between the spontaneous and the radiation mutations 



