: NATURE OF THE GENETIC EFFECTS 421 



replication process. Light might be thrown on these questions by means 

 of tracer studies. In the meantime, however, it must at least be admitted 

 that these mutations do change the "old genes," in the more Hmited sense 

 of not involving missteps in the formation of those daughter genes which 

 are manufactured for the chromosomes of daughter cells. 



Whatever interpretation of the above findings is preferred, there is one 

 established series of results which only by a stretching of assumptions 

 could be explained otherwise than by the occurrence of changes in already 

 formed gene material. These are the data showing that treatment of 

 mature Drosophila spermatozoa with ionizing radiation gives rise to 

 mutant genes which are inherited by all parts of the offspring. If the 

 mutations had been such as to involve only a change in the building of a 

 daughter gene, then the offspring would be a mosaic, for it would have 

 received a mutant daughter gene in one of the two nuclei of its ''two-cell 

 stage" and an unchanged mother gene in the other. In the case of some 

 "visible" gene mutations, expressed in virtually all parts of the epi- 

 dermis (and sometimes in some internal parts also) as a visible change in 

 morphology or pigmentation, this mosaicism would be evident by causing 

 a patchwork appearance of the mutant characteristic. Yet it has been 

 found that, on the contrary, such gene mutations in the great majority of 

 cases show their effects throughout all parts of the body which are capable 

 of expressing them. The further fact that, in the spermatozoon itself, 

 the gene lies in a dormant condition, makes it unlikely that the old gene 

 becomes changed within the spermatozoon through a misstep in some 

 otherwise normal process of turnover. It is true that a postponed turn- 

 over, occurring just after fertilization, might be invoked ad hoc, as an 

 improbable alternative, to escape this conclusion. In that case, however, 

 there would have to have been an intermediate step, in preparation for 

 the mutation, and this step must itself have persisted unchanged through- 

 out the spermatozoon stage; the subsequent misstep in turnover would 

 then have to be very precisely timed, so as to take place before gene 

 reproduction proper occurs in the fertilized egg. It is much simpler to 

 suppose that the radiation permanently changed the old gene, within the 

 spermatozoon itself. 



19. RELATION BETWEEN MUTAGENICITY AND CARCINOGENICITY 



As was indicated in Sect. 11, the question whether mutations (pre- 

 sumably "point mutations" of some kind) produced in somatic cells by 

 radiation form the basis of the carcinogenic effect of radiation constitutes 

 a part of the more general problem of whether somatic mutations, no 

 matter how caused, result in malignancies. A further consideration of 

 this topic has been deferred to this point because a number of the matters 

 bearing upon it have been presented in our treatment, in the two preced- 



