336 GENERAL DISCUSSION 



It has been proved that the intensity of the radiation alters the possi- 

 hihty of the re-combination of the fragments. Bnt for gene mutations 

 we speak of the repair of the primary damage. While not objecting to 

 the essence of the problem — the possibility of restoring gene mutations 

 — it is nevertheless appropriate to ask if this phenomenon has been 

 proved in experiments on the alteration of the radiation intensity. 



In work with Drosophila, Luning and others used recessive lethal 

 nnitations. At the same time, Dubinin, Khvostova and Mansurova 

 studied these mutations from the cytological point of view and showed 

 for example that for a dose of 2 kr about 30 per cent of the lethals are 

 connected with large-scale chromosome re-organizations. Only some 

 of the mutations in Luning's experiments were capable of genetic 

 repair. What is this part ? Can it be that these are precisely of the cate- 

 gory of chromosome arrangements? No analysis is available of this 

 question. 



Russel in his well-known experiments which have been described 

 here by Hollaender dealt with a family of loci in mice, which gave 

 recessive mutations with visible manifestations. They are recognized 

 as gene mutations. But is this so in all cases? There is no reason to 

 doubt that in this case the occurrence of many mutations is connected 

 with chromosome rearrangements. 



It has been shown in work with Drosophila that many mutations 

 previously regarded as classic examples of gene mutations, (e.g. yellowy 

 white, etc.) which arise after irradiation, in many cases turn out to 

 be connected with chromosome rearrangements. Certain hereditary 

 illnesses in man which have been regarded as gene mutations have also 

 proved to be connected with chromosome rearrangements. 



According to Russel's data, amongst the mutations studied, about 

 half fall into mutations of one locus "S" which possesses both recessive 

 and dominant manifestations. In the case where the mutations in this 

 one locus ai'e connected with chromosome rearrangements, the de- 

 l)endence upon the intensity of radiation, which does not concern gene 

 nnitations. can be connected wdth chromosome rearrangements. 



llussel has not studied the cytological nature of the radiation muta- 

 tions obtained by him in mice, and so the cj^uestion of the repair of 

 gene mutations in conditions of low radiation intensity remains 

 open. This question is so important that it is urgently necessary to 

 force a solution l)y using the category of true gene mutations. It must, 

 however, be said that this is a very difficult experiment. 



During recent months I have studied the influence of streptomycin on 

 spontaneous sex-linked recessive mutations in Drosophila and on the 

 s[)ontaneous emergence of chromosome aiTangements in the cells of 



