182 H. MARCO VICH 



close to each other and during the chromosome's injection introchice them- 

 selves together. You irradiate and pick out those recombinants which contain 

 both these loci, and obtain that curve which I have drawn. 



HERCIK: If you try to take only one of the loci, you would obtain approximately 

 the same curve. It does not mean that the locus itself is damaged but that the 

 probability of its reapj^earance in the recombinant is decreasing. Does it mean 

 that there is a break in tlie chromosome? 



MARCOViCH : It is possible. 



ZHUKOV-VEREZHNicov : What is the minimum radiation dose to which your sys- 

 tem can react with sufficient statistical reliability? 



MARCOVICH: The lowest dose accepted in these exi:)eriments was 5,000 rad. This 

 material is not suitable for studying smaller doses. 



ASTAUROV: Dominant lethals at least in the highest organisms, constitute a com- 

 plex group of genetic changes. The greater part of them consist of chromosome 

 lesions belonging to the type of deficiencies or breaks. Could it not be that 

 chromosomes with such great lesions would be unable to penetrate at all, and 

 that such disturbances should be left out in this process? 



MARCOVICH : It is possible that such breaks may indeed partially explain the death 

 of recombinants. But it seems to me that they could not explain the survival 

 curve. Most likely in bacteria the loss of chromosome material does not lead to 

 the cell's death. In "male" bacteria the loss of chromosome material does not harm 

 them since if crossing occurs the "males" lose part of their nucleus, but do not die. 



ARDASHNicov: Plcasc explain the correction factor. 



MARCOVICH: a is the sensitivity of the whole cell. If N is the chromosome num- 

 ber, and by accident only one chromosome would be isolated, then the expression 

 of this damage would be cc/N. 



ARDASHNicov: Why do you connect the exponential curve with the presence of 

 dominant lethals? Other factors may account for it, for example, damage to any 

 other unique structure, indispensable for the life of the given organism. 



MARCOVICH: I cjuite agree with you. The only thing I have said is that there are 

 two alternatives, one of them testing the chromosome dainage. But it seems to me 

 that my results sliow that it is not the cause and the damage should be sought 

 elsewhere. 



