MUTAGENS OF POTENTIAL SIGNIFICANCE 191 



Stern: How could you eliminate the possibility that homozygosity 

 would be the result not of mutation but of somatic crossing over or a 

 similar process? 



Atwood: That must be almost as rare as some of the mutations. 



Stern: Is that known? 



Atwood: I don't know, but at least we have evidence that it must be 

 very rare in the blood cells. 



Auerbach: I should like to say something about the possibility of 

 scoring sex-linked lethals in mice. Dr. Falconer and I worked out a 

 system for doing this and we have applied it to irradiated mice for 

 two years, but the result was disappointing. I had calculated that if 

 the lethal frequency in the screened section of the X chromosome was 

 the same as the average mutation frequency found by the Russells, 

 then we might obtain 2 per cent or more of lethals. In fact, we obtained 

 none in several hundred chromosomes. So it seems that the section of 

 the genome tested carries too few loci that can mutate to a lethal, and 

 we abandoned the test. Carter in Harwell had a similarly disappoint- 

 ing result when he tested for autosomal lethals by a method designed 

 by Haldane. On the other hand, Klein's system of treating tumor cells 

 did give what appeared to be mutations both in his experiments and 

 in those carried out in Edinburgh. 



Atwood: Klein's system has the advantage of very powerful screen- 

 ing, comparable to the kind of screening we can do in bacteria, but 

 could the induced compatibility be caused by chromosome loss? 



Auerbach: Dhalival did a few cytological observations and as far as 

 I remember he did get aneuploidy. 



Lederberg: I saw him a few weeks ago, and it looks as though it is 

 a crossing over, at least based on the observations. Where two com- 

 ponents can be recognized from two individuals who are doubly 

 heterozygous, so to speak, A and B, one gets out two types; one gets 

 out the type where both of these factors become pure homozygous 

 evidently, and types where one of these factors becomes homozygous 

 and not the other. This would be compatible with crossing over, and 

 it is a way of placing the relative order of the markers for the cen- 

 tromere. The technical possibilities of verifying this exist, because 

 there are additional components within the H-2 region, and if one can 

 obtain a seriation of these components by their pattern of homozygosity 

 in these evident somatic segregants that agrees with the mapping that 

 has already been conducted, I think it would pretty well establish the 

 result. But at the moment there is only the bias that they get one type 



