128 MUTATIONS 



Auerbach: But that should make it more uniform in spermatozoa. 



Russell: The absolute rate at all loci is much more uniform. 



Auerbach: Oh, yes, that's what I meant. 



Neel: Bill, would you expect that following spermatozoa irradiation, 

 the apparent "specific locus phenotypes" would tend to be accompanied 

 by other abnormalities if deletions are a significant part of the picture? 



Russell: Yes. 



Neel: And do you see accompanying abnormalities? 



Russell: Most of the specific locus mutations, even those induced in 

 spermatogonia, are completely lethal homozygous, so we would have 

 to look for this mainly in heterozygotes. I would say, in general, that 

 the ones induced in spermatozoa are not as viable as those induced in 

 spermatogonia. Some of them are associated with translocations and 

 some are clearly deficiencies, because we have two markers close 

 together which go out at the same time. I think there is a greater 

 amount of size reduction in the mutant animals themselves for those 

 induced in spermatozoa, but this hasn't really been put on a quantita- 

 tive basis. 



Auerbach: Then I come to microorganisms, where there is abundant 

 evidence for mutagen specificity at the observational level. I think 

 what I have to say will sound rather naive, but I am really interested 

 in discussing for a few examples at which stage of mutagenesis the 

 observed specificity is likely to arise. 



Glass: Before you begin with the microorganisms, would you say 

 something about the work of the Fahmys (22)? 



Auerbach: You will find a criticism of their latest claims in the 

 present number of Genetics (5). I wrote it together with a statistician, 

 and the conclusion we arrived at was that their experiments do not 

 disprove the possibility of mutagen specificity in Drosophila (21), but 

 that they do not prove it either. 



However, the Fahmys have meanwhile started on what to me seems 

 a much more promising line. Their work so far had been concerned 

 with testing whether certain visible mutations occur more frequently 

 after one type of treatment than after another. Now these visible 

 mutations are "forward" mutations and presumably occur at many 

 sites within a locus, so one hardly would expect much mutagen 

 specificity among them. But now the Fahmys have started to analyze 

 one particular locus, the "rudimentary" locus, into what they claim 

 are different cistrons. They have published a very short note (22) in 

 which they say that these cistrons may differ in their response to 

 mutagens. If this can be substantiated it would be more in line with 



