MUTAGENESIS 157 



combination frequencies and actual molecular distances, one could 

 say something from the number of sites. You would have to have 

 saturated the map at least within a factor of 3 to make this decis- 

 ion. 



Lederberg: Exactly. My impression was that you had not run the 

 map down to that level as yet. You could think that the map was 

 devised in triplets of nucleotides, and if recombination took place only 

 within the triplets — 



Neel: Seymour, if there were, in fact, three different reversion rates 

 at a well-studied locus which could be related to purine-pyrimidine 

 substitutions, doesn't it follow that for each reverse mutation there 

 should be two mutations, assuming a randomness, to an alternate 

 purine or pyrimidine which would not result in a change of phenotype 

 but would alter the rate of reverse mutation. Is it feasible to attack it 

 from that angle? 



Benzer: You want me to look for a change from one mutant to an- 

 other one that looks just like it, at the same site? 



Neel: That's right. But there is a change in the mutability state. 



Benzer: That is not an easy experiment. 



Summary of Discussion 



Demerec: The fact that Dr. Auerbach led the discussion in accord- 

 ance with a well-organized outline makes my assignment a great deal 

 easier. I do not intend to repeat in abbreviated form all the topics 

 covered by Dr. Auerbach but shall stress a few of the special prob- 

 lems that were taken up. 



Before going into a discussion of these problems, however, I wish to 

 say a few words about Dr. Auerbach 's introductory statement with 

 which I am in full agreement. She expressed concern lest the beautiful 

 findings about the nature of the genetic material obtained in chemical 

 studies of the transforming principle, of phages, and of viruses should 

 make us forget that this material resides in the cell where a great 

 many things happen that would not happen to anything in vitro. 

 We should try to avoid overemphasis on the results of work with simple 

 organisms to the neglect of prior information accumulated during 

 more than half a century of genetics research which, after all, forms a 

 solid foundation for the structure of knowledge we are trying to build. 

 We should be mindful that in many of the problems now being con- 

 sidered a backlog of reserve information already available might be 

 utilized to great advantage. 



In the brief review of the study of chemical mutagenesis it was 

 pointed out that attempts to induce mutations were first made with 



