Discussion 239 



called first-step mutants merely because they are distinguished in the 

 first attempt at selection? 



Cavalli-SJorza: If that happens after one exposure, I think yes. A 

 first-step mutant might on the other hand be the result of more than 

 one mutation: only genetic analysis could tell. This depends of course 

 on conditions of selection, and on mutation rates. 



Lederberg: This terminology was developed when our only method of 

 analysis was mutational; the first-step was just an operational statement 

 as to how many sequences of selection were made. From that you might 

 wish to infer that there was a one-gene change, which is not necessarily 

 true. We now have methods of finding out in some organisms how many 

 genes are involved at various levels of resistance. I think that is a much 

 more important question if what you are interested in is the genetic 

 structure of the resistant forms. I would not ordinarily rely too implicitly 

 on the number of steps with which you could get a highly resistant form 

 to tell how many genes are involved. Dr. Hotchkiss has some contra- 

 dictory evidence on this point. In one system with penicillin he was able 

 to correlate them very well, in another with sulphonamide resistance he 

 had a one-step isolation which has given him at least three loci. Ap- 

 parently there can be coincidences or accidents which will lead to some 

 discrepancy between the number of steps you think you made and the 

 number of mutations which have really accumulated. 



Dean : It becomes rather important when you calculate mutation rates 

 from the Lea-Coulson formulae, because in their mathematical analysis 

 the distribution is based by hypothesis on one genetic change. It should 

 not be applied to polygenic systems. 



Lederberg: You can make reasonable corrections for it, if you keep in 

 mind that what you are measuring is the summation of all genetic 

 changes that have the phenotype for which you are scoring. 



Fulton: It is a most difficult problem to isolate DNA in highly poly- 

 merized form from bacteria, and I should like to know what accompanies 

 the DNA in the bacteriologists' cell-extracts. Since even the elegant 

 experiments of McCarty on the isolation of DNA from pneumococci have 

 been criticized, do you think that products labelled DNA by the 

 bacteriologist are really DNA ? 



Yudkin: The important point is that the transforming activity is 

 destroyed by DNAse. 



Hotchkiss: Then you have another tube in the laboratory which one 

 labels DNAse ; how good is that DNAse ? 



Davis: Those interested in the genetics of drug resistance have 

 deliberately studied only increments of resistance large enough to permit 

 clean screening. It seems to me we pose impossible questions when we 

 take the techniques that were suitable for such large degrees of resistance 

 and apply them in the region of tiny-step resistance. As Dr. Gyorffy 

 pointed out, in this region there is an overlap between physiological 

 variations and mutations. A cell may survive a borderline concentration 

 of drug for physiological reasons and then continue to grow in a slow, 

 struggling manner. But before this cell has given rise to a colony of a 

 million cells the clone may have developed one or more mutations that 



