PEOBLEMS OF MEASUREMENT OF MUTATION RATES 55 



Novick: I am reminded of the very famous case of Zinder and 

 Lederberg (46) , where they were finally the same locus. 



Lederberg: No, I don't think so. They simply were closely linked. I 

 might say that we have been studying a system very much like what 

 Dr. Zamenhof has mentioned, in B. subtilis involving tryptophan and 

 histidine markers. They are behaving perfectly conventionally in our 

 case, not necessarily the same mutants (37) . They revert and conform 

 conventionally.* 



Zamenhof: We have three linkage cases (11). In two of them, we 

 have only 65 per cent linkage. But this one is 100 per cent. 



Auerbach: When you have them reverted, can you make them 

 mutate individually again? 



Zamenhof: We didn't try that. But they were obtained individually. 



Auerbach: Yes, but how could you see whether they were still there 

 as an individual mutable unit? Will you now try to get auxotrophs 

 again? 



Zamenhof: We haven't tried yet. 



Atwood: I want to close by saying that I think we have enough 

 reason to suppose that the excessive human mutation rates, as com- 

 pared with mutation rates in microorganisms do not necessarily involve 

 major differences in the frequencies of the individual microevents that 

 constitute mutation. 



The purpose to be served by a mutation rate has more to do with 

 its validity than the value itself in this sense: if you are interested 

 in the effect of mutation on human gene frequencies, then the rates as 

 estimated in man are appropriate, but if you are interested in under- 

 standing the mutation process, these rates are likely to be misleading 

 and the rates measured by better methods in microorganisms are more 

 appropriate. 



Auerbach: May I ask one question? It is really connected with what 

 Dr. Magni said. For years, I have been trying to find the evidence for 

 the statement that mutation does occur independently in two al- 

 lelomorphs in the same nucleus. Muller, for instance, writes that when 

 you have a nucleus, one gene mutates, and there is no more than a 

 random chance that the allelomorph on the opposite chromosome will 

 also mutate. This is not easy to establish, and I don't really know 

 what the evidence is. 



Stern: Formerly, the evidence was seen in the following facts. In 

 Drosophila, a female which is heterozygous for a normal and a white 

 allele has red eyes. If a mutation occurs from normal to white, a white 



* Sneath and Lederberg, 1961. 



