70 MUTATIONS 



Auerhach: But that wouldn't happen in Drosophila, by segregation 

 ratios. 



Neel: Yet a considerable number of little repeat regions are known 

 in D. melanog aster. 



Auerhach: Yes, that is true. 



Lederberg: Jim, did I understand correctly that you were suggesting 

 that the greatest number of mutations would reach their expression 

 in man and not, perhaps, in bacteria? You were implying that lethals 

 would not be the most common event of the genetic imbalance in man? 



Neel: I was not implying anything so much as raising a question, 

 whether the nature of Drosophila or some bacteria may be such that 

 any nucleotide change will result in a detectable effect whereas, in 

 another species, a nucleotide change will not have the same probability 

 of a phenotypic effect. 



Magni: It depends also to a great extent on the technique for scoring 

 mutants. For a bacterium unable to grow on a minimal medium, for 

 example, the number of mutants one could be able to pick up should 

 be much lower than for E. coli, and this would not be due to a 

 peculiarity of the organism but rather to our technical approach to the 

 problem. 



Glass: To answer Joshua Lederberg's question in another way, we 

 most certainly do not find lethals as the most common kind of 

 mutation in Drosophila. In those studies that have been carried out 

 to get comparable figures, detrimentals that do not produce lethal 

 effects are more common, much more common, than lethals (23, 41). 

 It is largely because of the nature of the technique of screening in 

 microbial genetics that you get lethals as the most common type of 

 mutation. Isn't that so? 



Lederberg: It's an interesting point, I think, that has been suggested 

 before — that the thousandfold difference in the information content 

 of bacteria and of man does not represent a whit of difference in bio- 

 chemical enzymatic information, but just in the more subtle controls, 

 and these may perhaps often be partial systematic effects. 



Goodgal: I don't think the definition of lethals in the microorganism 

 is quite valid, because I think a number of people are finding now 

 that you can separate mutation from lethal by controlling things like 

 RNA synthesis, so you get a very high frequency of mutation with 

 very little killing. 



Lederberg: The word "lethals" has been used here to mean lethal 

 mutation, and it should be distinguished from bactericidal effect. 



Goodgal: Yes, but if you're getting mutations without very much 

 killing, then you can't be getting lethal mutations, 



