PROBLEMS OF MEASUREMENT OF MUTATION RATES 53 



rate per cell division is actually higher when the cells are dividing 

 rapidly than when they are dividing slowly. The mature mussel grows 

 about a centimeter in length in a year, so that in a year it does not 

 even double the perimeter, if it is already of fair size. This means that, 

 barring turnover, the shell gland cells in large specimens divide less 

 than once a year. 



Freese: I think that you have to be very careful about the word 

 "mutation" in this connection, because you cannot even exclude 

 cytoplasmic inheritance, in which cytoplasmic particles segregate early 

 in the life of the mussel. 



Atwood: Yes, the genetic basis isn't known. 



Now we come to the problem of how the two alleles at the ABO 

 locus do not seem to be independent. Independence of mutational 

 events has seldom been tested. Perhaps others here can cite such 

 examples. One was that, many years ago, Szybalski and Bryson com- 

 pared single and double mutations in E. coli (39). The double muta- 

 tions, if I remember correctly, were at least ten times more frequent 

 than the product of the single frequencies, or perhaps more than that. 

 Does anyone recall that paper? 



Demerec: Yes, the material has been published only in abstract form, 

 in the annual reports of the Biological Laboratory (4, 5) . He was ob- 

 serving not reversions but changes from the wild type to the mutant 

 type. 



Magni: We recently had some evidence on an effect similar to the 

 one mentioned by Dr. Atwood. We have studied back mutation induced 

 by X-rays for two independent mutons in the same methionine cistron 

 in yeast. A diploid strain was used which was carrying the two mutants 

 in transposition, and because they were not complementing each other, 

 it was methionine dependent. After X-ray treatment methionine in- 

 dependent colonies were selected and genetically analyzed. The pre- 

 liminary result was that, with an X-ray dose inducing back mutations 

 at any one of the two sites with a frequency of the order of 10-^, we 

 found double mutants with a frequency of 2-3 X 10"''' while the ex- 

 pected one was 10"^. 



Freese: Was it hard or soft X-ray? 



Magni: It was very soft. The water half- value-layer was 2 mm. 



Auerbach: Did you ever see whether you got something more com- 

 parable to Dr. Atwood's story? Did you ever find both alleles of the 

 same locus mutate simultaneously in the same cell? 



Magni: I have no data on true alleles. The mutants I have used can 

 be more properly considered as pseudoalleles. 



