56 MUTATIONS 



spot will be formed in the eye on the red background. Such spots occur 

 with a certain frequency. In a fly which is homozygous for the normal 

 allele white spots will be formed only when both alleles have mutated. 

 The extremely low frequency of white spots in homozygous red eyes, 

 assuming that they occur at all, seemed to indicate that simultaneous 

 mutations of both alleles are much less frequent than mutation of a 

 single allele. The argument, however, has been greatly weakened by 

 the finding that white spots in heterozygotes may not be the result of 

 mutation but of somatic crossing over.* 



Auerbach: But I wonder if there is any other evidence for it. 



Magni: We have data in one case which is perhaps not perfectly 

 pertinent but nevertheless is worthwhile to be mentioned. Studying 

 the spontaneous back mutation rate of site meti-.i in yeast we have 

 observed a discrepancy between mutation rate in haploid strains and 

 in diploids homozygous for the same mutant, the rate in diploids being 

 higher than expected. 



Auerbach: It should be done in the same cell. 



Stern: There is similar evidence in Drosophila derived from the 

 frequency of white spots in hemizygous and homozygous red-eyed 

 individuals. In hemizygotes the spots are more frequent than sup- 

 posedly in homozygotes. Here, somatic crossing over is, of course, ruled 

 out as an explanation, but as in yeast the difference in mutation rates 

 is obtained in different types of cells, those with one and those with 

 two alleles. 



Lederberg: I think there are any number of cases in pericarp and in 

 the aleurone mutation in corn where it has been established that the 

 mutation goes from a homozygous to a heterozygous condition, by 

 examining the embryo in the rows. I don't think it can be a question 

 of the possibility of qualitative independence of mutation in one allele 

 and another. I don't know of any case outside of the microorganisms, 

 either, which would make any cjuantitative statements as to the fre- 

 quency of mutation. I remember looking quite hard for this a while ago. 



Auerbach: But it could be checked in plants, really, as to the muta- 

 tion frequency. 



Lederberg: I don't know where in plants you really have that good 

 a measurement of rate of mutation, outside of the somatic cell situa- 

 tion, and then there is always this question. 



* Addendum : The problem of somatic mutation of one or both alleles has first 

 been discussed by R. A. Emerson. The nature of bud variations as indicated by 

 their mode of inheritance, Am. Nat. 56: 64-79 (1922). He concluded that only one 

 of two alleles underwent mutation. 



