S. E. LURIA 



which cannot be recognized by inspection. A first approximation can be made by 

 assuming that all "doubles" include a clone of one mutant, and that they all 

 occur among the most frequent clone size classes observed (plates with 2, 3, 4 

 mutants). The resulting corrected distribution has a slight excess of ones, but 

 does not deviate significantly from Equation (2), in spite of the fact that the 

 correction is an extreme one, which concentrates all the distortion in the initial, 

 most critical portion of the frequency distribution curve. 



3. Failure of the plaque count method to reveal the full number of mutants is 

 unhkely to result from technical reasons, but might be due to intrinsic properties 

 of phage reproduction. For example, only a fraction of the gene copies produced 

 might appear in active phage particles because of loss or inactivation of a certain 

 proportion of phage within the bacteria before liberation. Such "sampling 

 losses," superimposed on a logarithmic distribution, would make the initial slope 

 of log Y versus log x steeper; a samphng loss of 50 per cent would give a quite 

 appreciable deviation from the initial slope of —1. This is in conflict with our 

 results. Sampling losses could be superimposed on any other distribution, but it 

 is hard to visualize how the results could simulate those of an exponential 

 distribution. 



A variety of alternate hypotheses was considered in an attempt to find one, 

 besides that of exponential reproduction, that could lead to the results found 

 experimentally. No sensible hypothesis could be devised. Altogether, the 

 hypothesis of mutations occurring at a constant rate in the course of expoiiential 

 nonsynchronized gene reduphcation appears adequate to account for our results. 



Mutation Rates 



Our results permit fairly accurate estimations of mutation rates. A total of 

 87.6 r mutations (85 mutant clones plus 2.6 coincidences) occurred in 23,000 

 bursts, producing approximately 1,850,000 active phage particles. The mutation 

 frequency per reduplication is around 5 X 10~^. 102.3 w mutations in 11,000 

 bursts, or 880,000 particles, correspond to a mutation frequency of 1.2 X 10~*. 

 These mutation rates, of course, are the sums of the mutation rates at all the 

 individual loci that can mutate to give either the r or the w phenotype. 



Incidental Observations 



1. The mutants of T2L classified as r or tt^ are generally clearly recognizable as 

 such. The r phenotype, however, is not uniform; different r mutations give 

 plaque types often distinguishable from one another. The w phenotype, though 

 generally sharply distinct from r and from wild-type, is even more variable. Only 

 one clone (consisting of one plaque), even after repeated replatings, could not be 

 classified with certainty as either w or r; since at any rate it seemed to represent 

 a novel phenotype, it was excluded from the analysis. 



2. Several other types of mutants were observed in the course of our experi- 

 ments, mainly "minute" or "sharp" plaque types. These were not included in 

 the results. 



148 



