Mutations at the rII locus (L = 3136) producing rapid plaque growth (phenotype r) were scored in single bursts (2). After discarding cases in which the mutant was probably present in the inoculum, 420 mutants were scored in 22615 bursts (c = 1), and it was determined that B = 82. This gives f / c = 420 / 22615 / 82 = 2.3 ´ 10^-4. Mutations were probably close to neutral (a » 1) and deviations from neutrality should not produce a strong bias, since c = 1. The mutational spectrum of this gene was analyzed for the closely related bacteriophage T4 (1). Fifteen nonsense (all of which should produce the phenotype) and 21 missense mutations were identified in a 435 base region of the locus, and nonsense mutations were expected to represent ca. 0.073 of all random substitutions. Hence, the expected number of substitutions (including those that did not produce the “r” phenotype) is 15 / 0.073 = 206, indicating that 21 / 206 = 0.102 of missense mutations produced the “r” phenotype. In another assay, it was shown that among 121 observed rII mutants, 27 were single-nucleotide substitutions and 94 were indels. T_s is not simply 3L because many mutations were not observable. Since nonsense mutations represented approximately 0.073 of all mutations and 0.102 of missense mutations were observable, T_s = 3L ´ (0.073 + 0.102 ´ (1 - 0.073)) = 1576. Thus m_s/n/c = 3 ´ 2.3 ´ 10^-4 ´ 27 / 121 / 1576 = 9.8 ´ 10^-8, m_i/n/c = 2.3 ´ 10^-4 ´ 94 / 121 / 3136 = 5.7 ´ 10^-8, and d = 0.37.

1. Drake, J. W. 1991. A constant rate of spontaneous mutation in DNA-based microbes. Proc. Natl. Acad. Sci. USA 88:7160-7164.

2. Luria, S. E. 1951. The frequency distribution of spontaneous bacteriophage mutants as evidence for the exponential rate of phage reproduction. Cold Spring Harb. Symp. Quant. Biol. 16:463-470.