The appearance of the “r” phenotype was scored using single-burst experiments (2). Given that replication is close to binary (2) and assuming that “r” mutants are neutral, the F method described by Drake can be used and yields a mutation rate to the phenotype of 4.8 ´ 10^-5 (1). However, it is also possible to use the null-class method: Out of 22615 bursts, 85 gave one or more mutants (after correcting for the presence of mutants in the inoculum), and B = 82. Hence, m = - log ((22615 - 85) / 22615) / 82 = 4.6 ´ 10^-5, which is consistent with the estimate obtained with the F method. T_s = 1576 and 27 / 121 mutations were nucleotide substitutions (see calculation of the mutation rate per cell infection cycle for this same phage). Hence, m_s/n/r = 3 ´ 4.6 ´ 10^-5 ´ 27 / 121 / 1576 = 2.0 ´ 10^-8.

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.