2.5 (1.8-3.4)

95% CI, 95% confidence interval.

95% CI, 95% confidence interval.

mice, mutation rates were estimated from five genes with observable effects on the phenotype of coat color (Table 5.1; Schlager & Dickie 1971). Rates of mutation per gene were between 1.8 and 16.6 mutations per 1 million gametes produced. This is equivalent to a mutation rate of (1.8-16.6) x 10-6 per locus per sexual generation. Very similar mutation rates for mice have been reported from more recent irradiation studies as well (Russell & Russell 1996). The rates of mutation from wild type to a novel allele (called forward mutations) are nearly a factor of 10 more common than mutations from a novel allele to wild type (termed reverse mutations). This asymmetry of forward and backward mutation rates per locus is a common observation in mutation experiments. It is a product of the fact that there are more ways mutation can cause a normal allele to malfunction than there are ways to exactly restore that function once it is disrupted. In this sense, forward and reverse mutation rates exist only because mutations are detected via their phenotypic effect.

Mutation rates can also be estimated in terms of the chance that a genome or a base pair mutates per replication or per sexual generation. At least in principle, the mutation rates for viruses and microbes can be estimated from direct examination of nucleotide sequences after correction for the error rate of the

DNA sequencing techniques, avoiding the underestimate of mutations that comes from detecting only those with a phenotypic effect. Table 5.2 summarizes mutation rate estimates presented in a comprehensive review of mutation rate data (Drake et al. 1998). These rates of mutation differ greatly across the taxonomic groups surveyed but tend to be very similar within the taxonomic groups. For microbes with DNA-based genomes, the rate of mutation per genome per replication clusters around 1/300. Since these organisms vary greatly in their genome sizes, the rate of mutation per base pair per generation ranges widely from 7.2 x 10-7 to 7.2 x 10-11. In eukaryotes, mutation rates per base pair per sexual generation all fall within a narrow range from 3.4 x 10-10to 5.0 x 10-11. If mutation rates per genome in eukaryotes are expressed in terms of the portion of the genome that contains coding genes (termed "effective genome size" by Drake et al. 1998), then the rate of mutation per effective genome in eukaryotes is statistically indistinguishable from the 1/300 rate of DNA-based microbes. The per-base-pair mutation rate estimates are in rough agreement with older per-locus estimates based on visible phenotypes. If the coat-color loci in Table 5.1 have around 1000 or 103 base pairs, then the per-base-pair mutation rates would be on the order of 10-8.

Table 5.2 Rates of spontaneous

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