deleterious mutation

García-Dorado A, López-Fanjul C & Caballero A 1999 Properties of spontaneous mutations affecting quantitative traits. Genet Res 74:341-350.

• although the rate (λ) and effect (s) of very slight deleterious mutation remain unknown, most mutational fitness decline would come from moderately deleterious mutation (s ≈ 0⋅2, λ ≈ 0⋅03)
• this situation would not qualitatively change in harsh environments
• estimates of the average coefficient of dominance (h) of non-severe deleterious mutations are controversial
• the typical value of h = 0⋅4 can be questioned
• a lower estimate (about 0⋅1) is suggested
• estimated mutational parameters are remarkably alike for morphological and fitness component traits (excluding lethals), indicating low mutation rates and moderate mutational effects

• Table 1

• the main interest is in the dominance of mutations with small effect on viability
• there seems to be general agreement about the dominance coefficient of lethal mutants (h = 0⋅01–0⋅03; Simmons & Crow, 1977)
• relevant results are therefore for chromosomes with viability greater than about 60%, called quasinormals

• the regression of heterozygous on homozygous viabilities
• h = cov(v_ii + v_jj, v_ij) / var(v_ii + v_jj) ... (2)
• (Mukai & Yamazaki, 1968)
• the denominator refers to the genotypic component of variance
• apparently, no estimates using (2) were computed by Ohnishi
• we have reanalysed Ohnishi's results for quasinormal chromosomes by means of (2)
• the estimated h is around 0⋅1 for both coupling and repulsion heterozygotes (A. García-Dorado & A. Caballero, unpublished)

• it is possible that the above estimate of 0⋅1 may be a little biased downwards
• the generally assumed estimate of about 0⋅4 for the dominance coefficient of mildly deleterious mutations affecting viability in D. melanogaster can be, at least, questioned