Measuring the coefficient of dominance, h, in particular, for mutations of small effect (s), which constitute the vast majority of all mutations, is difficult. Direct estimates can be obtained from spontaneous mutations in mutation-accumulation experiments of highly inbred lines. Such assays are very laborious. By assuming mutation-selection balance, indirect estimates can be obtained from the structure of segregating populations. These estimates, however, are based on additional assumptions that often cannot be checked and also face statistical challenges. Therefore, they are not very reliable, and the development of methods for measuring dominance coefficients is an active field of research.
All known data demonstrate widespread correlations between dominance and fitness effects of mutations; that is, mutations with a large deleterious effect tend to be more recessive than those with a small effect. This is true for coding genes and, more recently, has also been shown for noncoding genes. Because fitness and dominance effects vary across loci, and the locus at which a mutation occurs can often not be determined, in general, only average coefficients of dominance (h) and selection (h) can be inferred. Available data suggest that h is about 0.1-0.2 and decays approximately exponentially with increasing s.
Mutations affect fitness through phenotypic traits. In fact, the fitness effect is a consequence of a mutation's effect on one or several traits. If these are closely correlated with fitness, then the fitness effect may be large. Many traits, however, for instance morphological traits, influence fitness only weakly. Dominance effects have been estimated with respect to traits that can be measured on a metric scale. For such metric traits (e.g., body size, oil content in maize, or abdominal bristle number in Drosophila, the latter being extremely well studied genetically), average dominance coefficients appear to be close to 0.5 and depend only weakly on the size of the muta-tional effect. In particular, no significant skew has been observed.
Compared with intermediate dominance, over- and underdominance are rare. The probably best-known example for overdominance is sickle-cell anemia, which provides immunity against malaria in heterozygous state. Over- and underdominance often seem to be the consequence of frequency-dependent selection or spatially heterogeneous selection. The first occurs if the heterozygotes have an overall advantage, for instance because they are generalists and can use most or all available resources. Underdominance can easily occur by migration-selection balance if each of the two homozygotes is optimally adapted to a different ecological niche and heterozygotes are maladapted because they do not have their own niche.
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