A PLEIOTROPIC NONADDITIVE MODEL OF VARIATION IN QUANTITATIVE TRAITS

A model of mutation-selection-drift balance incorporating pleiotropic and dominance effects of new mutations on quantitative traits and fitn ess is investigated and used to predict the amount and nature of genet ic variation maintained in segregating populations. The model is based on recent information on the joint distribution of mutant effects on bristle traits and fitness in Drosophila melanogaster from experiments on the accumulation of spontaneous and P element-induced mutations. T hese experiments suggest a leptokurtic distribution of effects with an intermediate correlation between effects on the trait and fitness. Mu tants of large effect tend to be partially recessive while those with smaller effect are on average additive, but apparently with very varia ble gene action. The model is parameterized with two different sets of information derived from P element insertion and spontaneous mutation data, though the latter are not fully known. They differ in the numbe r of mutations per generation which is assumed to affect the trait. Pr edictions of the variance maintained for bristle number assuming param eters derived from effects of P element insertions, in which the propo rtion of mutations with an effect on the trait is small, fit reasonabl y well with experimental observations. The equilibrium genetic varianc e is nearly independent of the degree of dominance of new mutations. H eritabilities of between 0.4 and 0.6 are predicted with population siz es from 10(4) to 10(6), and most of the variance for the metric trait in segregating populations is due to a small proportion of mutations ( about 1% of the total number) with neutral or nearly neutral effects o n fitness and intermediate effects on the trait (0.1-0.5 sigma(P)). Mu ch of the genetic variance is contributed by recessive or partially re cessive mutants, but only a small proportion (about 10%) of the geneti c variance is dominance variance. The amount of apparent selection on the trait itself generated by the model is very small. If a model is a ssumed in which all mutation events have an effect on the quantitative trait, the majority of the genetic variance is contributed by deleter ious mutations with tiny effects on the trait. If such a model is assu med for viability, the heritability is about 0.1, independent of the p opulation size.