Evaluating the genetic architecture of quantitative traits via selection followed by inbreeding

The deleterious mutation model proposes that quantitative trait variation should be dominated by rare, partially recessive, deleterious mutations. Following artificial selection on a focal trait, the ratio of the difference in inbreeding effects between control and selected populations (Delta B), to the difference in trait means caused by directional selection (Delta M), can inform the extent to which deleterious mutations cause quantitative trait variation. Here, we apply the Delta B/Delta M ratio test to two quantitative traits (male mating success and body size) in Drosophila melanogaster. For both traits, Delta B/Delta M ratios suggested that intermediate-frequency alleles, rather than rare, partially recessive alleles (i.e. deleterious mutations), caused quantitative trait variation. We discuss these results in relation to viability data, exploring how differences between regimens in segregating (measured through inbreeding) and fixed (measured through population crosses) mutational load could affect the ratio test. Finally, we present simulations that test the statistical power of the ratio test, providing guidelines for future research.