Genetic Architecture of Quantitative Variation

Abstract The genetic architecture of quantitative variation plays an important role in evolutionary processes for complex traits. There are two aspects of the genetic architecture of quantitative variation that need to be considered. First, there is the genetic architecture of the quantitative traits themselves, including: (1)the number of loci that affect trait variation; (2)the typical size of genotypic effects; (3) whether alleles at a single locus display dominance interactions; (4) whether alleles at multiple loci engage in epistatic interactions; and (5) the range and pleiotropic patterns of gene effects. These features describe the genetic architecture of a trait and are specified in quantitative genetic theory by the genotypic values at a locus or set of loci (Falconer & Mackay 1996). Genotypic values are simply the mean phenotypic value of individuals carrying a specified genotype (see Box 19.1). These genotypic value definitions do not involve allele frequencies. The second aspect of the genetic architecture of quantitative variation is the genotype and allele frequencies at the loci in question. Genotypic values and allele frequencies are combined to define the population genie values that in turn specify the components of genetic variation and, hence, help to determine the response to evolutionary forces such as selection and genetic drift (Box 19.2; Falconer & Mackay 1996). It is through its manifold effects on heritable genetic variation that genetic architecture impacts evolutionary processes.