Models of gene action for complex traits. Phenotypic values are shown for two diploid host loci under different patterns of gene action. (a) Additive effects at locus A and B. Each allele contributes a fixed metric value to the trait, independent of the effects of other alleles at the same or different loci. Human height is a classic example of a complex trait where approximately 80% of the variation in height amongst individuals is due to additive genetic effects . (b) Dominance at locus A. Both loci are independent, but with dominance occurring at locus A, as the phenotypic value of the heterozygote is not midway between the values of the two homozygotes. Complete dominance, as shown here, is typical of Mendelian genetic disorders such as Huntington's disease where an affected individual need only inherit one copy of the mutant allele. (c) Epistatic interactions between locus A and B. Epistasis is estimated as the deviance from the additive combination of two loci and can take many forms, depending on whether an allele combination is more or less fit than expected. Well known examples of epistasis include the interaction between genes in shaping coat color in mice, or the occurrence of synthetic lethality seen when mutations occur in two genes with redundant functions .
Hall and Ebert BMC Biology 2013 11:79 doi:10.1186/1741-7007-11-79