Three possible models can be specified for the effects of the MAOA locus in females using the classical biometrical-genetic approach (Fisher, 1918; Mather & Jinks, 1982): (1) an ‘additive’ model, or one with no dominance in which the phenotypic mean of the heterozygotes is between the means of the two homozygotes; (2) a model of ‘complete dominance’ in which the heterozygote mean is the same as either of the two homozygotes; and (3) a model of ‘incomplete dominance’ in which the mean phenotype of heterozygotes resides between the two homozygotes, while differing from the mid-point. Both additive and dominance effects may interact with the environment if genotypes differ in their sensitivity to the environment.
