In the absence of experimental data to determine the levels of MAOA expression of homozygotes and heterozygotes, it remains possible to specify their phenotypic differences by dummy coding of the homozygous differences and heterozygous deviations between genotypes at the locus (Mather & Jinks, 1982; Falconer & Mackay, 1996). Thus, the phenotype midway between the homozygous phenotypes is defined as m. The value h is used to identify the phenotypic departure of the heterozygote from m. +d and −d are the phenotypic differences of the homozygotes from the mid-point. Thus, d refers to the fixable or additive genetic variation, and h reflects the unfixable heritable variation or dominance properties. Adapting the framework to reflect a dichotomous phenotype, the contribution associated with one homozygous genotype (AA) to the phenotype can be denoted as 1, and the contribution of the other homozygous genotype (aa) is defined as −1 and the heterozygote as 0 (Fig. 1) (Fisher et al. 1932).
