Genetic modeling takes advantage of the differing degrees of genetic relatedness among MZ versus DZ twin pairs. MZ twins share all of their additive and non-additive genetic effects, while DZ pairs share, on average, 50% of the additive and 25% of their non-additive genetic effects. Shared environmental effects are assumed to correlate 1.0 between members of both MZ and DZ pairs. Consequently, the phenotypic correlation between MZ twin pairs is determined by rMZ = a2 + d2 + c2 and the phenotypic correlation between members of DZ pairs is determined by rDZ = .5a2 + .25d2 + c2. Examining the pattern of MZ and DZ correlations can provide guidance on model fitting strategy, such that (a) .5rMZ = rDZ, suggests that the phenotype is due to additive genetic influences; (b) .5rMZ < rDZ, suggests that the phenotype is due to both additive genetic and shared environmental influences; or (c) .5rMZ > rDZ, suggests that the phenotype is due to additive and non-additive genetic influences.
