Data from MZ and DZ twin pairs can be utilized to quantify the relative contribution of additive genetic (A), shared environmental (C) and non-shared environmental (E) factors to a phenotype. Non-additive (including dominance, and thus denoted as D) genetic influences can be estimated in place of, but not jointly with C when data from twins alone are used. The decision to fit an ACE vs. ADE model is based on the ratio of DZ to MZ twin correlations on the phenotype of interest. A DZ correlation greater than half the MZ correlation indicates that environmental influences that make members of a twin pair similar (C) are contributing to the phenotype and thus provides support for an ACE model. Conversely, a DZ correlation less than half the MZ correlation supports an ADE model. (See Martin et al. (1997) for an overview of twin methodology).
