In the classical twin model, it is assumed that the amount of genetic material shared by a pair of monozygotic (MZ) twins is 100% and by dizygotic (DZ) twins is 50% (Neale and Maes 2005). One of the strengths of the twin model above other types of familial relatedness is the potential to separate out similarity due to shared genes from similarity due to being raised in the same environment. When twins are raised together, the environment is assumed to be identical for both, whether MZ or DZ. Therefore increased similarity between MZ twins compared to DZ for a given trait suggest that variation in the trait is being affected more strongly by genetic factors (A), while increased similarity between DZ twins suggests a prominent effect of the shared environment (C). Non-additive genetic effects will also tend to further increase the degree of similarity between MZ twins compared to DZ, providing an estimate of dominance (D). The environmental factors unique to each individual and measurement error are combined in the residual variance term (E).
