Fisher (1918), Wright (1968), and others in the field of quantitative genetics derived sophisticated methods of estimating variance components from correlations of traits in family members with different degrees of relatedness. These techniques make it possible to parcellate variance in the absence of more precise data regarding what the specific operative genetic and environmental factors actually are. Twin studies are one of the most powerful study designs for this purpose. The classic twin model assumes that the amount of genetic material shared by two family members is 100% in monozygotic (MZ) twins and 50% in dizygotic (DZ) twins. When twins are raised together, the environment is also assumed to be identical for both individuals. Increases in phenotypic similarity between MZ twins compared to DZ is ascribed to genetic factors. Similarity because of shared environmental effects (C) will increase the similarity between DZ twins. Nonadditive genetic effects will tend to increase the similarity in MZ twins relative to DZ, providing an estimate of dominance (D). The residual variance (E) is a combination of environmental factors unique to the individual combined with measurement error.
