a family more similar to one another. Nonshared or unique environmental effects (E) make members of twin pairs different. Important to note, E also includes measurement error. Considering proportions of variance, we denote the following: a2 for the proportion of total variance due to additive genetic effects, d2 for the proportion of total variance due to non-additivity, c2 for shared environmental contributions, and e2 for nonshared environmental variance.
