Classic path model for analyzing a sample of monozygotic (MZ) and dizygotic (DZ) twins.

The single-headed arrows (h, c, d, e) reflect the additive genetic (A), dominance genetic (D), common environmental (C), and specific environmental (E) effects on the phenotypes of twins (P_T1and P_T2). The double-headed arrows across the top show the expected relations among the additive genetic, dominance genetic, and common environmental effects for MZ and DZ twins.