Estimates of these genetic and environmental variance components can be obtained because A, C and E each predict different patterns of MZ and DZ twin pair correlations. MZ twins share all their genes, while DZ twins share on average 50% of their genes. Hence, if A were the sole source of variance in a trait, twin correlations of 1.0 for MZ pairs and 0.5 for DZ pairs are expected. If C were the sole source of variance in a trait, a twin correlation of 1.0 for both MZ and DZ pairs is expected, and if E would be the sole source of variance in a trait, a twin correlation of 0.0 for both MZ and DZ pairs is expected.
