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Figure 2. Bivariate twin model for genetic and environmental influences on systolic blood pressure (SBP). Biometrical genetic theory specifies that the additive genetic factors (denoted by A and As) of (monozygotic) MZ twins are perfectly correlated (1.0), whereas those of dizygotic (DZ) twins are correlated 0.5. Common environmental factors shared by twins from the same family (denoted by C and Cs) are correlated unity for both types of twins, whereas the unique environmental influences (E and Es) are always uncorrelated. Path coefficient a11 quantifies the effect of genetic influence A on SBP at rest, a21 quantifies the effect of A on SBP during stress, and a22 quantifies the effect of emergent genes in As on SBP during stress. In a similar way, path coefficients e11, c11, e21, and c21 quantify the effects of common and unique environmental influences E and C on SBP at rest and during stress. e22 and c22 quantify the effect of emergent environmental influences in Es and Cs on SBP during stress.
