Figure 1
Path diagram for a bivariate model. For clarity only one twin is depicted. A1, A2=genetic variance components; C1, C2=common environmental variance components; E1, E2=unique environmental variance components; a11 through a22=genetic path coefficients (or factor loadings) of which a22 represents specific genetic influences on SBP; c11 through c22=common environmental path coefficients (or factor loadings) of which c22 represents specific common environmental influences on SBP; e11 through e22=unique environmental path coefficients (or factor loadings), of which e22 represents specific unique environmental influences on SBP. Formulas for the different heritability estimates are as follows: 
