Path diagram for a bivariate model.
For clarity only one twin is depicted. A1, A2 = Genetic variance components, E1, E2 = Non-shared environmental variance components, a11 through a22 = genetic path coefficients (or factor loadings) of which a22 represents novel genetic influences on visit 2, e11 through e22 = non-shared environmental path coefficients. (or factor loadings) of which e22 represents novel environmental influences on visit 2. Delta = difference in levels of measured phenotype between visit 2 and 1. Subtracting the level of the measured phenotype at time 2 from the level of the measured phenotype at time 1 is established by setting the path coefficients originating from the two measured phenotypes (and pointing towards delta) to -1 (time 1) and 1 (time 2).
Formulas for the different heritability estimates (listed in Table 5) are as follows:
h2 total (visit1) = a112 / (a112 + e112)
h2 total (visit2) = (a212 + a222) / (a212 + a222 + e212 + e222)
h2 specific (visit2) = a222 / (a212 + a222 + e212 + e222)
h2 delta (visit2 – visit1) = ((a21- a11)2+ a222) / ((a21 - a11)2+ a222 + (e21- e11)2+ e222)