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Figure 1. Path diagram for a univariate scalar model. An opposite-sex twin pair is shown, twin 1 being male and twin 2 female. Observed phenotypes (P) for twin 1 and twin 2 are shown in squares; latent (ie, unmeasured) factors are shown in circles. Correlations between additive genetic factors are 1 in MZ twins and 0.5 in DZ twins. Path coefficients (or factor loadings) of observed variables on the different latent factors are shown in lower case: h = additive genetic effect; c = shared environmental effect; e = unique environmental effect;k = scalar factor. D, the dominance genetic influence, was also tested but is omitted to simplify the diagram.
