Figure 1. Hypothetical model of the underlying genetic and environmental factors, and their interactions, giving rise to a complex phenotype. Modeling approximations in terms of a basic model are shown at the bottom. The bottom left diagram depicts the generic model involving an aggregate familial component (T, which includes both genetic and familial environmental effects) and a residual (R). The bottom right diagram depicts a specific model with latent genetic (G), familial environmental (C) and residual (R) sources of phenotypic (P) variation in an individual. Path coefficients are defined in the text. (Reproduced with permission from Rao DC and Rice T (1998) Path analysis in genetics. In: Armitage P and Colten T (eds.) Encyclopedia of Biostatistics, vol. 4. Copyright John Wiley & Sons Ltd.)

Figure 2. TAU path model of resemblance between relatives in nuclear families (generic model). P denotes a phenotype, T indicates transmissible influences, and the subscripts F, M and C1 and C2 denote father, mother and two children respectively. R denotes the nontransmitted residual environment. Path coefficients are defined in the text. (Reproduced with permission from Rao DC and Rice T (1998) Path analysis in genetics. In: Armitage P and Colten T (eds.) Encyclopedia of Biostatistics, vol. 4. Copyright John Wiley & Sons Ltd.)

Figure 3. Path model of twin resemblance (under a specific model). P₁ and P₂ are the observed quantitative phenotypes for two members of a twin pair; G₁ and G₂ denote latent additive genetic effects which are correlated (1 for monozygotic (MZ) twins and ½ for dizygotic (DZ) twins); C is the latent familial environment shared by both members of a twin pair; R₁ and R₂ are latent residual environmental influences not shared by the twins. Path coefficients are defined in the text. (Reproduced with permission from Rao DC and Rice T (1998) Path analysis in genetics. In: Armitage P and Colten T (eds.) Encyclopedia of Biostatistics, vol. 4. Copyright John Wiley & Sons Limited.)