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Figure 1. Parent–offspring genetic ADEi model. The figure represents an opposite sex DZ pair where the first born is a male and the second born is a female. Measured phenotypes are represented into rectangles: twin 1—first born, twin 2—second born, mother, and father. Latent variables representing sources of variance are depicted into circles: A = additive genetic effects; D = dominance genetic effects; E = nonshared environment. 
represents the scalar parameter to account for the difference in variance in the parental generation. Path coefficients with the subscript m are those for males and the subscript f is for female parameters. The arrows connecting the twins represent the sibling interaction parameter, i3 in the diagram is the sibling interaction for opposite sex twin pairs; i1 would be the interaction parameter for males, and i2 for females. Parents and offspring are connected by paths with a 0.5 that represents the 50% of genetic variance that they share.
