Fig. 1.

Schematic diagram of direct and indirect genetic effects. $G_M,G_P,G_O$ represents the maternal, paternal, and offspring genotypes, respectively. $\alpha =\mathrm{Corr}\left(G_M,G_P\right)$ is the correlation between spousal genotypes at a locus. Effect size 0.5 is due to the fact that half of the parent’s genome is randomly transmitted to the offspring $G_O$. $Y_O$ is the offspring’s phenotype. $T$ and $NT$ represent transmitted and nontransmitted alleles from a parent to the offspring. In general, both offspring and parental genotypes could affect the offspring’s phenotype with effect sizes of ${\beta }_{dir},{\beta }_{ind\_mt,}$ and ${\beta }_{ind\_pt}$, respectively.