The model in Figure 3 assumes that all genetic effects that have an impact on EP either directly or indirectly affect PD. This is probably an unrealistic assumption as well. Rather, it is more likely that, among all the possible genetic effects on EP and PD, some will influence only EP, others only PD and yet others will influence both. This kind of relationship is well captured in quantitative genetics in a standard bivariate correlated liability model,19 which can be understood as an expanded version of the ‘liability-index’ model for EP (Figure 4a). Such a model assumes two sets of genetic risk factors: one for EP and one for PD. The magnitude of the association between these two sets of risk factors is captured by the genetic correlation (rg). The correlation will be lowered both by genes that affect EP but not PD, and genes that affect PD but not EP.
