Under the LCV model, the genetic causality proportion is defined as the number x such that: (1)q22q12=(ρg2)x, where qk is the normalized effect of L on trait k (Figure 1a), and ρg is the genetic correlation[16] (note that ρg = q1q2). When the gcp is equal to 1, trait 1 is fully genetically causal for trait 2; when it is positive but less than 1, trait 1 is partially genetically causal. When it is negative, trait 2 is partially genetically causal. The gcp can be defined without making LCV (or other) model assumptions (see Methods).
