2. is the genetic covariance between traits. Since the two traits are measured on different sets of samples, the residual covariance is ignored because it is assumed that there is no covariance between the unrelated individuals apart from that caused by genetic factors. The genetic correlation is also estimated as , however, the sampling variance of is different from that described above. Since the traits are measured in different sets of samples, . Therefore, from a second order Taylor series approximation [15] (8)This approximation involves the sampling variance of . We show below an equivalent approach to obtain .
