monozygotic twin recurrence risk, assuming there is no common environmental component to the recurrence risk. is not a normally reported statistic because of its dependence on disease prevalence [15]. If the relatives are siblings (R = S) then λS is the sibling risk ratio and Cov(X,S) = [11], where the variance subscripts A and D denote additive and dominance terms, and in combination denote epistatic variance terms. Thus, although λS is an estimable quantity, it is not simply related to the genetic variances on the observed binary scale.
