For disease traits, the phenotype has two possible values, either affected or not affected. On this observed scale, the directly measurable genetic parameters are those of recurrence risks to relatives, λR for relatives of type R, which is the ratio of the prevalence of disease in the relatives of affected individuals (KR) compared to the prevalence in the population (K),where cov(X, R) the covariance in disease status between diseased individuals X and their relatives on the observed disease risk scale [12]. For example, when the relatives are monozygous twins (R = MZ), Cov(X,MZ) = the genetic variance, with the subscript “01” denoting the all-or-none disease risk scale. On this scale, the majority of the genetic variance is non-additive, especially when disease prevalence is low [13],[14]. The broad sense heritability on this scale is = (λMZ -1)K/(1-K) where λMZ is the 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
