The genetic properties of disease are much more easily understood by using the threshold liability model [11], in which risk of disease is transformed to a normally distributed liability scale P ∼N(0, 1) and P = A + E, where A∼N(0, ) are the genetic effects on the liability scale. On this scale the genetic effects combine in an additive way; is the narrow sense heritability on the liability scale (or heritability of liability) and on this scale broad sense and narrow sense heritability are equal. E are independent environmental effects, E∼N(0,1-). The biological plausibility of an underlying normally distributed liability to disease is based on the assumption that complex traits are influenced by many variables; the central limit theorem states that the distribution of the sum of independent random variables approaches normality as the number of variables increases. Under the threshold liability model individuals are affected when P >T, where T is the threshold on the normal distribution which truncates the proportion of affected individuals or disease prevalence (i.e., K), T = Φ−1(1-K), Φ(T) = 1-K, where Φ(T) is
