The sibling relative risk attributable to a given SNP was calculated using the formula(9): λ*=p(pr2+qr1)2+q(pr1+q)2[p2r2+2pqr1+q2]2 where p is the population frequency of the minor allele, q = 1−p, and r1 and r2 are the relative risks (estimated as OR) for heterozygotes and rare homozygotes, relative to common homozygotes. Assuming a multiplicative interaction the proportion of the familial risk attributable to a SNP was calculated as log(λ∙)/log(λ0), where λ0 is the overall familial relative risk estimated from epidemiological studies, assumed here to be 1.8(10).
