= Sibling IBD ascertainment under multiplicative penetrance
{title2=$(z_0,z_1,z_2)=\dfrac{(\alpha^4,2\alpha^2\beta,\beta^2)}{(\alpha^2+\beta)^2}$}
For <full siblings> in an <outbred pedigree>, condition on both being affected under <multiplicative diallelic penetrance> and on residual disease outcomes being independent given their <genotypes>. The usual <Mendelian IBD sharing of full siblings> probabilities $(1/4,1/2,1/4)$ are weighted by $p^2\alpha^4,p^2\alpha^2\beta,p^2\beta^2$, respectively, giving the displayed conditional sharing probabilities. Since $\beta-\alpha^2=\phi(1-\phi)(\theta-1)^2\geq0$, $z_1\leq1/2$, strictly for a polymorphic locus with a nontrivial effect.
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