Necessary and sufficient conditions for equality of sibling and parent-offspring risk of a disease endophenotype in case families.

A large number of linkage and association studies of complex diseases focus on analysis of a more common or more easily measured disease endophenotype. The motivation for this approach is that there is a pleiotropic locus common to both the disease and the endophenotype and that this locus is a major genetic determinant of the endophenotype. In this paper, we determine the conditions under which the risk of the endophenotype in siblings of affected probands with disease equals the risk of the endophenotype in the offspring (parents) of affected parents (offspring) with disease. In doing so we prove that this equality holds if and only if the penetrance of either the endophenotype or the disease (but not necessarily both) is additive.