The affected sib pair IBD distribution for HLA-linked disease susceptibility genes.

The distribution of identity by descent (IBD) scores for sib pairs affected with a disease determined by a disease susceptibility (DS) locus tightly linked to the HLA complex is derived. It is shown that the sib pair IBD distribution differs from its a priori distribution and, moreover, is completely specified by three observable population parameters--the additive and dominance variances and the prevalence of the disease in the population. An application of the model is illustrated using data on juvenile diabetes mellitus.