HLA sharing in multiplex sibships

For an arbitrary number of affected sibs with unaffected parents, the proportion that share both marker haplotypes identical by descent is derived for the generalized single locus model. This proportion is shown to depend on the underlying parameters of the disease transmission model, on the total sibship size and on the number of sibs who are affected. The variety of identity by descent patterns suggests that any attempt to infer the presence of other independent disease susceptibility loci solely on the basis of an observed inverse relationship between the degree of haplotype sharing and the number of affected sibs, is liable to result in spurious conclusions.