Departure from the triangle constraints in discordant sib pairs: A test for genetic heterogeneity

For any genetic model, Holmans showed that the proportions of affected sibs sharing 2, 1, or 0 identical-by-descent parental marker alleles are constr ained to belong to a specific triangle. The triangle constraints do not hol d when the sib phenotypes are determined by different models. We test the r ejection of triangle constraints on affected sib pairs discordant for sever ity, to determine whether different models control the severe and mild form s of the disease in the simulated data. With this method we show that a loc us on chromosome 5 plays a different role in the two forms of the disease. (C) 1999 Wiley-Liss, Inc.