Consider a sibship with n sibs. Let Tj be the random variable of age at disease onset for the jth sib. Let (tj,δj) be the observed data where tj is the observed age at onset if δj = 1, and age at censoring if δj = 0. Consider a marker locus d in the test chromosomal region. We assume that the hazard function of developing disease for the jth individuals at age tj is modelled by the proportional hazards model with random effect Zj,
