Recall that the kinship coefficient is the probability that two identical alleles will be sampled from a pair of individuals when we select one allele at random from each. The self-kinship coefficient is the probability that two alleles sampled from one individual, with replacement, are identical. We used a recursive formulation to estimate kinship coefficients for X-linked genes, analogous to the conventional approach described in Lange [64] for autosomal genes. First, we ordered all individuals in a pedigree such that for any two individuals i and j, i > j implies that i is not an ancestor of j (any ordering where ancestors precede their descendants is suitable).
