Calculation of parent-specific IBD for a hypothetical family containing five siblings. Individual identification numbers are shown above the symbols, and alleles (designated “a,” “b,” “ c,” or “d”) at a hypothetical marker locus are shown within the symbols. For each of the siblings numbered “4”–“7,” calculation of allele sharing with individual 3 is shown for maternally derived alleles (πMO3j), paternally derived alleles (πFA3j), and all alleles (π3j).