Condition 1 is often the limiting factor. As indicated in Table 1, for untyped probands, the average number chip-typed relatives expected to share the paternal/maternal chromosome IBD exceeds 15. By contrast, for the un-typed probands born at or after 1900, the expected number of typed descendants carrying the paternal/maternal allele IBD is only 0.082. We define the legacy coefficient of an individual as the probability that the paternal/maternal haplotype is transmitted to at least one typed child or grandchild. For example, the legacy coefficient is 0.5 if exactly one child of the proband is chip-typed, and 0.25 if exactly one grandchild is typed (Supplementary Methods). The legacy coefficient, with some discount, should be about the probability that the paternal/maternal haplotype of an untyped proband can be imputed.
