of the deletion carriers do not have a ‘first-generation’ de novo mutation, we investigated the relationship between the 63 chromosomes with the deletion through long-range phasing. The deletion resides in a difficult region. Chromosome 15 has a very short p-arm. Among the SNPs used, only two are on the left of the deletion (51 are inside), meaning that the information in determining whether two chromosomes are IBD comes mostly from SNPs on the right side. This region also has a high recombination rate so that less SNPs than average are included for a specific genetic distance. Nonetheless, substantial progress was made and Fig. 4 displays some of the results. Probands 1 and 2 (P1 and P2) carry the deletion IBD (sharing a haplotype ~ 1500 SNPs in length not including the 51 SNPs in the deleted region). Relatives 1 to 3 (R1, R2 and R3) also share a long haplotype with the probands (over 4000 SNPs with P1 and ~ 1500 SNPs with P2), but they do not have the deletion. One can deduce from the family relationship that the grandfather (GF) has the deletion, and the mutation event occurred either at the meiosis of GF or GGM. By phasing
