Figure 1.

Expected distributions of IBD chromosomal segments between pairs of individuals. (A) The process underlying the pattern of IBD segments. Two homologous autosomal chromosomes are shown for two parents, each colored differently. Meiosis and recombination occur, and two sibling offspring inherit recombinant chromosomes (just one crossover per homologous pair for each meiosis event is depicted, marked by an X). For some segments of the chromosome in question, the siblings share a stretch that was inherited from one of the four parental chromosomes. The three IBD segments are identifiable as regions that share the same color (boxed and marked at right by black bars). The siblings mate with unrelated individuals, and the offspring each inherit an unrelated chromosome (tan or gray) and one that is a recombinant patchwork of the grandparental chromosomes. These first cousins share one segment IBD at this chromosome (red, boxed). (B) The number of segments that a pair of individuals shares IBD, across all chromosomes, is approximately Poisson-distributed with a mean that depends on the number of meioses d on the path relating the individuals (d = 2, 4, 6, 8, corresponding to siblings through third cousins). (C) The lengths of the IBD segments are approximately exponentially distributed, with mean length depending on the relationship between individuals (theoretical distributions shown for d = 2, 4, 6, 8).