The rate of identical-by-descent segment sharing between close and distant relatives

Genetic relatives share long stretches of DNA they co-inherited from a common ancestor in identical-by-descent (IBD) segments. Because children inherit half their parents' genomes, the expected amount of DNA relatives share drops by $\frac{1}{2}$ for each generation that separates them, being 2^-d for d-degree relatives. Even so, there is substantial variance in sharing rates, such that most distant relatives share zero IBD segments. We characterized IBD segment sharing between relatives by simulating 100,000 pairs for each of first through eighth cousins, including once-removed and half-cousins, while modeling both crossover interference and sex-specific genetic maps. Our results show that 98.5% of third cousins share at least one IBD segment, while only 32.7% of fifth cousins and 0.961% of eighth cousins have such sharing. These sharing rates are substantially higher than those that arise from models that ignore the more elaborate crossover features. The resulting segment count distributions are available with an interactive segment length threshold at https://hapi-dna.org/ibd-sharing-rates/.