Relatedness

Relatedness is a measure of the genetic or social connection between individuals that arises from shared ancestry. In genetics, it is commonly quantified by the coefficient of relatedness, r, defined as the expected proportion of genes that two individuals share by descent. Values: parent-offspring and full siblings r ≈ 0.5; half-siblings r ≈ 0.25; grandparent–grandchild and avuncular relationships around 0.25; first cousins around 0.125. Relatedness can extend to more distant relatives and to nonhuman organisms, and it depends on the pedigree considered and on recombination.

Relatedness can be estimated from pedigrees or from molecular data, using identity by descent (IBD) segments, or from population statistics such as F-statistics. Pedigree-based estimates assume known ancestry; marker-based estimates use genotypes to infer relatedness. For population genetics, r is often complemented by kinship coefficients and metrics such as the coefficient of kinship (phi) and Wright's F statistics.

High relatedness among individuals facilitates kin selection, where genes that confer benefits to related individuals can spread even at a cost to the actor, consistent with Hamilton's rule (rB > C). Relatedness also informs studies of social structure, mating systems, and inbreeding.

Beyond biology, relatedness can describe social or cultural ties—kinship networks, shared ancestry, or group belonging—used in anthropology, sociology, and genealogy to understand family structure and social organization.