Coefficient of relationship

In population genetics, the coefficient of relationship is a value between 0 and 1 that gives the probability that any two individuals share an allele at a given locus by virtue of being descended from a common ancestor. It is often denoted r.

For instance, the coefficient of relationship between a parent and child is 0.5, since half of the genetic material of the child (23 of the 46 chromosomes in the case of humans) is descended from that parent. By a similar token, the coefficient of relationship between a grandparent and grandchild is 0.25, and a great-grandparent and great-grandchild is 0.125.

The coefficient of relationship between normal siblings is 0.5, since half of their genes are identical by descent. Of identical siblings the value is 1, but of half-siblings the value is 0.25 (since the half-siblings have a coefficient of 0.5 by descent of the common parent, and a coefficient of 0 by descent of their different parents, and the average of these is 0.25).

The coefficient of relationship is relative to a base population. It does not factor in that all organisms in fact have common ancestors; it instead takes the parents from the base population, and calculates relatedness by descent only from that point.

All of the above probabilities assume a diploid species where there has been no consanguinity.