Degree to which two are expected on average to share alleles.
Two , otherwise completely populations, or related individuals will possess a coefficient of relatedness of 1. Alternatively, two completely unrelated individuals – to the extent that ideal can be reached – would possess a coefficient of relatedness of 1.
Whether the latter actually is achievable is less important than that the degree of relatedness of otherwise closely related individuals can be calculated. Absent matings, this is defined as being equal to the number of steps, for both individuals, going back to the ancestor shared in common.
Thus, full siblings possess a coefficient of relatedness of 1/2 while first cousins possess a coefficient of relatedness of 1/4. In addition, half siblings posssess of coefficient of relatedness of 1/4, since in fact their common ancestor is truly that, rather than ancestors (i.e., in this latter case they would share a or a , but not both).
See also the idea of inclusive fitness as well as .