Inbreeding, the mating of individuals related by ancestry, is a phenomenon closely related to consanguinity Opens in new window.

Inbreeding describes the situation in which individuals from a small population tend to choose their mates from within the same population for cultural, geographical, or religious reasons.

In this situation, the parents may consider themselves unrelated but still may have common ancestry within the past few generations.

In small closed populations, matings among relatives is inevitable. With time, every individual becomes related so that matings between unrelated individuals are impossible.

This is not a result of active choice of matings among relatives, but smply a consequence of the small number of founders and the small population size.

The study of inbreeding is of profound importance as it leads to reductions in heterozygosity, to reduced reproduction and survival (reproductive fitness), and to increased risk of extinction.

Loss of reproductive fitness as a consequence of inbreeding is known as inbreeding depression. For example, in a study of 44 captive mammal population, inbred individuals experienced higher juvenile mortality than outbred individuals in 41 cases. On average, brother-sister mating resulted in a 33% reduction in juvenile survival.

The primary consequence of matings between relatives is that their offspring have an increased probability of inheriting alleles that are recent copies of the same DNA sequence (identical by descent).

The inbreeding coefficient (F) is used to measure inbreeding. The inbreeding coefficient of an individual is the probability that both alleles at a locus are identical by descent. As F is a probability, it ranges from 0 to 1, the former being outbreds and the latter completely inbred.

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    Adapted from Thompson & Thompson Genetics in Medicine E-Book By Robert L. Nussbaum, Roderick R. McInnes, Huntington F Willard