However, because the increased proportion of deleterious homozygotes exposes the allele to natural selection, in the long run its frequency decreases more rapidly in inbred population. Contrary to common belief, inbreeding does not in itself alter allele frequencies, but rather increases the relative proportion of homozygotes to heterozygotes. However, because close relatives share a large fraction of their alleles, the probability that any such rare deleterious allele present in the common ancestor will be inherited from both related parents is increased dramatically with respect to non-inbred couples. Because most such alleles are rare in populations, it is unlikely that two unrelated marriage partners will both be heterozygous carriers. Such offspring have a higher possibility (see Coefficient of relationship) of congenital birth defects because it increases the proportion of zygotes that are homozygous for deleterious recessive alleles that produce such disorders (see Inbreeding depression).
Offspring of biologically related parents are subject to the possible impact of inbreeding.
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