Mating among fairly close relatives such that heterozygosity is decreased.
Inbreeding can be a consequence either of specific "attraction" between relatives, due to low levels of dissemination by organisms away from their point of "birth", or as a consequence of assortative mating. Inbreeding also can be forced upon a population give founder effects and genetic bottlenecking.
That is, specifically seeking out relatives for mating (which tends to be somewhat rare), being limited to only locally available potential mates which tend to be more closely related than more distantly located mates, or attraction to individuals who are phenotypically similar. The latter can be useful towards avoiding hybridization as due to mating with a different species, but otherwise these tendencies are often at least somewhat avoided within otherwise reasonably well mixed as well as relatively large populations.
More generally, inbreeding is a consequence of nonrandom mating. Contrast panmixis, i.e., random mating.
By decreasing heterozygosity, the frequency of homozygotes among offspring increases, which can have the effect of increasing the frequency of expression of deleterious, recessive alleles.
Inbreeding can also give rise to a fixation of alleles within isolated, local populations that otherwise are somewhat rare within larger populations. Such fixation, as a consequence of genetic drift, in fact can be of deleterious alleles.
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