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Inbred redirects here. For the 2011 British film, see Inbred (film).

Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely related genetically, in contrast to outcrossing, which refers to mating unrelated individuals.[1] By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders and other consequences that may arise from incestuous sexual relationships and consanguinity.

Inbreeding results in homozygosity, which can increase the chances of offspring being affected by recessive or deleterious traits.[2] This generally leads to a decreased biological fitness of a population[3][4] (called inbreeding depression), which is its ability to survive and reproduce. An individual who inherits such deleterious traits is referred to as inbred. The avoidance of such deleterious recessive alleles caused by inbreeding, via inbreeding avoidance mechanisms, is the main selective reason for outcrossing.[5][6] Crossbreeding between populations also often has positive effects on fitness-related traits.[7]

Inbreeding is a technique used in selective breeding. In livestock breeding, breeders may use inbreeding when, for example, trying to establish a new and desirable trait in the stock, but will need to watch for undesirable characteristics in offspring, which can then be eliminated through further selective breeding or culling. Inbreeding is used to reveal deleterious recessive alleles, which can then be eliminated through assortative breeding or through culling. In plant breeding, inbred lines are used as stocks for the creation of hybrid lines to make use of the effects of heterosis. Inbreeding in plants also occurs naturally in the form of self-pollination.

Offspring of biologically related persons are subject to the possible impact of inbreeding, such as congenital birth defects. The chances of such disorders is increased the closer the relationship of the biological parents. (See coefficient of inbreeding.) This is because such pairings increase the proportion of homozygous zygotes in the offspring, in particular deleterious recessive alleles, which produce such disorders.[8] (See inbreeding depression.) Because most recessive alleles are rare in populations, it is unlikely that two unrelated marriage partners will both be carriers of the alleles. However, because close relatives share a large fraction of their alleles, the probability that any such deleterious allele is inherited from the common ancestor through both parents is increased dramatically. Contrary to common belief, inbreeding does not in itself alter allele frequencies, but rather increases the relative proportion of homozygotes to heterozygotes. 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. In the short term, incestuous reproduction is expected to produce increases in spontaneous abortions of zygotes, perinatal deaths, and postnatal offspring with birth defects.[9] The advantages of inbreeding may be the result of a tendency to preserve the structures of alleles interacting at different loci that have been adapted together by a common selective history.[10]

Malformations or harmful traits can stay within a population due to a high homozygosity rate and it will cause a population to become fixed for certain traits, like having too many bones in an area, like the vertebral column in wolves on Isle Royale or having cranial abnormalities in Northern elephant seals, where their cranial bone length in the lower mandibular tooth row has changed. Having a high homozygosity rate is bad for a population because it will unmask recessive deleterious alleles generated by mutations, reduce heterozygote advantage, and it is detrimental to the survival of small, endangered animal populations.[11] When there are deleterious recessive alleles in a population it can cause inbreeding depression. The authors think that it is possible that the severity of inbreeding depression can be diminished if natural selection can purge such alleles from populations during inbreeding.[12] If inbreeding depression can be diminished by natural selection than some traits, harmful or not, can be reduced and change the future outlook on a small, endangered populations.

There may also be other deleterious effects besides those caused by recessive diseases. Thus, similar immune systems may be more vulnerable to infectious diseases (see Major histocompatibility complex and sexual selection).[13]

Inbreeding history of the population should also be considered when discussing the variation in the severity of inbreeding depression between and within species. With persistent inbreeding, there is evidence that shows inbreeding depression becoming less severe. This is associated with the unmasking and eliminating of severely deleterious recessive alleles. It is not likely, though, that eliminating can be so complete that inbreeding depression is only a temporary phenomenon. Eliminating slightly deleterious mutations through inbreeding under moderate selection is not as effective. Fixation of alleles most likely occurs through Mullers Ratchet, when an asexual populations genomes accumulate deleterious mutations that are irreversible.[14]

Autosomal recessive disorders occur in individuals who have two copies of the gene for a particular recessive genetic mutation.[15] Except in certain rare circumstances, such as new mutations or uniparental disomy, both parents of an individual with such a disorder will be carriers of the gene. These carriers do not display any signs of the mutation and may be unaware that they carry the mutated gene. Since relatives share a higher proportion of their genes than do unrelated people, it is more likely that related parents will both be carriers of the same recessive gene, and therefore their children are at a higher risk of a genetic disorder. The extent to which the risk increases depends on the degree of genetic relationship between the parents: The risk is greater when the parents are close relatives and lower for relationships between more distant relatives, such as second cousins, though still greater than for the general population.[16] A study has provided the evidence for inbreeding depression on cognitive abilities among children, with high frequency of mental retardation among offspring in proportion to their increasing inbreeding coefficients.[17]

Children of parent-child or sibling-sibling unions are at increased risk compared to cousin-cousin unions.[18]

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