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Inbreeding is the sexual reproduction of offspring from the mating or breeding of individuals or organisms that are closely related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders and other consequences that 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. This generally leads to a decreased biological fitness of a population (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.〔Michod RE. Eros and Evolution: A Natural Philosophy of Sex. (1994) Perseus Books, ISBN 020140754X〕 Crossbreeding between populations also often has positive effects on fitness-related traits.〔Lynch, Michael. (1991). The Genetic Interpretation of Inbreeding Depression and Outbreeding Depression. Oregon: Society for the Study of Evolution.〕 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. ==Overview== Offspring of biologically related persons are subject to the possible effect 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 have a 25% probability of producing homozygous zygotes in the offspring, containing 2 recessive alleles, which produce disorders. (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. It should also be noted that, in terms of probability, for each homozygous recessive individual formed, there is an equal chance of producing a homozygous dominant individual, one completely devoid of the harmful allele. 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. 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.〔Shields, W. M. 1982. Philopatry, Inbreeding, and the Evolution of Sex. Print. 50-69.〕 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. 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. 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). 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 Muller’s ratchet, when an asexual population’s genomes accumulate deleterious mutations that are irreversible. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Inbreeding」の詳細全文を読む スポンサード リンク
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