Editing Inbreeding (section)

==Overview==
[[File:Averages of individuals inbreeding coefficients per population or group of.png|thumb|right|Inbreeding coefficients of various populations in Europe and Asia]]

Offspring of biologically related persons are subject to the possible effects of inbreeding, such as [[Congenital disorder|congenital birth defects]]. The [[Coefficient of inbreeding|chances of such disorders]] are increased when the biological parents are more closely related. This is because such pairings have a 25% probability of producing [[homozygous]] zygotes, resulting in offspring with two [[recessive allele]]s, which can [[Inbreeding depression|produce disorders]] when these alleles are deleterious.<ref>{{cite journal | vauthors = Livingstone FB |year=1969 | title =Genetics, Ecology, and the Origins of Incest and Exogamy |journal=Current Anthropology |volume=10 |pages=45–62 |doi= 10.1086/201009|s2cid=84009643 }}</ref> Because most recessive alleles are rare in populations, it is unlikely that two unrelated partners will both be carriers of the same deleterious allele; 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. 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 populations.  In the short term, incestuous reproduction is expected to increase the number of spontaneous abortions of zygotes, perinatal deaths, and postnatal offspring with birth defects.<ref>{{cite book | last = Thornhill | first = Nancy Wilmsen |title=The Natural History of Inbreeding and Outbreeding: Theoretical and Empirical Perspectives |publisher = [[University of Chicago Press]] |location=Chicago |year=1993 |isbn=978-0-226-79854-7 | url = https://books.google.com/books?id=ZFXYeHxwD10C }}</ref> 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.<ref>Shields, W. M. 1982. Philopatry, Inbreeding, and the Evolution of Sex. Print. 50–69.</ref>

Malformations or harmful traits can stay within a population due to a high homozygosity rate, and this will cause a population to become fixed for certain traits, like having too many bones in an area, like the vertebral column of wolves on [[Isle Royale, Michigan|Isle Royale]] or having cranial abnormalities, such as in [[Northern elephant seal]]s, where their cranial bone length in the lower mandibular tooth row has changed. Having a high homozygosity rate is problematic 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.<ref>{{cite journal | vauthors = Meagher S, Penn DJ, Potts WK | title = Male-male competition magnifies inbreeding depression in wild house mice | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 97 | issue = 7 | pages = 3324–9 | date = March 2000 | pmid = 10716731 | pmc = 16238 | doi = 10.1073/pnas.97.7.3324 | bibcode = 2000PNAS...97.3324M | doi-access = free }}</ref> When deleterious recessive alleles are unmasked due to the increased homozygosity generated by inbreeding, this can cause inbreeding depression.<ref>{{cite journal | vauthors = Swindell WR, etal | year = 2006 | title = Selection and Inbreeding Depression: Effects of Inbreeding Rate and Inbreeding Environment | journal = Evolution | volume = 60 | issue = 5| pages = 1014–1022 | doi=10.1554/05-493.1| pmid = 16817541 | s2cid = 198156086 }}</ref>

There may also be other deleterious effects besides those caused by recessive diseases. Thus, similar [[immune system]]s may be more vulnerable to infectious diseases (see [[Major histocompatibility complex and sexual selection]]).<ref name=moral>{{cite journal | vauthors = Lieberman D, Tooby J, Cosmides L | title = Does morality have a biological basis? An empirical test of the factors governing moral sentiments relating to incest | journal = Proceedings. Biological Sciences | volume = 270 | issue = 1517 | pages = 819–26 | date = April 2003 | pmid = 12737660 | pmc = 1691313 | doi = 10.1098/rspb.2002.2290 }}</ref>

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 that inbreeding depression becomes less severe. This is associated with the unmasking and elimination of severely deleterious recessive alleles. However, inbreeding depression is not a temporary phenomenon because this elimination of deleterious recessive alleles will never be complete.  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 genome accumulates deleterious mutations that are irreversible.<ref name="Pusey, A. 1996">{{cite journal | vauthors = Pusey A, Wolf M | title = Inbreeding avoidance in animals | journal = Trends in Ecology & Evolution | volume = 11 | issue = 5 | pages = 201–6 | date = May 1996 | pmid = 21237809 | doi = 10.1016/0169-5347(96)10028-8 | bibcode = 1996TEcoE..11..201P }}</ref>

Despite all its disadvantages, inbreeding can also have a variety of advantages, such as ensuring a child produced from the mating contains, and will pass on, a higher percentage of its mother/father's genetics, reducing the [[Genetic load#Recombination/segregation load|recombination load]],<ref>{{cite book|last1=Shields|first1=William M.  |title=Philopatry, inbreeding, and the evolution of sex|date=1982|publisher=State University of New York Press|location=Albany|isbn=978-0-87395-618-5}}</ref> and allowing the expression of recessive advantageous phenotypes. Some species with a [[Haplodiploidy]] mating system depend on the ability to produce sons to mate with as a means of ensuring a mate can be found if no other male is available.  It has been proposed that under circumstances when the advantages of inbreeding outweigh the disadvantages, preferential breeding within small groups could be promoted, potentially leading to [[speciation]].<ref>{{cite journal | vauthors = Joly E | title = The existence of species rests on a metastable equilibrium between inbreeding and outbreeding. An essay on the close relationship between speciation, inbreeding and recessive mutations | journal = Biology Direct | volume = 6 | pages = 62 | date = December 2011 | pmid = 22152499 | pmc = 3275546 | doi = 10.1186/1745-6150-6-62 | doi-access = free }}</ref>