View source for Inbreeding

{{Short description|Reproduction by closely related organisms}}
{{redirect|Inbred}}
{{For|reproduction by which offspring arise from a single organism|Asexual reproduction}}
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[[File:Child_first_cousins_Homozy_BD.png|thumb|right|The passage of [[homozygous]] alleles through an inbred pedigree]]

'''Inbreeding''' is the production of [[offspring]] from the [[mating]] or breeding of individuals or [[organism]]s that are closely [[genetic distance|related genetically]].<ref>{{Britannica|284509|Inbreeding}}</ref> By analogy, the term is used in [[human reproduction]], but more commonly refers to the genetic disorders and other consequences that may arise from expression of [[wikt:deleterious|deleterious]] [[dominance (genetics)|recessive]] traits resulting from [[incest]]uous sexual relationships and [[consanguinity]]. 

Inbreeding results in [[homozygous|homozygosity]] which can increase the chances of offspring being affected by recessive [[phenotypic trait|traits]].<ref>{{cite journal | vauthors = Nabulsi MM, Tamim H, Sabbagh M, Obeid MY, Yunis KA, Bitar FF | title = Parental consanguinity and congenital heart malformations in a developing country | journal = American Journal of Medical Genetics. Part A | volume = 116A | issue = 4 | pages = 342–7 | date = February 2003 | pmid = 12522788 | doi = 10.1002/ajmg.a.10020 | s2cid = 44576506 }}</ref> In extreme cases, this usually leads to at least temporarily decreased [[Fitness (biology)|biological fitness]] of a [[population]]<ref>{{cite journal|author-link2=Kimberly Hughes | vauthors = Jiménez JA, Hughes KA, Alaks G, Graham L, Lacy RC | title = An experimental study of inbreeding depression in a natural habitat | journal = Science | volume = 266 | issue = 5183 | pages = 271–3 | date = October 1994 | pmid = 7939661 | doi = 10.1126/science.7939661 | bibcode = 1994Sci...266..271J }}</ref><ref>{{cite journal| vauthors = Chen X |doi=10.1038/hdy.1993.163|title=Comparison of inbreeding and outbreeding in hermaphroditic ''Arianta arbustorum'' (L.) (land snail)|year=1993|journal=Heredity|volume=71|issue=5 |pages=456–461|doi-access=free|bibcode=1993Hered..71..456C }}</ref> (called [[inbreeding depression]]), which is its ability to survive and reproduce. An individual who inherits such deleterious traits is colloquially referred to as ''inbred''. The avoidance of expression of such deleterious recessive [[allele]]s caused by inbreeding, via [[inbreeding avoidance]] mechanisms, is the main selective reason for outcrossing.<ref name="Bernstein H 1985">{{cite journal | vauthors = Bernstein H, Byerly HC, Hopf FA, Michod RE | title = Genetic damage, mutation, and the evolution of sex | journal = Science | volume = 229 | issue = 4719 | pages = 1277–81 | date = September 1985 | pmid = 3898363 | doi = 10.1126/science.3898363 | bibcode = 1985Sci...229.1277B }}</ref><ref name="Michod RE 1994">Michod RE. Eros and Evolution: A Natural Philosophy of Sex. (1994) Perseus Books, {{ISBN|0-201-40754-X}}</ref> Crossbreeding between populations sometimes has positive effects on fitness-related traits,<ref>{{cite journal | last = Lynch | first = Michael | year =  1991 | title = The Genetic Interpretation of Inbreeding Depression and Outbreeding Depression. | journal = Evolution; International Journal of Organic Evolution | volume = 45 | issue = 3 | pages = 622–629 | location = Oregon | publisher = Society for the Study of Evolution | doi = 10.1111/j.1558-5646.1991.tb04333.x | pmid = 28568822 | s2cid = 881556 | doi-access = free }}{{page needed|date=August 2018}}</ref> but also sometimes leads to negative effects known as [[outbreeding depression]]. However, increased homozygosity increases the probability of fixing beneficial alleles and also slightly decreases the probability of fixing deleterious alleles in a population.<ref name="pmid12807795">{{cite journal |vauthors=Whitlock MC |title=Fixation probability and time in subdivided populations |journal=Genetics |volume=164 |issue=2 |pages=767–79 |date=June 2003 |doi=10.1093/genetics/164.2.767 |pmid=12807795 |pmc=1462574 }}</ref> Inbreeding can result in purging of deleterious alleles from a population through [[purifying selection]].<ref name="nih">{{cite journal | vauthors = Tien NS, Sabelis MW, Egas M | title = Inbreeding depression and purging in a haplodiploid: gender-related effects | journal = Heredity | volume = 114 | issue = 3 | pages = 327–32 | date = March 2015 | pmid = 25407077 | pmc = 4815584 | doi = 10.1038/hdy.2014.106 | bibcode = 2015Hered.114..327T }}</ref><ref>{{cite journal | vauthors = Peer K, Taborsky M | title = Outbreeding depression, but no inbreeding depression in haplodiploid Ambrosia beetles with regular sibling mating | journal = Evolution; International Journal of Organic Evolution | volume = 59 | issue = 2 | pages = 317–23 | date = February 2005 | pmid = 15807418 | doi=10.1554/04-128| s2cid = 198156378 }}</ref><ref>{{cite journal | vauthors = Gulisija D, Crow JF | title = Inferring purging from pedigree data | journal = Evolution; International Journal of Organic Evolution | volume = 61 | issue = 5 | pages = 1043–51 | date = May 2007 | pmid = 17492959 | doi = 10.1111/j.1558-5646.2007.00088.x | s2cid = 24302475 }}</ref>

Inbreeding is a technique used in [[selective breeding]]. For example, in [[livestock breeding]], breeders may use inbreeding when trying to establish a new and desirable [[Phenotypic trait|trait]] in the stock and for producing distinct families within a breed, but will need to watch for undesirable characteristics in offspring, which can then be eliminated through further selective breeding or [[culling]]. Inbreeding also helps to ascertain the type of gene action affecting a trait. Inbreeding is also used to reveal deleterious recessive alleles, which can then be eliminated through [[assortative mating|assortative breeding]] or through culling. In [[plant breeding]], [[inbred line]]s are used as stocks for the creation of [[Hybrid (biology)|hybrid]] lines to make use of the effects of [[heterosis]]. Inbreeding in plants also occurs naturally in the form of [[self-pollination]].

Inbreeding can significantly influence [[gene expression]] which can prevent inbreeding depression.<ref name="Gene-Expression Changes">{{cite journal | title = Gene-Expression Changes Caused by Inbreeding Protect Against Inbreeding Depression in Drosophila | pmc=3430533 | pmid=22714404 | doi=10.1534/genetics.112.142687 | volume=192 | issue=1 | year=2012 | journal=Genetics | pages=161–72 | vauthors=García C, Avila V, Quesada H, Caballero A}}</ref>

==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 | page = 62 | date = December 2011 | pmid = 22152499 | pmc = 3275546 | doi = 10.1186/1745-6150-6-62 | doi-access = free }}</ref>

== Genetic disorders ==
[[File:Animation of uniparental isodisomy.ogv|thumb|Animation of uniparental isodisomy]]

[[Autosomal recessive]] disorders occur in individuals who have [[homozygous|two copies of an allele]] for a particular recessive genetic [[mutation]].<ref>Hartl, D.L., Jones, E.W. (2000) Genetics: Analysis of Genes and Genomes. Fifth Edition. Jones and Bartlett Publishers Inc., pp. 105–106. {{ISBN|0-7637-1511-5}}.</ref> 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 allele, and therefore their children are at a higher risk of inheriting an autosomal recessive 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.<ref name="pmid2497870">{{cite journal | vauthors = Kingston HM | title = ABC of clinical genetics. Genetics of common disorders | journal = BMJ | volume = 298 | issue = 6678 | pages = 949–52 | date = April 1989 | pmid = 2497870 | pmc = 1836181 | doi =  10.1136/bmj.298.6678.949}}</ref>

Children of parent-child or sibling-sibling unions are at an increased risk compared to cousin-cousin unions.<ref name=WolfDurham2005>{{cite book|title=Inbreeding, incest, and the incest taboo: the state of knowledge at the turn | editor1-last = Wolf | editor1-first = Arthur P. | editor2-last = Durham | editor2-first = William H. |publisher =Stanford University Press|year= 2005|isbn=978-0-8047-5141-4 }}</ref>{{rp|3}} Inbreeding may result in a greater than expected phenotypic expression of deleterious [[Dominance (genetics)|recessive alleles]] within a population.<ref>{{cite book | last1 = Griffiths | first1 = Anthony J. F. | first2 = Jeffrey H. | last2 = Miller | first3 = David T. | last3 = Suzuki | first4 = Richard C. | last4 = Lewontin | first5 = William M. | last5 = Gelbart | title = An introduction to genetic analysis | publisher = W. H. Freeman | year = 1999 | location = New York | pages = 726–727 | isbn = 978-0-7167-3771-1 }}</ref> As a result, first-generation inbred individuals are more likely to show physical and health defects,<ref name="Bittles 2012">{{cite journal | vauthors = Bittles AH, Black ML | title = Evolution in health and medicine Sackler colloquium: Consanguinity, human evolution, and complex diseases | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 107 Suppl 1 | issue = suppl 1 | pages = 1779–86 | date = January 2010 | pmid = 19805052 | pmc = 2868287 | doi = 10.1073/pnas.0906079106 | bibcode = 2010PNAS..107.1779B | doi-access = free }}</ref><ref name="Fareed 2014">{{cite journal | vauthors = Fareed M, Afzal M | title = Evidence of inbreeding depression on height, weight, and body mass index: a population-based child cohort study | journal = American Journal of Human Biology | volume = 26 | issue = 6 | pages = 784–95 | year = 2014 | pmid = 25130378 | doi = 10.1002/ajhb.22599 | s2cid = 6086127 }}</ref> including:
{{columns-list|colwidth=30em|
* Lower [[intelligence quotient]] levels and higher incidence rates of being affected by an [[intellectual disability]]
* Reduced [[fertility]] both in litter size and [[Spermatozoon|sperm]] viability
* Increased [[genetic disorder]]s
* Fluctuating [[facial asymmetry]]
* Lower [[birth rate]]
* Higher [[infant mortality]] and [[child mortality]]<ref name="Fareed 2016a">{{cite journal | vauthors = Fareed M, Kaisar Ahmad M, Azeem Anwar M, Afzal M | title = Impact of consanguineous marriages and degrees of inbreeding on fertility, child mortality, secondary sex ratio, selection intensity, and genetic load: a cross-sectional study from Northern India | journal = Pediatric Research | volume = 81 | issue = 1 | pages = 18–26 | date = January 2017 | pmid = 27632780 | doi = 10.1038/pr.2016.177 | doi-access = free }}</ref>
* Smaller [[adult]] size
* Loss of [[immune system]] function
* Increased [[cardiovascular disease|cardiovascular risks]]<ref name="Fareed 2016b">{{cite journal | vauthors = Fareed M, Afzal M | title = Increased cardiovascular risks associated with familial inbreeding: a population-based study of adolescent cohort | journal = Annals of Epidemiology | volume = 26 | issue = 4 | pages = 283–92 | date = April 2016 | pmid = 27084548 | doi = 10.1016/j.annepidem.2016.03.001 }}</ref>
}}

The isolation of a small population for a period of time can lead to inbreeding within that population, resulting in increased genetic relatedness between breeding individuals. Inbreeding depression can also occur in a large population if individuals tend to mate with their relatives, instead of mating randomly.{{citation needed|date=July 2025}}

Due to higher [[perinatal mortality|prenatal]] and [[child mortality|postnatal]] mortality rates, some individuals in the first generation of inbreeding will not live on to reproduce.<ref>{{cite journal | vauthors = Bittles AH, Grant JC, Shami SA | title = Consanguinity as a determinant of reproductive behaviour and mortality in Pakistan | journal = International Journal of Epidemiology | volume = 22 | issue = 3 | pages = 463–7 | date = June 1993 | pmid = 8359962 | doi = 10.1093/ije/22.3.463 | url = http://researchrepository.murdoch.edu.au/id/eprint/12550/ | type = Submitted manuscript | url-access = subscription }}</ref> Over time, with isolation, such as a [[population bottleneck]] caused by purposeful ([[Assortative mating|assortative]]) breeding or natural [[Environment (biophysical)|environmental]] factors, the deleterious inherited traits are culled.<ref name="Bernstein H 1985"/><ref name="Michod RE 1994"/><ref>{{cite journal | vauthors = Kirkpatrick M, Jarne P | title = The Effects of a Bottleneck on Inbreeding Depression and the Genetic Load | journal = The American Naturalist | volume = 155 | issue = 2 | pages = 154–167 | date = February 2000 | pmid = 10686158 | doi = 10.1086/303312 | bibcode = 2000ANat..155..154K | s2cid = 4375158 }}</ref>

Island species are often very inbred, as their isolation from the larger group on a mainland allows natural selection to work on their population. This type of isolation may result in the formation of [[Race (biology)|race]] or even [[speciation]], as the inbreeding first removes many deleterious genes, and permits the expression of genes that allow a population to adapt to an [[ecosystem]]. As the adaptation becomes more pronounced, the new species or race radiates from its entrance into the new space, or dies out if it cannot adapt and, most importantly, reproduce.<ref name=Leck1980>{{cite journal|jstor = 4512538|url = http://sora.unm.edu/sites/default/files/journals/jfo/v051n02/p0168-p0173.pdf|title = Establishment of New Population Centers with Changes in Migration Patterns|pages = 168–173|journal = Journal of Field Ornithology|year = 1980|volume = 51|issue = 2| vauthors = Leck CF }}</ref>

The reduced [[genetic diversity]], for example due to a bottleneck will unavoidably increase inbreeding for the entire population. This may mean that a [[species]] may not be able to adapt to changes in environmental conditions. Each individual will have similar immune systems, as immune systems are genetically based. When a species becomes [[endangered species|endangered]], the population may [[minimum viable population|fall below a minimum]] whereby the forced interbreeding between the remaining animals will result in [[extinction]].{{citation needed|date=July 2025}}

Natural breedings include inbreeding by necessity, and most animals only migrate when necessary. In many cases, the closest available mate is a mother, sister, grandmother, father, brother, or grandfather. In all cases, the environment presents stresses to remove from the population those individuals who cannot survive because of illness.{{Citation needed|date = July 2020}}

There was an assumption{{By whom|date = July 2020}} that wild populations do not inbreed; this is not what is observed in some cases in the wild. However, in species such as [[horse]]s, animals in [[wild horse|wild]] or [[feral]] conditions often drive off the young of both sexes, thought to be a mechanism by which the species instinctively avoids some of the genetic consequences of inbreeding.<ref>"[http://www.advs.usu.edu/files/uploads/ADVS3910WildHorses.pdf ADVS 3910 Wild Horses Behavior]", College of Agriculture, Utah State University.</ref> In general, many mammal species, including humanity's closest [[primate]] relatives, avoid close inbreeding possibly due to the deleterious effects.<ref name=WolfDurham2005/>{{rp|6}}

===Examples===
Although there are several examples of inbred populations of wild animals, the negative consequences of this inbreeding are poorly documented.{{citation needed|date=September 2016}} In the [[South American sea lion]], there was concern that recent population crashes would reduce genetic diversity. Historical analysis indicated that a population expansion from just two matrilineal lines was responsible for most of the individuals within the population. Even so, the diversity within the lines allowed great variation in the gene pool that may help to protect the South American sea lion from extinction.<ref>{{cite web | vauthors = Freilich S, Hoelzel AR, Choudhury SR | url = http://www.dur.ac.uk/anthropology.journal/vol13/iss1/posters/freilich.pdf | title = Genetic diversity and population genetic structure in the South American sea lion (''Otaria flavescens'') | publisher = Department of Anthropology and School of Biological & Biomedical Sciences, University of Durham, U.K. }}</ref>
[[File:Heterozygous.jpg|thumb|Heterozygous]]
In lions, [[Lion#Group organisation|prides]] are often followed by related males in bachelor groups. When the dominant male is killed or driven off by one of these bachelors, a father may be replaced by his son. There is no mechanism for preventing inbreeding or to ensure outcrossing. In the prides, most lionesses are related to one another. If there is more than one dominant male, the group of [[alpha male]]s are usually related. Two lines are then being "line bred". Also, in some populations, such as the [[Ngorongoro Crater|Crater]] lions, it is known that a population bottleneck has occurred. Researchers found far greater genetic [[heterozygosity]] than expected.<ref name="autogenerated1">{{cite journal | vauthors = Gilbert DA, Packer C, Pusey AE, Stephens JC, O'Brien SJ | title = Analytical DNA fingerprinting in lions: parentage, genetic diversity, and kinship | journal = The Journal of Heredity | volume = 82 | issue = 5 | pages = 378–86 | date = 1991-10-01 | pmid = 1940281 | doi=10.1093/oxfordjournals.jhered.a111107}}</ref> In fact, predators are known for low genetic variance, along with most of the top portion of the trophic levels of an [[ecosystem]].<ref>{{cite journal |doi=10.1351/pac199870112079 |title=Biodiversity and intraspecific genetic variation |journal=Pure and Applied Chemistry |volume=70 |issue=11 |pages=2079–2084 |year=1998 |last1=Ramel |first1=C |citeseerx=10.1.1.484.8521 |s2cid=27867275 }}</ref> Additionally, the alpha males of two neighboring prides can be from the same litter; one brother may come to acquire leadership over another's pride, and subsequently mate with his 'nieces' or cousins. However, killing another male's cubs, upon the takeover, allows the new selected gene complement of the incoming alpha male to prevail over the previous male. There are genetic [[assay]]s being scheduled for lions to determine their genetic diversity. The preliminary studies show results inconsistent with the outcrossing paradigm based on individual environments of the studied groups.<ref name="autogenerated1" />

In Central California, [[sea otters]] were thought to have been driven to extinction due to over hunting, until a small colony was discovered in the Point Sur region in the 1930s.<ref>{{Cite journal|last=Kenyon|first=Karl W. |date=August 1969 |title=The sea otter in the eastern Pacific Ocean |journal=North American Fauna|language=en-US|volume=68|page=13 |doi=10.3996/nafa.68.0001 |doi-access=free |bibcode=1969usgs.rept...13K }}</ref> Since then, the population has grown and spread along the central Californian coast to around 2,000 individuals, a level that has remained stable for over a decade. Population growth is limited by the fact that all Californian sea otters are descended from the isolated colony, resulting in inbreeding.<ref>{{Cite journal| vauthors = Bodkin JL, Ballachey BE, Cronin MA, Scribner KT | date=December 1999|title=Population Demographics and Genetic Diversity in Remnant and Translocated Populations of Sea Otters |journal=Conservation Biology |volume=13 |issue=6 |pages=1378–85 |doi=10.1046/j.1523-1739.1999.98124.x | bibcode=1999ConBi..13.1378B| s2cid=86833574}}</ref>

Cheetahs are another example of inbreeding. Thousands of years ago, the cheetah went through a population bottleneck that reduced its population dramatically so the animals that are alive today are all related to one another. A consequence from inbreeding for this species has been high juvenile mortality, low fecundity, and poor breeding success.<ref>{{cite journal |doi=10.1002/(SICI)1098-2361(1996)15:4<353::AID-ZOO1>3.0.CO;2-A |title=Reassessing the relationship between juvenile mortality and genetic monomorphism in captive cheetahs |journal=Zoo Biology |volume=15 |issue=4 |pages=353–369 |year=1996 |last1=Wielebnowski |first1=Nadja }}</ref>

In a study on an island population of song sparrows, individuals that were inbred showed significantly lower survival rates than outbred individuals during a severe winter weather related population crash. These studies show that inbreeding depression and ecological factors have an influence on survival.<ref name="Pusey, A. 1996"/>

The [[Florida panther]] population was reduced to about 30 animals, so inbreeding became a problem. Several females were imported from Texas and now the population is better off genetically.<ref>[https://www.ucf.edu/news/genetic-rescue-of-florida-panthers-may-have-had-unintended-consequences/#:~:text=any%20future%20introductions.-,The%20Florida%20panther's%20population%20reached%20a%20dangerously%20low%20number%20of,as%20cryptorchidism%2C%20and%20other%20diseases. UCF report on complex genetic health of Florida panther]</ref><ref>Johnson, Warren E., David P. Onorato, Melody E. Roelke, E. Darrell Land, Mark Cunningham, Robert C. Belden, Roy McBride et al. "Genetic restoration of the Florida panther." ''Science'' 329, no. 5999 (2010): 1641-1645.</ref>

== Measures ==

A measure of inbreeding of an individual A is the probability ''F''(A) that both alleles in one locus are derived from the same allele in an ancestor.  These two identical alleles that are both derived from a common ancestor are said to be [[identical by descent]]. This probability F(A) is called the "[[coefficient of inbreeding]]".<ref name=":0">{{cite journal | vauthors = Wright S | year = 1922 | title = Coefficients of inbreeding and relationship | url = https://zenodo.org/record/1431365| journal = American Naturalist | volume = 56 | issue = 645| pages = 330–338 | doi=10.1086/279872| bibcode = 1922ANat...56..330W | s2cid = 83865141 }}</ref>

Another useful measure that describes the extent to which two individuals are related (say individuals A and B) is their coancestry coefficient f(A,B), which gives the probability that one randomly selected allele from A and another randomly selected allele from B are identical by descent.<ref>{{cite journal | vauthors = Reynolds J, Weir BS, Cockerham CC | title = Estimation of the coancestry coefficient: basis for a short-term genetic distance | journal = Genetics | volume = 105 | issue = 3 | pages = 767–79 | date = November 1983 | doi = 10.1093/genetics/105.3.767 | pmid = 17246175 | pmc = 1202185 }}</ref> This is also denoted as the kinship coefficient between A and B.<ref>{{Cite journal| vauthors = Casas AM, Igartua E, Valles MP, Molina-Cano JL |date=November 1998|title=Genetic diversity of barley cultivars grown in Spain, estimated by RFLP, similarity and coancestry coefficients |journal=Plant Breeding |volume=117|issue=5|pages=429–35 |doi=10.1111/j.1439-0523.1998.tb01968.x |bibcode=1998PBree.117..429C |hdl=10261/121301}}</ref>

A particular case is the self-coancestry of individual A with itself, f(A,A), which is the probability that taking one random allele from A and then, independently and with replacement, another random allele also from A, both are identical by descent. Since they can be identical by descent by sampling the same allele or by sampling both alleles that happen to be identical by descent, we have f(A,A) = 1/2 + F(A)/2.<ref>{{cite book | vauthors = Malecot G | page = 1048 | title = Les Mathématiques de l'hérédité. | publisher = Masson et Cie | location = Paris }}</ref>

Both the inbreeding and the coancestry coefficients can be defined for specific individuals or as average population values. They can be computed from genealogies or estimated from the population size and its breeding properties, but all methods assume no selection and are limited to neutral alleles.{{citation needed|date=July 2025}}

There are several methods to compute this percentage. The two main ways are the path method<ref>[http://www.braquedubourbonnais.info/en/inbreeding-calculation.htm How to compute and inbreeding coefficient (the path method)], Braque du Bourbonnais.</ref><ref name=":0" /> and the tabular method.<ref>{{cite web | first = Knud | last = Christensen | url = http://www.husdyr.kvl.dk/htm/kc/popgen/genetics/4/5.htm | title = 4.5 Calculation of inbreeding and relationship, the tabular method | work = Genetic calculation applets and other programs | publisher = Genetics pages | access-date = 2010-10-07 | archive-date = 2010-03-27 | archive-url = https://web.archive.org/web/20100327075815/http://www.husdyr.kvl.dk/htm/kc/popgen/genetics/4/5.htm }}</ref><ref>{{Cite journal| vauthors = García-Cortés LA, Martínez-Ávila JC, Toro MA |date=2010-05-16|title=Fine decomposition of the inbreeding and the coancestry coefficients by using the tabular method |journal=Conservation Genetics |volume=11 |issue=5 |pages=1945–52 |doi=10.1007/s10592-010-0084-x |bibcode=2010ConG...11.1945G |s2cid=2636127}}</ref>

Typical coancestries between relatives are as follows:
* Father/daughter or mother/son → 25% ({{frac|1|4}})
* Brother/sister → 25% ({{frac|1|4}})
* Grandfather/granddaughter or grandmother/grandson → 12.5% ({{frac|1|8}})
* Half-brother/half-sister, Double cousins → 12.5% ({{frac|1|8}})
* Uncle/niece or aunt/nephew → 12.5% ({{frac|1|8}})
* Great-grandfather/great-granddaughter or great-grandmother/great-grandson → 6.25% ({{frac|1|16}})
* Half-uncle/niece or half-aunt/nephew → 6.25% ({{frac|1|16}})
* First cousins → 6.25% ({{frac|1|16}})

== Animals ==
[[File:White tiger-Gunma Safari Park.jpg |thumb|right| [[White tiger]] in Gunma Safari Park]]

=== Wild animals ===
<!-- Merge from https://en.wikipedia.org/wiki/Category:Incestuous_animals -->
* [[Banded mongoose]] females regularly mate with their fathers and brothers.<ref name="mammals" />
* [[Bed bug]]s: [[North Carolina State University]] found that bedbugs, in contrast to most other insects, tolerate incest and are able to genetically withstand the effects of inbreeding quite well.<ref>{{cite web|url=http://www.laboratoryequipment.com/news/2011/12/insect-incest-produces-healthy-offspring|title=Insect Incest Produces Healthy Offspring|date=8 December 2011|access-date=11 February 2017|archive-date=16 May 2017|archive-url=https://web.archive.org/web/20170516080757/http://www.laboratoryequipment.com/news/2011/12/insect-incest-produces-healthy-offspring}}</ref>
* [[Common fruit fly]] females prefer to mate with their own brothers over unrelated males.<ref name="Scottish-families">{{cite journal |vauthors=Loyau A, Cornuau JH, Clobert J, Danchin E |title=Incestuous sisters: mate preference for brothers over unrelated males in Drosophila melanogaster |journal=PLOS ONE |volume=7 |issue=12 |article-number=e51293 |year=2012 |pmid=23251487 |pmc=3519633 |doi=10.1371/journal.pone.0051293|bibcode=2012PLoSO...751293L |doi-access=free }}</ref>
* [[Cottony cushion scale]]s: 'It turns out that females in these hermaphrodite insects are not really fertilizing their eggs themselves, but instead are having this done by a parasitic tissue that infects them at birth,' says Laura Ross of [[Department of Zoology, University of Oxford|Oxford University's Department of Zoology]]. 'It seems that this infectious tissue derives from left-over sperm from their father, who has found a sneaky way of having more children by mating with his daughters.'<ref>{{cite journal | vauthors = Gardner A, Ross L | title = The evolution of hermaphroditism by an infectious male-derived cell lineage: an inclusive-fitness analysis | journal = The American Naturalist | volume = 178 | issue = 2 | pages = 191–201 | date = August 2011 | pmid = 21750383 | doi = 10.1086/660823 | bibcode = 2011ANat..178..191G | hdl = 10023/5096 | s2cid = 15361433 | url = https://pure.rug.nl/ws/files/67294482/660823.pdf | hdl-access = free }}</ref><ref name="pure.rug.nl 2019">{{cite web | title=The Evolution of Hermaphroditism by an Infectious Male-Derived Cell Lineage | website=pure.rug.nl | date=April 26, 2019 | url=https://pure.rug.nl/ws/files/67294482/660823.pdf | archive-url=https://web.archive.org/web/20230422025204/https://pure.rug.nl/ws/files/67294482/660823.pdf | archive-date=April 22, 2023 | url-status=live}}</ref>
* ''[[Adactylidium]]'': The single male offspring mite mates with all the daughters when they are still in the mother. The females, now impregnated, cut holes in their mother's body so that they can emerge. The male emerges as well, but does not look for food or new mates, and dies after a few hours. The females die at the age of 4 days, when their own offspring [[cannibalism (zoology)|eat them alive from the inside]].<ref>{{cite book | first1 = Scott | last1 = Freeman | first2 = Jon C. | last2 = Herran |year=2007 |chapter=Aging and other life history characters | page=484 |title=Evolutionary Analysis |edition=4th  |publisher=[[Pearson Education, Inc]] |isbn=978-0-13-227584-2}}</ref>

[[File:Mangoustes rayées - Banded Mongooses.jpg |thumb|left| [[Banded mongoose]] females regularly mate with their fathers and brothers.<ref name="mammals">{{cite journal | vauthors = Nichols HJ, Cant MA, Hoffman JI, Sanderson JL | title = Evidence for frequent incest in a cooperatively breeding mammal | journal = Biology Letters | volume = 10 | issue = 12 | article-number = 20140898 | date = December 2014 | pmid = 25540153 | pmc = 4298196 | doi = 10.1098/rsbl.2014.0898 }}</ref>]]

[[File:Shaded silver Persian Cat Missionhill Cosmic Rainstorm.jpg |thumb|right| Hereditary [[polycystic kidney disease]] is prevalent in the [[Persian cat]] breed, affecting almost half the population in some countries.<ref>{{Cite web|url=http://icatcare.org/advice/polycystic-kidney-disease|title=Polycystic kidney disease {{!}} International Cat Care|website=icatcare.org|access-date=2016-07-08}}</ref><ref>{{Cite web|url=https://www.vet.cornell.edu/fhc/Health_Information/PolycysticKidneyDisease.cfm|title=Polycystic Kidney Disease|website=www.vet.cornell.edu|access-date=2016-07-08}}</ref>]]

{{clear left}}

=== Domestic animals ===
[[File:Intense inbreeding - Continuous sire to daughter mating.svg |thumb|right| An intensive form of inbreeding where an individual '''S''' is mated to his daughter '''D1''', granddaughter '''D2''' and so on, in order to maximize the percentage of '''S''''s genes in the offspring. 87.5% of '''D3''''s genes would come from '''S''', while '''D4''' would receive 93.75% of their genes from '''S'''.<ref name="TaveInbreeding">{{cite book| last = Tave | first = Douglas |title=Inbreeding and brood stock management|year=1999|publisher=Food and Agriculture Organization of the United Nations|isbn=978-92-5-104340-0|page=50|url=https://books.google.com/books?id=UdvIpkQOf5MC&pg=PA50}}</ref>]]

Breeding in domestic animals is primarily [[assortative mating|assortative]] breeding (see [[selective breeding]]). Without the sorting of individuals by trait, a breed could not be established, nor could poor genetic material be removed.
[[Homozygosity]] is the case where similar or identical alleles combine to express a trait that is not otherwise expressed (recessiveness). Inbreeding exposes recessive alleles through increasing homozygosity.<ref>{{Cite journal|title=Deleterious alleles in the context of domestication, inbreeding, and selection|year=2019|doi=10.1111/eva.12691|last1=Bosse|first1=Mirte|last2=Megens|first2=Hendrik-Jan|last3=Derks|first3=Martijn F. L.|last4=Cara|first4=Ángeles M. R.|last5=Groenen|first5=Martien A. M.|journal=Evolutionary Applications|volume=12|issue=1|pages=6–17|pmid=30622631|pmc=6304688|bibcode=2019EvApp..12....6B }}</ref>

Breeders must avoid breeding from individuals that demonstrate either homozygosity or heterozygosity for disease causing alleles.<ref>[http://extension.missouri.edu/publications/DisplayPub.aspx?P=G2036 G2036 Culling the Commercial Cow Herd: BIF Fact Sheet, MU Extension] {{Webarchive|url=https://web.archive.org/web/20160416142415/http://extension.missouri.edu/publications/DisplayPub.aspx?P=G2036 |date=2016-04-16 }}. Extension.missouri.edu. Retrieved on 2013-03-05.</ref> The goal of preventing the transfer of deleterious alleles may be achieved by reproductive isolation, [[Neutering|sterilization]], or, in the extreme case, [[culling]]. Culling is not strictly necessary if genetics are the only issue in hand. Small animals such as cats and dogs may be sterilized, but in the case of large agricultural animals, such as cattle, culling is usually the only economic option.{{citation needed|date=July 2025}}

The issue of casual breeders who inbreed irresponsibly is discussed in the following quotation on cattle:
<blockquote>Meanwhile, milk production per cow per lactation increased from 17,444&nbsp;lbs to 25,013&nbsp;lbs from 1978 to 1998 for the Holstein breed. Mean breeding values for milk of Holstein cows increased by 4,829&nbsp;lbs during this period.<ref>{{cite web |title=Genetic Evaluation Results |url=http://aipl.arsusda.gov/main/data.html#gtrend
 |archive-url=https://web.archive.org/web/20010827074038/http://aipl.arsusda.gov/main/data.html
 |archive-date=August 27, 2001}}</ref> High producing cows are increasingly difficult to breed and are subject to higher health costs than cows of lower genetic merit for production (Cassell, 2001).

Intensive selection for higher yield has increased relationships among animals within breed and increased the rate of casual inbreeding.
Many of the traits that affect profitability in crosses of modern dairy breeds have not been studied in designed experiments. Indeed, all crossbreeding research involving North American breeds and strains is very dated (McAllister, 2001) if it exists at all.<ref>[http://www.nimss.umd.edu/homepages/home.cfm?trackID=2354 S1008: Genetic Selection and Crossbreeding to Enhance Reproduction and Survival of Dairy Cattle (S-284)] {{Webarchive|url=https://web.archive.org/web/20060910181548/http://nimss.umd.edu/homepages/home.cfm?trackID=2354 |date=2006-09-10 }}. Nimss.umd.edu. Retrieved on 2013-03-05.</ref></blockquote>

As a result of long-term cooperation between [[USDA]] and dairy farmers which led to a revolution in dairy cattle productivity, the United States has since 1992 been the world's largest supplier of dairy bull semen.<ref name=USdairy>{{cite web |url=https://worksinprogress.co/issue/how-big-data-created-the-modern-dairy-cow/ |title=How big data created the modern dairy cow |date=December 5, 2024 |access-date=December 6, 2024 |first=Jared |last=Hutchens |work=Works In Progress |archive-url=https://archive.today/20241206185854/https://worksinprogress.co/issue/how-big-data-created-the-modern-dairy-cow/ |archive-date=December 6, 2024}}</ref>  However, US genomic technology has resulted in the US dairy cattle population becoming "the most inbred it's ever been" and the rate of increase in US national milk yield has tapered off.  Efforts are now being made to identify desirable genes in cattle breeds not yet optimized by US dairy breeders in order to apply [[hybrid vigor]] to the US dairy cattle population and thus propel US dairy technology to even higher levels of productivity.{{citation needed|date=July 2025}}

The BBC produced two documentaries on dog inbreeding titled ''[[Pedigree Dogs Exposed]]'' and ''[[Pedigree Dogs Exposed: Three Years On]]'' that document the negative health consequences of excessive inbreeding.{{citation needed|date=July 2025}}

==== Linebreeding ====

Linebreeding is a form of inbreeding. There is no clear distinction between the two terms, but linebreeding may encompass crosses between individuals and their descendants or two cousins.<ref name="TaveInbreeding" /><ref name="UMissouri">{{cite web | last1 = Vogt | first1 = Dale | last2 = Swartz | first2 = Helen A. | last3 = Massey | first3 = John | title = Inbreeding: Its Meaning, Uses and Effects on Farm Animals | url = http://extension.missouri.edu/publications/DisplayPub.aspx?P=G2911 | work = MU Extension | publisher = [[University of Missouri]] | access-date = April 30, 2011 | date = October 1993 | archive-date = March 8, 2012 | archive-url = https://web.archive.org/web/20120308235825/http://extension.missouri.edu/publications/DisplayPub.aspx?P=G2911 }}</ref> This method can be used to increase a particular animal's contribution to the population.<ref name="TaveInbreeding" /> While linebreeding is less likely to cause problems in the first generation than does inbreeding, over time, linebreeding can reduce the genetic diversity of a population and cause problems related to a too-small gene pool that may include an increased prevalence of genetic disorders and inbreeding depression.<ref>{{Cite web |last=admin-science |date=2023-12-20 |title=Understanding the Genetic Issues Associated with Inbreeding |url=https://scienceofbiogenetics.com/blog/the-detrimental-genetic-issues-associated-with-inbreeding-understanding-the-consequences-and-implications |access-date=2025-08-31 |website=Genetics |language=en-US}}</ref>

==== Outcrossing ====

Outcrossing is where two unrelated individuals are crossed to produce progeny. In outcrossing, unless there is verifiable genetic information, one may find that all individuals are distantly related to an ancient progenitor. If the trait carries throughout a population, all individuals can have this trait. This is called the [[founder effect]]. In the well established breeds, that are commonly bred, a large gene pool is present. For example, in 2004, over 18,000 Persian cats were registered.<ref>[http://www.petplace.com/cats/top-cat-breeds-for-2004/page1.aspx Top Cat Breeds for 2004]. Petplace.com. Retrieved on 2013-03-05.</ref> A possibility exists for a complete outcross, if no barriers exist between the individuals to breed. However, it is not always the case, and a form of distant linebreeding occurs. Again it is up to the assortative breeder to know what sort of traits, both positive and negative, exist within the diversity of one breeding. This diversity of genetic expression, within even close relatives, increases the variability and diversity of viable stock.{{citation needed|date=July 2025}}

=== Laboratory animals ===

Systematic inbreeding and maintenance of inbred strains of [[laboratory mice]] and rats is of great importance for biomedical research. The inbreeding guarantees a consistent and uniform [[animal model]] for experimental purposes and enables genetic studies in [[congenic]] and knock-out animals. In order to achieve a mouse strain that is considered inbred, a minimum of 20 sequential generations of sibling matings must occur. With each successive generation of breeding, homozygosity in the entire genome increases, eliminating heterozygous loci. With 20 generations of sibling matings, homozygosity is occurring at roughly 98.7% of all loci in the genome, allowing for these offspring to serve as animal models for genetic studies.<ref>Taft, Robert et al. "Know thy mouse." Science Direct. Vol. 22, No. 12, Dec. 2006, pp. 649-653. Trends in Genetics. {{doi|10.1016/j.tig.2006.09.010}}</ref> The use of inbred strains is also important for genetic studies in animal models, for example to distinguish genetic from environmental effects. The mice that are inbred typically show considerably lower survival rates.{{citation needed|date=July 2025}}

==Humans==
{{See also|Incest|Incest taboo|Pedigree collapse|Cousin marriage}}
[[File:Global prevalence of consanguinity.svg|center|frame|Consanguineous marriages (second-degree cousins or closer) in the world, in percentage (%).<ref>{{cite journal | vauthors = Hamamy H | title = Consanguineous marriages: Preconception consultation in primary health care settings | journal = Journal of Community Genetics | volume = 3 | issue = 3 | pages = 185–92 | date = July 2012 | pmid = 22109912 | pmc = 3419292 | doi = 10.1007/s12687-011-0072-y }}</ref>]]

===Effects===
Inbreeding increases homozygosity, which can increase the chances of the expression of deleterious or beneficial recessive alleles  and therefore has the potential to either decrease or increase the fitness of the offspring. Depending on the rate of inbreeding, natural selection may still be able to eliminate deleterious alleles.<ref>{{Cite journal |last1=Reed |first1=David H. |last2=Lowe |first2=Edwin H. |last3=Briscoe |first3=David A. |last4=Frankham |first4=Richard |date=2003-05-01 |title=Inbreeding and extinction: Effects of rate of inbreeding |journal=Conservation Genetics |language=en |volume=4 |issue=3 |pages=405–410 |doi=10.1023/A:1024081416729 |bibcode=2003ConG....4..405R |issn=1572-9737}}</ref> With continuous inbreeding, genetic variation is lost and homozygosity is increased, enabling the expression of recessive deleterious alleles in homozygotes. The [[coefficient of inbreeding]], or the degree of inbreeding in an individual, is an estimate of the percent of homozygous alleles in the overall genome.<ref name="Woodley 2009">{{cite journal |doi=10.1016/j.intell.2008.10.007 |title=Inbreeding depression and IQ in a study of 72 countries |journal=Intelligence |volume=37 |issue=3 |pages=268–276 |year=2009 |last1=Woodley |first1=Michael A }}</ref> The more biologically related the parents are, the greater the coefficient of inbreeding, since their genomes have many similarities already. This overall homozygosity becomes an issue when there are deleterious recessive alleles in the gene pool of the family.<ref name="Kamin 1980">{{cite journal |doi=10.1037/0033-2909.87.3.469 |title=Inbreeding depression and IQ |journal=Psychological Bulletin |volume=87 |issue=3 |pages=469–478 |year=1980 |last1=Kamin |first1=Leon J |pmid=7384341 }}</ref> By pairing chromosomes of similar genomes, the chance for these recessive alleles to pair and become homozygous greatly increases, leading to offspring with autosomal recessive disorders.<ref name="Kamin 1980" /> However, these deleterious effects are common for very close relatives but not for those related on the 3rd cousin or greater level, who exhibit increased fitness.<ref name="nature.com">{{Cite journal |last1=de Boer |first1=Raïssa A. |last2=Vega-Trejo |first2=Regina |last3=Kotrschal |first3=Alexander |last4=Fitzpatrick |first4=John L. |date=July 2021 |title=Meta-analytic evidence that animals rarely avoid inbreeding |url=https://www.nature.com/articles/s41559-021-01453-9 |journal=Nature Ecology & Evolution |language=en |volume=5 |issue=7 |pages=949–964 |doi=10.1038/s41559-021-01453-9 |pmid=33941905 |bibcode=2021NatEE...5..949D |s2cid=233718913 |issn=2397-334X|url-access=subscription }}</ref>

Inbreeding is especially problematic in small populations where the genetic variation is already limited.<ref name="Tadmouri 2009">{{cite journal | vauthors = Tadmouri GO, Nair P, Obeid T, Al Ali MT, Al Khaja N, Hamamy HA | title = Consanguinity and reproductive health among Arabs | journal = Reproductive Health | volume = 6 | issue = 1 | article-number = 17 | date = October 2009 | pmid = 19811666 | pmc = 2765422 | doi = 10.1186/1742-4755-6-17 | doi-access = free }}</ref> By inbreeding, individuals are further decreasing genetic variation by increasing homozygosity in the genomes of their offspring.<ref name="Roberts 1967">{{cite journal | vauthors = Roberts DF | title = Incest, inbreeding and mental abilities | journal = British Medical Journal | volume = 4 | issue = 5575 | pages = 336–7 | date = November 1967 | pmid = 6053617 | pmc = 1748728 | doi = 10.1136/bmj.4.5575.336 }}</ref> Thus, the likelihood of deleterious recessive alleles to pair is significantly higher in a small inbreeding population than in a larger inbreeding population.<ref name="Tadmouri 2009" />

The fitness consequences of consanguineous mating have been studied since their scientific recognition by [[Charles Darwin]] in 1839.<ref name="Berg 1983">{{Cite journal|title = Human inbreeding avoidance: Culture in nature|journal = Behavioral and Brain Sciences|doi = 10.1017/S0140525X00014850|volume=6 |pages=91–102|year = 2010|last1 = Van Den Berghe|first1 = Pierre L|s2cid = 146133244}}</ref><ref name="Bittles 2010">{{cite book| veditors = Speicher MR, Motulsky AG, Antonarakis SE, Bittles AH |title=Vogel and Motulsky's human genetics problems and approaches|date=2010|publisher=Springer-Verlag|location=Berlin|isbn=978-3-540-37654-5|pages=507–528|edition=4th |chapter=Consanguinity, Genetic Drift, and Genetic Diseases in Populations with Reduced Numbers of Founders}}</ref>  Some of the most harmful effects known from such breeding includes its effects on the mortality rate as well as on the general health of the offspring.<ref name="Ober 1999">{{cite journal | vauthors = Ober C, Hyslop T, Hauck WW | title = Inbreeding effects on fertility in humans: evidence for reproductive compensation | journal = American Journal of Human Genetics | volume = 64 | issue = 1 | pages = 225–31 | date = January 1999 | pmid = 9915962 | pmc = 1377721 | doi = 10.1086/302198 }}</ref> Since the 1960s, there have been many studies to support such debilitating effects on the human organism.<ref name="Roberts 1967" /><ref name="Berg 1983" /><ref name="Ober 1999" /><ref name="Morton 1978">{{cite journal | vauthors = Morton NE | title = Effect of inbreeding on IQ and mental retardation | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 75 | issue = 8 | pages = 3906–8 | date = August 1978 | pmid = 279005 | pmc = 392897 | doi = 10.1073/pnas.75.8.3906 | bibcode = 1978PNAS...75.3906M | doi-access = free }}</ref><ref name="Bittles 2002">{{cite journal | vauthors = Bittles AH, Grant JC, Sullivan SG, Hussain R | title = Does inbreeding lead to decreased human fertility? | journal = Annals of Human Biology | volume = 29 | issue = 2 | pages = 111–30 | date = 2002-01-01 | pmid = 11874619 | doi = 10.1080/03014460110075657 | s2cid = 31317976 }}</ref> Specifically, inbreeding has been found to decrease fertility as a direct result of increasing homozygosity of deleterious recessive alleles.<ref name="Bittles 2002" /><ref>{{cite journal | vauthors = Ober C, Elias S, Kostyu DD, Hauck WW | title = Decreased fecundability in Hutterite couples sharing HLA-DR | journal = American Journal of Human Genetics | volume = 50 | issue = 1 | pages = 6–14 | date = January 1992 | pmid = 1729895 | pmc = 1682532 }}</ref> Fetuses produced by inbreeding also face a greater risk of spontaneous abortions due to inherent complications in development.<ref>{{cite journal | vauthors = Diamond JM | title = Causes of death before birth | journal = Nature | volume = 329 | issue = 6139 | pages = 487–8 | year = 1987 | pmid = 3657971 | doi = 10.1038/329487a0 | bibcode = 1987Natur.329..487D | s2cid = 4338257 }}</ref> Among mothers who experience stillbirths and early infant deaths, those that are inbreeding have a significantly higher chance of reaching repeated results with future offspring.<ref>{{cite journal | vauthors = Stoltenberg C, Magnus P, Skrondal A, Lie RT | title = Consanguinity and recurrence risk of stillbirth and infant death | journal = American Journal of Public Health | volume = 89 | issue = 4 | pages = 517–23 | date = April 1999 | pmid = 10191794 | pmc = 1508879 | doi = 10.2105/ajph.89.4.517 }}</ref> Additionally, consanguineous parents possess a high risk of premature birth and producing underweight and undersized infants.<ref>{{cite journal | vauthors = Khlat M | title = Inbreeding effects on fetal growth in Beirut, Lebanon | journal = American Journal of Physical Anthropology | volume = 80 | issue = 4 | pages = 481–4 | date = December 1989 | pmid = 2603950 | doi = 10.1002/ajpa.1330800407 | bibcode = 1989AJPA...80..481K }}</ref> Viable inbred offspring are also likely to be inflicted with physical deformities and genetically inherited diseases.<ref name="Woodley 2009" /> Studies have confirmed an increase in several genetic disorders due to inbreeding such as blindness, hearing loss, [[neonatal diabetes]], limb malformations, [[disorders of sex development]], [[schizophrenia]] and several others.<ref name="Woodley 2009" /><ref>{{cite journal | vauthors = Bener A, Dafeeah EE, Samson N | title = Does consanguinity increase the risk of schizophrenia? Study based on primary health care centre visits | journal = Mental Health in Family Medicine | volume = 9 | issue = 4 | pages = 241–8 | date = December 2012 | pmid = 24294299 | pmc = 3721918 }}</ref> Moreover, there is an increased risk for [[congenital heart disease]] depending on the inbreeding coefficient (See [[coefficient of inbreeding]]) of the offspring, with significant risk accompanied by an F =.125 or higher.<ref name="Bittles 2012"/>

=== Prevalence ===
The general negative outlook and eschewal of inbreeding that is prevalent in the [[Western world]] today has roots from over 2000 years ago. Specifically, written documents such as the Bible illustrate that there have been laws and social customs that have called for the abstention from inbreeding. Along with cultural taboos, parental education and awareness of inbreeding consequences have played large roles in minimizing inbreeding frequencies in areas like Europe. That being so, there are less urbanized and less populated regions across the world that have shown continuity in the practice of inbreeding.{{Citation needed|date=January 2025}}

The continuity of inbreeding is often either by choice or unavoidably due to the limitations of the geographical area. When by choice, the rate of consanguinity is highly dependent on religion and culture.<ref name="Tadmouri 2009" /> In the Western world, some [[Anabaptism|Anabaptist]] groups are highly inbred because they originate from small founder populations that have bred as a closed population.<ref name="k666">{{cite journal | last1=Agarwala | first1=Richa | last2=Schäffer | first2=Alejandro A. | last3=Tomlin | first3=James F. | title=Towards a Complete North American Anabaptist Genealogy II: Analysis of Inbreeding | journal=Human Biology | publisher=Wayne State University Press | volume=73 | issue=4 | year=2001 | issn=0018-7143 | jstor=41466828 | pages=533–545 | doi=10.1353/hub.2001.0045 | pmid=11512680 }}</ref>

Of the practicing regions, [[Cousin marriage in the Middle East|Middle Eastern and northern African]] nations show the greatest frequencies of consanguinity.<ref name="Tadmouri 2009" />

Among these populations with high levels of inbreeding, researchers have found several disorders prevalent among inbred offspring. In [[Lebanon]], [[Saudi Arabia]], [[Egypt]], and [[Israel]], the offspring of consanguineous relationships have an increased risk of congenital malformations, congenital heart defects, congenital [[hydrocephalus]] and [[neural tube defect]]s.<ref name="Tadmouri 2009" />  Furthermore, among inbred children in [[Palestine]] and Lebanon, there is a positive association between consanguinity and reported [[Cleft palate|cleft lip/palate]] cases.<ref name="Tadmouri 2009" />  Historically, populations of [[Qatar]] have engaged in consanguineous relationships of all kinds, leading to high risk of inheriting genetic diseases. As of 2014, around 5% of the Qatari population suffered from hereditary hearing loss; most were descendants of a consanguineous relationship.<ref>{{cite journal | vauthors = Girotto G, Mezzavilla M, Abdulhadi K, Vuckovic D, Vozzi D, Khalifa Alkowari M, Gasparini P, Badii R | title = Consanguinity and hereditary hearing loss in Qatar | journal = Human Heredity | volume = 77 | issue = 1–4 | pages = 175–82 | date = 2014-01-01 | pmid = 25060281 | doi = 10.1159/000360475 | doi-access = free | hdl = 11577/3455561 | hdl-access = free }}</ref> In 2017-2019, congenital anomalies due to inbreeding was the most common cause of death of babies belonging to the Pakistani and Bangladeshi ethnic groups in England and Wales.<ref>{{Cite web |title=Births and infant mortality by ethnicity in England and Wales - Office for National Statistics |url=https://www.ons.gov.uk/peoplepopulationandcommunity/healthandsocialcare/childhealth/articles/birthsandinfantmortalitybyethnicityinenglandandwales/2007to2019 |access-date=2025-01-27 |website=www.ons.gov.uk}}</ref>

=== Royalty and nobility ===
[[File:Carlos II de España, por Juan Carreño de Miranda (Museo del Prado).jpg|thumb|[[Charles II of Spain]], a member of the famously inbred [[House of Habsburg|Habsburgs]] with an [[Habsburg jaw|extremely pronounced lower jaw]]. ]]
{{Main|Royal intermarriage}}
{{see also|List of coupled cousins}}

Inter-nobility [[marriage]] was used as a method of forming [[political alliance]]s among elites. These ties were often sealed only upon the birth of progeny within the [[arranged marriage]]. Thus marriage was seen as a union of lines of nobility and not as a contract between individuals.{{citation needed|date=July 2025}}

[[Royal intermarriage]] was often practiced among European royal families, usually for interests of state. Over time, due to the relatively limited number of potential consorts, the [[gene pool]] of many ruling families grew progressively smaller, until all European royalty was related. This also resulted in many being descended from a certain person through many lines of descent, such as the numerous European royalty and nobility descended from the British [[Victoria of the United Kingdom|Queen Victoria]] or [[King Christian IX of Denmark]].<ref name="beechegotha1">{{cite book | title=The Gotha: Still a Continental Royal Family, Vol. 1 | publisher=Kensington House Books | last = Beeche | first = Arturo | year=2009 | location=Richmond, US | pages=1–13 | isbn=978-0-9771961-7-3}}</ref> The House of [[Habsburg]] was known for its intermarriages;  the [[Prognathism|Habsburg lip]] often cited as an ill-effect.  The closely related houses of Habsburg, [[House of Bourbon|Bourbon]], [[House of Braganza|Braganza]] and [[House of Wittelsbach|Wittelsbach]] also frequently engaged in first-cousin unions as well as the occasional [[Cousin#Multiplicities|double-cousin]] and uncle–niece marriages.{{citation needed|date=July 2025}}

In [[ancient Egypt]], royal women were believed to carry the bloodlines and so it was advantageous for a [[pharaoh]] to marry his sister or half-sister;<ref>{{cite web | title = Women in Ancient Egypt, Women and Law | last = Seawright | first = Caroline | work = thekeep.org | url = http://www.thekeep.org/~kunoichi/kunoichi/themestream/women_egypt.html | access-date = 2010-12-29 | archive-date = 2010-12-27 | archive-url = https://web.archive.org/web/20101227173747/http://www.thekeep.org/~kunoichi/kunoichi/themestream/women_egypt.html }}</ref> in such cases a special combination between [[endogamy]] and [[polygamy]] is found. Normally, the old ruler's eldest son and daughter (who could be either siblings or half-siblings) became the new rulers. All rulers of the [[Ptolemaic dynasty]] uninterruptedly from [[Ptolemy IV]] ([[Ptolemy II]] married his sister but had no issue) were married to their brothers and sisters, so as to keep the Ptolemaic blood "pure" and to strengthen the line of succession. King [[Tutankhamun]]'s mother is reported to have been the half-sister to his father,<ref>[https://web.archive.org/web/20100218120151/http://news.nationalgeographic.com/news/2010/02/100216-king-tut-malaria-bones-inbred-tutankhamun/ King Tut Mysteries Solved: Was Disabled, Malarial, and Inbred]</ref> [[Cleopatra VII]] (also called Cleopatra VI) and [[Ptolemy XIII]], who married and became co-rulers of [[ancient Egypt]] following their father's death, are the most widely known example.<ref>{{cite web|title= The House of Ptolomey | vauthors = Bevan ER | work =  uchicago.edu | url = https://penelope.uchicago.edu/Thayer/E/Gazetteer/Places/Africa/Egypt/_Texts/BEVHOP/13*.html}}</ref>

{{clear}}

== See also ==
{{columns-list|colwidth=20em|
* [[Álvarez incest case]]
* [[Coefficient of relationship]]
* [[Consanguinity]]
* [[Cousin marriage]]
* [[Cousin marriage in the Middle East]]
* [[Evolution of sexual reproduction]]
* [[Exogamy]]
* [[Founder effect]]
* [[F-statistics|''F''-statistics]]
* [[Fritzl case]]
* [[Genetic diversity]]
* [[Genetic purging]]
* [[Genetic sexual attraction]]
* [[Heterozygote advantage]]
* [[Identical ancestors point]]
* [[Inbreeding depression]]
* [[Inbreeding in fish]]
* [[Incest]]
* [[Incest taboo]]
* [[Insular dwarfism]]
* [[Intellectual inbreeding]]
* [[Legality of incest]]
* [[List of coupled cousins]]
* [[Mahram]]
* [[Outbreeding depression]]
* [[Outcrossing]]
* [[Proximity of blood]]
* [[Prohibited degree of kinship]]
* [[Selective breeding]]
* [[Self-incompatibility in plants]] (the way that some plants avoid inbreeding)
}}

== References ==
{{Reflist|30em}}

== External links ==
* [http://extension.missouri.edu/publications/DisplayPub.aspx?P=G2911 Dale Vogt, Helen A. Swartz and John Massey, 1993. ''Inbreeding: Its Meaning, Uses and Effects on Farm Animals''. University of Missouri, Extension.] {{Webarchive|url=https://web.archive.org/web/20120308235825/http://extension.missouri.edu/publications/DisplayPub.aspx?P=G2911 |date=2012-03-08 }}
* [https://web.archive.org/web/20120302174519/http://www.consang.net/index.php/Summary Consanguineous marriages with global map]
* {{Cite Americana|wstitle=Cross-Fertilization in Animals and in Man | first = Ernest | last = Ingersoll |author-link = Ernest Ingersoll |short=x}}

{{Genetics}}
{{Incest}}

[[Category:Population genetics]]
[[Category:Breeding]]
[[Category:Incest]]
[[Category:Kinship and descent]]