Editing Inbreeding (section)

==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 |url=https://doi.org/10.1023/A:1024081416729 |journal=Conservation Genetics |language=en |volume=4 |issue=3 |pages=405–410 |doi=10.1023/A:1024081416729 |bibcode=2003ConG....4..405R |issn=1572-9737|url-access=subscription }}</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"/>

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 | pages = 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 }}</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 | url=http://www.jstor.org/stable/41466828 | access-date=2024-08-25| url-access=subscription }}</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===
{{Main|Royal intermarriage}}
{{see also|List of coupled cousins}}
[[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]]]]
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.

[[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#Additional terms|double-cousin]] and uncle–niece marriages.

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 | url-status = dead }}</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>