Editing Inbreeding (section)

==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.

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 fall below a minimum whereby the forced interbreeding between the remaining animals will result in [[extinction]].

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>