Editing Incest (section)

==Inbreeding==
{{Main|Inbreeding}}
Offspring of biologically related parents are subject to the possible impact of inbreeding. Such offspring have a higher possibility of [[Congenital disorder|congenital birth defects]] (see [[Coefficient of relationship]]), because it increases the proportion of zygotes that are [[homozygous]] for deleterious [[recessive allele]]s that produce such disorders<ref>{{cite journal |last=Livingstone |first=F. B. |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> (see [[Inbreeding depression]]). Because most such [[allele]]s are rare in populations, it is unlikely that two unrelated marriage partners will both be heterozygous carriers. However, because close relatives [[Coefficient of relationship|share a large fraction of their alleles]], the probability that any such rare deleterious allele present in the common ancestor will be inherited from both related parents is increased dramatically with respect to non-inbred couples. Contrary to common belief, inbreeding does not in itself alter allele frequencies, but rather increases the relative proportion of homozygotes to heterozygotes. This has two contrary effects:<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>
* In the short term, because incestuous reproduction increases [[zygosity]], deleterious recessive alleles will express themselves more frequently, leading to increases in [[Miscarriage|spontaneous abortions]] of zygotes, perinatal deaths, and postnatal offspring with birth defects.
* In the long run, however, because of this increased exposure of deleterious recessive alleles to [[natural selection]], their frequency decreases more rapidly in inbred population, leading to a "healthier" population (with fewer deleterious recessive alleles).
The closer the relationship between two persons, the higher the zygosity, and thus the more severe the biological costs of inbreeding. This fact probably explains why inbreeding between close relatives, such as siblings, is less common than inbreeding between cousins.<ref>{{cite journal |last1=Antfolk |first1=Jan |last2=Lieberman |first2=Debra |last3=Santtila |first3=Pekka |title=Fitness Costs Predict Inbreeding Aversion Irrespective of Self-Involvement: Support for Hypotheses Derived from Evolutionary Theory |journal=PLOS ONE |volume=7 |issue=11 |doi=10.1371/journal.pone.0050613 |pages=e50613 |pmid=23209792 |pmc=3509093|year=2012 |bibcode=2012PLoSO...750613A |doi-access=free }}</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 |last1=Lieberman |first1=D. |last2=Tooby |first2=J. |last3=Cosmides |first3=L. |doi=10.1098/rspb.2002.2290 |title=Does morality have a biological basis? An empirical test of the factors governing moral sentiments relating to incest |journal=Proceedings of the Royal Society B: Biological Sciences |volume=270 |issue=1517 |pages=819–826 |year=2003 |pmid= 12737660|pmc=1691313}}</ref>

A 1994 study found a mean excess mortality with inbreeding among first cousins of 4.4%.<ref>{{cite web |last=Bittles |first=A.H. |title=A Background Summary of Consaguineous marriage |url=http://www.consang.net/images/d/dd/01AHBWeb3.pdf |publisher=consang.net |year=2001 |access-date=19 January 2010 |archive-date=27 September 2018 |archive-url=https://web.archive.org/web/20180927023329/http://www.consang.net/images/d/dd/01AHBWeb3.pdf }}, citing {{Cite journal |last1=Bittles |first1=A. H. |last2=Neel |first2=J.V. |year=1994 |title=The costs of human inbreeding and their implications for variation at the DNA level |journal=Nature Genetics |issue=2 |pages=117–121 |pmid=7842008 |volume=8 |doi=10.1038/ng1094-117|s2cid=36077657 }}</ref> A 2008 study also found decreased lifespan among offspring of first cousins, but no difference between lifespans after the second cousin level.<ref>{{Cite journal |last1=Helgason |first1=Agnar |last2=Pálsson |first2=Snæbjörn |last3=Guðbjartsson |first3=Daníel F. |last4=Kristjánsson |first4=þórður |last5=Stefánsson |first5=Kári |date=2008-02-08 |title=An Association Between the Kinship and Fertility of Human Couples |url=https://www.science.org/doi/10.1126/science.1150232 |journal=Science |language=en |volume=319 |issue=5864 |pages=813–816 |doi=10.1126/science.1150232 |pmid=18258915 |bibcode=2008Sci...319..813H |s2cid=17831162 |issn=0036-8075}}</ref> A 1990 study conducted in South India found that the incidence of malformations was slightly higher in uncle-niece progeny (9.34%) compared to the first cousin progeny (6.18%).<ref>{{Cite journal |last1=Kulkarni |first1=M L |last2=Kurian |first2=M |date=June 1990 |title=Consanguinity and its effect on fetal growth and development: a south Indian study. |journal=Journal of Medical Genetics |volume=27 |issue=6 |pages=348–352 |doi=10.1136/jmg.27.6.348 |issn=0022-2593 |pmc=1017129 |pmid=2359095}}</ref> Stillbirth rates were significantly higher among consanguineous couples irrespective of the mother's socioeconomic status, and were higher in uncle-niece mating's compared to first cousin and beyond first cousin unions in both the poor and middle/upper class. Children of parent{{ndash}}child or sibling{{ndash}}sibling unions are at increased risk compared to cousin{{ndash}}cousin unions. Studies suggest that 20–36% of these children will die or have major disability due to the inbreeding.<ref name="WolfDurham2005" /> A study of 29 offspring resulting from brother{{ndash}}sister or father{{ndash}}daughter incest found that 20 had congenital abnormalities, including four directly attributable to autosomal recessive alleles.<ref name="Baird">{{cite journal |doi=10.1016/S0022-3476(82)80347-8 |last1=Baird |first1=P. A. |last2=McGillivray |first2=B. |year=1982 |title=Children of incest |journal=The Journal of Pediatrics |volume=101 |issue=5 |pages=854–7 |pmid=7131177}}</ref>