Editing Sibling (section)

==Consanguinity and genetics==
[[Consanguinity]] is the measure of how closely people are related.<ref>{{Cite web|last=nivens|date=2015-01-09|title=Consanguinity|url=https://adoption.umn.edu/consanguinity|access-date=2021-03-02|website=Adoption Medicine Clinic – University of Minnesota|language=en|archive-date=2021-03-05|archive-url=https://web.archive.org/web/20210305135928/https://adoption.umn.edu/consanguinity|url-status=dead}}</ref> Genetic relatedness measures how many genes a person shares. As all humans share over 99% of the same genes, consanguinity only matters for the small fraction of genes which vary between different people.<ref name="Ask a Geneticist 2006">{{cite web|last=Shafer |first=Aaron |url=https://www.thetech.org/ask-a-geneticist/articles/2006/ask166/ |title=How are siblings 50% genetically identical while humans and mice are 85% identical? |website=[[The Tech Interactive]] |series=Ask a Geneticist |date=17 March 2006 |access-date=5 August 2024}}</ref> Inheritance of genes has a random element to it,<ref name="Ask a Geneticist 2006"/> and these two concepts are different.<ref name="Ask a Geneticist 2013">{{cite web |last=Starr |first=D. Barry |url=https://www.thetech.org/ask-a-geneticist/why-siblings-share-around-fifty-percent-their-dna/ |title=Why do half-siblings share 25% of their genes? |website=[[The Tech Interactive]] |series=Ask a Geneticist |date=5 September 2013 |access-date=5 August 2024}}</ref> Consanguinity decreases by half for every generation of reproductive separation through their most recent common ancestor. Siblings are 50% related by consanguinity as they are separated from each other by two generation (sibling to parent to sibling), and they share two parents as common ancestors (<math>\left ( \tfrac{1}{2} \right )^2 + \left ( \tfrac{1}{2} \right )^2</math>).

A fraternal twin is a sibling and, therefore, is related by 50% consanguinity.<ref>{{Cite web|title=Coefficient of relationship – ISOGG Wiki|url=https://isogg.org/wiki/Coefficient_of_relationship|access-date=2021-03-02|website=isogg.org}}</ref> [[Fraternal twins]] are no more genetically similar than regular siblings. As [[identical twin]]s come from the same zygote, their most recent common ancestor is each other. They’re genetically identical and 100% consanguineous as they’re separated by zero generations (<math>\left ( \tfrac{1}{2} \right )^0</math>).<ref name="Ask a Geneticist 2006"/> [[Twin study|Twin studies]] have been conducted by scientists to examine the roles that [[Nature versus nurture|genetics and environment]] play in the development of various traits. Such studies examine how often identical twins possess the same behavioral trait and compare it to how often fraternal twins possess the same trait.<ref>{{cite web |title=Insights From Identical Twins |url=https://learn.genetics.utah.edu/content/epigenetics/twins/ |website=Genetic Science Learning Center, University of Utah}}</ref> In other studies twins are raised in separate families, and studies compare the passing on of a behavioral trait by the family environment and the possession of a common trait between identical twins. This kind of study has revealed that for personality traits which are known to be [[Heritability|heritable]], genetics play a substantial role throughout life and an even larger role during early years.<ref>{{cite journal|last1=Plomin|first1=R|last2=Pederson|first2=N.L.|last3=McClearn|first3=G.E.|last4=Nesselroade|first4=J.R.|last5=Bergeman|first5=C.S.|title=EAS temperaments during the last half of the life span: Twins reared apart and twins reared together|journal=Psychology and Aging|date=1988|volume=3|issue=1|pages=43–50|doi=10.1037/0882-7974.3.1.43|pmid=3268242}}</ref>

Half-siblings are 25% related by consanguinity as they share one parent and separated from each other by two generations (<math>\left ( \tfrac{1}{2} \right )^2</math>).

A person may share more than the standard consanguinity with their sibling if their parents are related (the [[coefficient of inbreeding]] is greater than zero).{{citation needed|date=June 2020}} {{anchor|3quarters}}Interestingly, half-siblings can be related by as "three-quarters siblings" (related by 3/8) if their unshared parents have a consanguinity of 50%. This means the unshared parents are either siblings, making the half-siblings [[cousin]]s, or parent and child, making them half- [[aunt]]-[[uncle]] and [[niece]]-[[nephew]].<ref>{{cite web |last=Benitez |first=Cecil |url=https://www.thetech.org/ask-a-geneticist/articles/2011/ask430/ |title=How related are three-fourths siblings? |date=29 September 2011 |website=[[The Tech Interactive]] |series=Ask a Geneticist |access-date=5 August 2024}}</ref>

===Percentage distribution===
In practice, full siblings do not share exactly 50% of their DNA, as [[chromosomal crossover]] only occurs a limited number of times and, therefore, large chunks of a chromosome are shared or not shared at one time. In fact, the mean DNA fraction shared is 50.28% with a standard deviation of 3.68%,<ref>{{cite journal | pmc=4037293 | year=2013 | last1=Vinkhuyzen | first1=A. A. | last2=Wray | first2=N. R. | last3=Yang | first3=J. | last4=Goddard | first4=M. E. | last5=Visscher | first5=P. M. | title=Estimation and Partitioning of Heritability in Human Populations using Whole Genome Analysis Methods | journal=Annual Review of Genetics | volume=47 | pages=75–95 | doi=10.1146/annurev-genet-111212-133258 | pmid=23988118 }}</ref> meaning approximately 1/4 of sibling pairs share more than 52.76% of their DNA, while 1/4 share less than 47.8%.<ref>Calculated based on the [[Standard deviation#Rules for normally distributed data|normal distribution]]</ref>

There is a very small chance that two half-siblings might not share any genes if they didn't inherit any of the same chromosomes from their shared parent. This is possible for full-siblings as well, though even more unlikely. But because of how [[homologous chromosomes]] swap genes (due to [[chromosomal crossover]] during [[meiosis]]) during the development of an egg or sperm cell, however, the odds of this ever actually occurring are practically non-existent.<ref name="Ask a Geneticist 2013"/>