Editing Eye color (section)

===Blue===
{{Redirect|Blue eyes}}
[[File:Blue eye 2.jpg|thumb|A light blue iris with [[limbal ring]]]]
[[File:A blue eye.jpg|thumb|Blue iris]]
There is no intrinsically blue pigmentation either in the iris or in the [[vitreous body]]; in fact, a form of melanin that would produce a blue coloration does not currently exist in the bodies of most mammals. Rather, blue eyes result from [[structural color]] in combination with certain concentrations of non-blue pigments. The iris pigment [[epithelium]] is brownish black due to the presence of [[melanin]].<ref name=Menon>{{cite journal |vauthors=Menon IA, Basu PK, Persad S, Avaria M, Felix CC, Kalyanaraman B |title=Is there any difference in the photobiological properties of melanins isolated from human blue and brown eyes? |journal=Br J Ophthalmol |volume=71 |issue=7 |pages=549–52 |year=1987|pmid=2820463 |pmc=1041224 |doi=10.1136/bjo.71.7.549}}</ref> Unlike brown eyes, blue eyes have low concentrations of melanin in the stroma of the iris, which lies in front of the dark epithelium. Longer wavelengths of light tend to be absorbed by the dark underlying epithelium, while shorter wavelengths are reflected and undergo [[Rayleigh scattering]] in the [[Turbidity|turbid]] medium of the stroma.<ref name="Wang"/> This is the same scattering that accounts for the blue appearance of the sky.<ref name="Fox1978"/>{{rp|9}}<ref name="Mason"/> The result is a "[[Tyndall effect|Tyndall]] blue" structural color that varies with external lighting conditions.
 	
Blue eyes are a highly [[sexually dimorphic]] eye color. Studies from various populations in Europe have shown that men are substantially more likely to have blue eyes than women.<ref name="Martinez-Cadenas Peña-Chilet Ibarrola-Villava Ribas 2013 pp. 453–460" />

The inheritance pattern followed by blue eyes was previously assumed to be a [[Mendelian inheritance|Mendelian recessive]] trait, though this has been shown to be incorrect. Eye color inheritance is now recognized as a [[polygenic trait]], meaning that it is controlled by the interactions of several genes.<ref>{{harvnb|Duffy|Montgomery|Chen|Zhao|Le|Hayward|Martin|Sturm|2007|quote=The genetics of human eye color has been studied for over a century,52 and, for most of that time, it was considered a simple Mendelian recessive trait, with brown eye color dominant over blue. More recently, eye color has been accepted as being a polygenic trait, with multiple genes contributing to the expressivity of eye color.18,35}}</ref>

In 2008, a team of researchers from the [[University of Copenhagen]] located a single mutation that causes the phenomenon of blue eyes. The research was published in the ''[[Journal of Human Genetics]]''. The same DNA sequence of the ''OCA2'' gene among blue-eyed people suggests they may have a single common ancestor. The researchers hypothesized that the ''OCA2'' mutation responsible for blue eyes arose in an individual who lived in the northwestern part of the [[Black Sea]] region in [[Europe]] sometime between 6,000 and 10,000 years ago, during the [[Neolithic]] period.<ref name="MSNBC">{{cite news|first=Jeanna|last=Bryner|title= Genetic mutation makes those brown eyes blue|publisher=[[NBC News]] |date=31 January 2008|access-date=19 October 2009|url=http://www.nbcnews.com/id/22934464|archive-url=https://web.archive.org/web/20130701044221/http://www.nbcnews.com/id/22934464|url-status=dead|archive-date=1 July 2013}}</ref><ref>{{cite web |date=2008-01-31 |title=How one ancestor helped turn our brown eyes blue |url=https://www.independent.co.uk/news/science/how-one-ancestor-helped-turn-our-brown-eyes-blue-776170.html |access-date=2015-12-21 |website=The Independent |language=en-GB}} "Everyone with blue eyes alive today can trace their ancestry back to one person who probably lived about 10,000 years ago in the Black Sea region, a study has found."</ref> However, more recent [[ancient DNA]] research has identified human remains much older than the Neolithic period which possess the ''OCA2'' mutation for blue eyes. It is now believed that the ''OCA2'' allele responsible for blue eyes dates back to the migration of modern humans [[Recent African origin of modern humans|out of Africa]] roughly 50,000 years ago, and entered Europe from western Asia.<ref name="Cberg" />

Eiberg and colleagues suggested in a study published in ''Human Genetics'' that a mutation in the 86th [[intron]] of the ''[[HERC2]]'' gene, which is hypothesized to interact with the ''OCA2'' gene [[Promoter (biology)|promoter]], reduced expression of ''OCA2'' with subsequent reduction in melanin production.<ref name="MSNBC" /><ref name="blue mutation">{{cite journal |last1=Eiberg |first1=Hans |last2=Troelsen |first2=Jesper |last3=Nielsen |first3=Mette |last4=Mikkelsen |first4=Annemette |last5=Mengel-From |first5=Jonas |last6=Kjaer |first6=Klaus W. |last7=Hansen |first7=Lars |year=2008 |title=Blue eye color in humans may be caused by a perfectly associated founder mutation in a regulatory element located within the HERC2 gene inhibiting OCA2 expression |journal=Hum. Genet. |volume=123 |issue=2 |pages=177–87 |doi=10.1007/s00439-007-0460-x |pmid=18172690 |s2cid=9886658}}</ref><ref name="telegraph.co.uk">{{cite news |url=https://www.telegraph.co.uk/science/science-news/3323607/Blue-eyes-result-of-ancient-genetic-mutation.html |archive-url=https://web.archive.org/web/20091101055254/http://www.telegraph.co.uk/science/science-news/3323607/Blue-eyes-result-of-ancient-genetic-mutation.html |archive-date=1 November 2009 |location=London |work=The Daily Telegraph |title=Blue eyes result of ancient genetic 'mutation' |first=Roger |last=Highfield |date=30 January 2008 |access-date=19 October 2011}}</ref>

It has been proposed that blue eyes may have been adaptive to shorter day lengths at higher latitudes, as blue eyes increase intraocular light scattering, which suppresses [[melatonin]] release from the [[pineal gland]], perhaps reducing psychological depression (which is linked to the short day length of higher latitudes).<ref name="e300">{{cite journal | last=Lucock | first=Mark D. | title=The evolution of human skin pigmentation: A changing medley of vitamins, genetic variability, and <scp>UV</scp> radiation during human expansion | journal=American Journal of Biological Anthropology | publisher=Wiley | volume=180 | issue=2 | date=2022-06-25 | issn=2692-7691 | doi=10.1002/ajpa.24564 | pages=252–271| pmid=36790744 |quote=This may also be a direct counter‐measure to a short winter photoperiod at high latitudes; blue eyes increase intraocular light scattering and thereby suppress melatonin release from the pineal gland. This may be an adaptive trait to reduce/prevent depression, a condition linked to short day length (Higuchi et al., 2007; Lucock et al., 2021).| pmc=10083917 }}</ref>

Blue eyes are predominant in northern and eastern Europe, particularly around the [[Baltic Sea]]. Blue eyes are also found in [[Southern Europe]], [[Central Asia]], [[South Asia]], [[North Africa]], and [[West Asia]].<ref name="Cavalli-Sforza, L. L. 1994">{{cite book |last1=Cavalli-Sforza |first1=Luigi Luca |last2=Cavalli-Sforza |first2=Luca |last3=Menozzi |first3=Paolo |last4=Piazza |first4=Alberto |title=The History and Geography of Human Genes |url=https://archive.org/details/historygeography0000cava_g9l7 |url-access=registration |date=1994 |publisher=Princeton University Press |isbn=978-0-691-08750-4 }}{{page needed|date=May 2020}}</ref><ref name="altervista1">{{cite web|url=http://carnby.altervista.org/troe/08-05.htm |title=Distribution of Bodily Characters. Pigmentation, the Pilous System, and Morphology of the Soft Parts |archive-url=https://web.archive.org/web/20110726095519/http://carnby.altervista.org/troe/08-05.htm |archive-date=26 July 2011 }}</ref>
<!---
<gallery widths="200" heights="160px">
File:Bimbam.jpg|A [[Birman|Birman kitten]] with distinctive [[sapphire]] blue eyes
File:Karasu2a.JPG|[[Jungle crow]]
File:FrankieakaLogan.jpg|The first blue-eyed [[koala]] known to be born in captivity<ref>[http://www.adelaidenow.com.au/worlds-only-blue-eyed-koala/story-e6frea6u-1111115302021 Blue eyed Koala]. Adelaidenow.com.au (11 January 2008). Retrieved on 2011-12-23.</ref>
File:Blue-eyed black lemur.jpg|[[Sclater's lemur]], also known as the blue-eyed black lemur
</gallery>
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Approximately 8% to 10% of the global population have blue eyes.<ref name=":0" /> A 2002 study found the prevalence of blue eye color among the [[White Americans|white population]] in the United States to be 33.8% for those born from 1936 through 1951, compared with 57.4% for those born from 1899 through 1905.<ref name="Grant" /> {{as of|2006}}, one out of every six Americans, or 16.6% of the total US population, has blue eyes,<ref>{{Cite news|last=Belkin|first=Douglas|date=2006-10-18|title=Blue eyes are increasingly rare in America - Americas - International Herald Tribune (Published 2006)|language=en-US|work=The New York Times|url=https://www.nytimes.com/2006/10/18/world/americas/18iht-web.1018eyes.3199975.html|access-date=2021-02-02|issn=0362-4331}}</ref> including 22.3% of whites. The incidence of blue eyes continues to decline among American children.<ref name="BostonGlobe">{{cite web |url=http://www.boston.com/yourlife/articles/2006/10/17/dont_it_make_my_blue_eyes_brown/ |title=Don't it make my blue eyes brown Americans are seeing a dramatic color change|work=The Boston Globe |date=17 October 2006 |first=Douglas |last=Belkin |archive-url=https://web.archive.org/web/20141022125010/http://www.boston.com/yourlife/articles/2006/10/17/dont_it_make_my_blue_eyes_brown/?page=2 |archive-date=22 October 2014}}</ref> Of [[Slovenes]], 56% have blue/green eyes.<ref>{{cite journal|title= Prediction of eye color in the Slovenian population using the IrisPlex SNPs| pmc=3760663 | pmid=23986280 | volume=54 | issue=4 | year=2013 | journal=Croat. Med. J. | pages=381–6 | last1 = Kastelic | first1 = V | last2 = Pośpiech | first2 = E | last3 = Draus-Barini | first3 = J | last4 = Branicki | first4 = W | last5 = Drobnič | first5 = K | doi=10.3325/cmj.2013.54.381}}</ref> In a series of 221 photographs of Spanish subjects, 16.3% of the subjects were determined to have blue-gray eyes.<ref name="pubmed">{{Cite journal |last1=Muiños Díaz |first1=Yerena |last2=Saornil |first2=Maria A. |last3=Almaraz |first3=Ana |last4=Muñoz-Moreno |first4=Mari F. |last5=García |first5=Ciro |last6=Sanz |first6=Ruperto |date=2009 |title=Iris color: validation of a new classification and distribution in a Spanish population-based sample |url=https://pubmed.ncbi.nlm.nih.gov/19551689/ |journal=European Journal of Ophthalmology |volume=19 |issue=4 |pages=686–689 |doi=10.1177/112067210901900427 |issn=1120-6721 |pmid=19551689|s2cid=40940828 }}</ref>