View source for Eye color

{{short description|Polygenic phenotypic characteristic}}
{{redirect|Iris color|the shade of purple|Iris (color)}}
{{pp|small=yes}}
{{Use dmy dates|date=December 2021}}
[[File:Farbverlauf Augenfarben.jpg|thumb|upright=1.35|The irises of human eyes exhibit a wide spectrum of colours]]
'''Eye color''' is a [[polygene|polygenic]] [[phenotypic trait]] determined by two factors: the [[pigment]]ation of the [[eye]]'s [[Iris (anatomy)|iris]]<ref name=Wielgus>{{cite journal |vauthors=Wielgus AR, Sarna T|title=Melanin in human irides of different color and age of donors |journal=Pigment Cell Res. |volume=18 |issue=6|pages=454–64 |year=2005 |pmid=16280011 |doi=10.1111/j.1600-0749.2005.00268.x  |issn = 0893-5785}}</ref><ref name=Prota>{{cite journal |vauthors=Prota G, Hu DN, Vincensi MR, McCormick SA, Napolitano A|title=Characterization of melanins in human irides and cultured uveal melanocytes from eyes of different colors |journal=Exp. Eye Res.|volume=67|issue=3 |pages=293–9 |year=1998|pmid=9778410 |doi=10.1006/exer.1998.0518 |doi-access=free }}</ref> and the frequency-dependence of the [[scattering]] of light by the [[Turbidity|turbid]] medium in the [[Stroma of iris|stroma of the iris]].<ref name="Fox1978">{{cite book |last= Fox |first= Denis Llewellyn |title=Biochromy: Natural Coloration of Living Things |publisher= University of California Press |year= 1979 |url=https://books.google.com/books?id=c2xyxwlm2UkC&pg=PA9|isbn= 978-0-520-03699-4}}</ref>{{rp|9}}

In humans, the pigmentation of the [[Iris (anatomy)|iris]] varies from light brown to black, depending on the concentration of [[melanin]] in the [[iris pigment epithelium]] (located on the back of the iris), the melanin content within the iris stroma (located at the front of the iris), and the cellular density of the stroma.<ref name="Wang">{{cite book |doi=10.1109/ICCV.2005.215 |chapter=Separating reflections in human iris images for illumination estimation |title=Tenth IEEE International Conference on Computer Vision (ICCV'05) Volume 1 |year=2005 |last1=Huiqiong Wang |last2=Lin |first2=S. |last3=Xiaopei Liu |last4=Sing Bing Kang|author4-link=Sing Bing Kang |pages=1691–1698 Vol. 2 |isbn=978-0-7695-2334-7 |citeseerx=10.1.1.87.418 |s2cid=2215768 }}</ref> The appearance of blue, green, and hazel eyes results from the [[Tyndall effect|Tyndall scattering]] of light in the stroma, a phenomenon similar to [[Rayleigh scattering]] which accounts for the blue sky.<ref>Sturm R.A. & Larsson M., Genetics of human iris colour and patterns, Pigment Cell Melanoma Res, 22:544-562, 2009.</ref> Neither blue nor green pigments are present in the human iris or [[vitreous humour]].<ref name="Fox1978"/><ref name="Mason">{{cite journal |last=Mason |first=Clyde W. |title=Blue Eyes |journal= Journal of Physical Chemistry | volume=28 |issue=5 |pages=498–501 |year=1924 | doi=10.1021/j150239a007}}</ref> This is an example of [[structural color]], which depends on the lighting conditions, especially for lighter-colored eyes.

The brightly colored eyes of many bird species result from the presence of other pigments, such as [[pteridine]]s, [[purine]]s, and [[carotenoid]]s.<ref name="Oliphant 1">{{cite journal |author=Oliphant LW |title=Pteridines and purines as major pigments of the avian iris |journal=Pigment Cell Res. |volume=1 |issue=2 |pages=129–31 |year=1987 |pmid=3507666 |doi=10.1111/j.1600-0749.1987.tb00401.x }}</ref> Humans and other animals have many [[phenotypic]] variations in eye color.<ref name="Morris">Morris, PJ. [http://www.athro.com/evo/gen/eyecols.html "Phenotypes and Genotypes for human eye colors."] Athro Limited website. Retrieved 10 May 2006.</ref>

The genetics and inheritance of eye color in humans is complicated. {{as of|2010}}, as many as 16 genes have been associated with eye color inheritance. Some of the eye-color genes include ''[[OCA2]]'' and ''[[HERC2]]''.<ref name="Kayser" /><ref name=pmid20944644>{{cite journal |last1=White |first1=Désirée |last2=Rabago-Smith |first2=Montserrat |title=Genotype–phenotype associations and human eye color |journal=Journal of Human Genetics |date=14 October 2010 |volume=56 |issue=1 |pages=5–7 |doi=10.1038/jhg.2010.126 |pmid=20944644 |doi-access=free }}</ref> The earlier belief that blue eye color is a [[recessive trait]] has been shown to be incorrect, and the genetics of eye color are so complex that almost any parent-child combination of eye colors can occur.<ref>[https://www.sciencedaily.com/releases/2007/02/070222180729.htm No Single Gene For Eye Color, Researchers Prove]. Sciencedaily.com (22 February 2007). Retrieved on 2011-12-23.</ref><ref>{{cite web |url=http://www.medterms.com/script/main/art.asp?articlekey=25003 |title=Eye color definition – Medical Dictionary definitions of popular medical terms easily defined on MedTerms |publisher=Medterms.com |date=29 October 2003 |access-date=19 October 2011 |archive-date=5 June 2011 |archive-url=https://web.archive.org/web/20110605231509/http://www.medterms.com/script/main/art.asp?articlekey=25003 |url-status=dead }}</ref><ref name=OCA2>{{cite journal |title=A three-single-nucleotide polymorphism haplotype in intron 1 of OCA2 explains most human eye-color variation |journal=Am. J. Hum. Genet. |volume=80 |issue=2 |pages=241–52 |year=2007|pmid=17236130 |pmc=1785344 |doi=10.1086/510885 |last1=Duffy |first1=David L. |last2=Montgomery |first2=Grant W. |last3=Chen |first3=Wei |last4=Zhao |first4=Zhen Zhen |last5=Le |first5=Lien |last6=James |first6=Michael R. |last7=Hayward |first7=Nicholas K. |last8=Martin |first8=Nicholas G. |last9=Sturm |first9=Richard A. <!--exactly 9 authors--> }}</ref> 

==Genetic determination==
Eye color is an inherited trait determined by multiple [[gene]]s.<ref name="Sturm">{{cite journal |vauthors=Sturm RA, Frudakis TN|title=Eye colour: portals into pigmentation genes and ancestry |journal=Trends Genet. |volume=20 |issue=8 |pages=327–32 |year=2004 |pmid=15262401 |doi=10.1016/j.tig.2004.06.010 |url=http://www.biosci.ohio-state.edu/~pfuerst/courses/eeobmg640/reading1eyecolor.pdf |archive-url=https://web.archive.org/web/20060909190624/http://www.biosci.ohio-state.edu/~pfuerst/courses/eeobmg640/reading1eyecolor.pdf |archive-date=9 September 2006 }}</ref><ref name="Grant">{{cite journal|vauthors=Grant MD, Lauderdale DS|title=Cohort effects in a genetically determined trait: eye colour among US whites |journal=Ann. Hum. Biol. |volume=29 |issue=6 |pages=657–66 |year=2002 |pmid=12573082 |doi=10.1080/03014460210157394 |s2cid=25364754 }}</ref> These genes are sought by studying small changes in the genes themselves and in neighboring genes, called [[single-nucleotide polymorphism]]s or SNPs. The total number of genes that contribute to eye color is unknown, but there are a few likely candidates. A study in [[Rotterdam]] (2009) found that it was possible to predict eye color with more than 90% accuracy for brown and blue using just six SNPs.<ref>[https://www.newscientist.com/article/mg20126992.100-dna-test-for-eye-colour-could-help-fight-crime.html "DNA test for eye colour could help fight crime"], ''[[New Scientist]]'' 14 March 2009.</ref><ref>{{cite journal |last1=Liu |first1=Fan |last2=Van Duijn |first2=Kate |last3=Vingerling |first3=Johannes R. |last4=Hofman |first4=Albert |last5=Uitterlinden |first5=André G. |last6=Janssens |first6=A. Cecile J.W. |author-link6=A. Cecile J.W. Janssens |last7=Kayser |first7=Manfred |year=2009 |title=Eye color and the prediction of complex phenotypes from genotypes |journal=[[Current Biology]] |volume=19 |issue=5 |pages=R192–R193 |doi=10.1016/j.cub.2009.01.027 |pmid=19278628 |doi-access=free|bibcode=2009CBio...19.R192L }}</ref>

In humans, eye color is a highly sexually dimorphic trait.<ref name="Martinez-Cadenas Peña-Chilet Ibarrola-Villava Ribas 2013 pp. 453–460">{{cite journal | last1=Martinez-Cadenas | first1=Conrado | last2=Peña-Chilet | first2=Maria | last3=Ibarrola-Villava | first3=Maider | last4=Ribas | first4=Gloria | title=Gender is a major factor explaining discrepancies in eye colour prediction based on HERC2/OCA2 genotype and the IrisPlex model | journal=Forensic Science International: Genetics | publisher=Elsevier BV | volume=7 | issue=4 | year=2013 | issn=1872-4973 | doi=10.1016/j.fsigen.2013.03.007 | pages=453–460| pmid=23601698 | hdl=10234/86709 | hdl-access=free }} "This effect is what may explain the fact that there seem to be comparatively higher frequencies of blue-eyed males than blue-eyed females in populations of European origin such as Iceland [10], Holland [10], Australia [18] or Poland [17], as well as in this study (see Fig. 2)."</ref> Several studies have shown that men are more likely to have blue eyes than women, while women are more likely to have darker eye colors (green and brown eyes) than men.<ref name="Martinez-Cadenas Peña-Chilet Ibarrola-Villava Ribas 2013 pp. 453–460" /><ref name="t967">{{cite book | last1=Pilli | first1=E. | last2=Berti | first2=A. | title=Forensic DNA Analysis: Technological Development and Innovative Applications | publisher=Apple Academic Press | year=2021 | isbn=978-1-000-06454-4 | url=https://books.google.com/books?id=A9MREAAAQBAJ&pg=PA207 | access-date=2024-05-22 | page=207}} "Several research groups have demonstrated that females have a darker eye color than males, given the same SNP profile (Martinez-Cadenas et al., 2013; Pietroni et al., 2014; Pospiech et al., 2016)."</ref> [[Sex]] is therefore a major factor in the expression of eye color genotypes.<ref name="Martinez-Cadenas Peña-Chilet Ibarrola-Villava Ribas 2013 pp. 453–460" /> One study suggested that women's higher levels of the sex hormone [[estrogen]] may explain why women tend to have darker eyes than men.<ref name="x873">{{cite journal | last1=Hernando | first1=Barbara | last2=Ibarrola-Villava | first2=Maider | last3=Fernandez | first3=Lara P. | last4=Peña-Chilet | first4=Maria | last5=Llorca-Cardeñosa | first5=Marta | last6=Oltra | first6=Sara S. | last7=Alonso | first7=Santos | last8=Boyano | first8=Maria Dolores | last9=Martinez-Cadenas | first9=Conrado | last10=Ribas | first10=Gloria | title=Sex-specific genetic effects associated with pigmentation, sensitivity to sunlight, and melanoma in a population of Spanish origin | journal=Biology of Sex Differences | volume=7 | issue=1 | date=2016 | page=17 | issn=2042-6410 | doi=10.1186/s13293-016-0070-1 | doi-access=free | pmid=26998216 | pmc=4797181 | quote=The results of this study suggest that there are indeed sex-specific genetic effects in human pigmentation, with larger effects for darker pigmentation in females compared to males. A plausible cause might be the differentially expressed melanogenic genes in females due to higher oestrogen levels. These sex-specific genetic effects would help explain the presence of darker eye and skin pigmentation in females, as well as the well-known higher melanoma risk displayed by males.| hdl=10810/32358 | hdl-access=free }}</ref> 

People of European descent show the greatest variety in eye color of any population worldwide. Recent advances in [[ancient DNA]] technology have revealed some of the history of eye color in Europe. Through the analysis of ancient DNA, a 2020 study published in ''Experimental Dermatology'' suggested that the common gene for blue eye color likely originated in the [[Near East]] and arrived in Europe around 42,000 years ago, after the exodus [[Recent African origin of modern humans|out of Africa]].<ref name="Cberg">{{cite journal |last1=Hanel |first1=Andrea |last2=Carlberg |first2=Carsten |title=Skin colour and vitamin D: An update |journal=Experimental Dermatology |date=September 2020 |volume=29 |issue=9 |pages=864–875 |doi=10.1111/exd.14142 |pmid=32621306 |s2cid=220335539 |language=en |issn=0906-6705|doi-access=free }} "The genetic history of today's European populations is based on continuous migrations over the past 40 000 years. Homo sapiens arrived in Europe from Near East some 42 000 years ago.[48, 49] Like in their African origin, these humans had dark skin but due to variations of their OCA2 gene (causing iris depigmentation) many of them had blue eyes[48,50] (Figure 2, left)." Figure 2: "Phenotype information was retrieved from supplementary files of the according publications or additionally assessed following the instructions of HirisPlex-S, which is a forensic DNA phenotyping tool based on an array of different marker SNPs providing additional support to the SNPs of the genes SLC24A5, SLC45A2 and OCA2."</ref>

There is evidence that as many as 16 different genes could be responsible for eye color in humans; however, the main two genes associated with eye color variation are ''[[OCA2]]'' and ''[[HERC2]]'', and both are localized in [[chromosome 15]].<ref name=pmid20944644/>

The gene ''OCA2'' ({{OMIM|203200||short}}), when in a variant form, causes the pink eye color and [[hypopigmentation]] common in human [[Albinism in humans|albinism]]. (The name of the gene is derived from the disorder it causes, oculocutaneous albinism type II.) Different SNPs within ''OCA2'' are strongly associated with blue and green eyes as well as variations in [[freckling]], [[Mole (skin marking)|mole]] counts, [[hair color|hair]] and [[Human skin color|skin tone]]. The polymorphisms may be in an ''OCA2'' [[regulatory sequence]], where they may influence the expression of the gene product, which in turn affects pigmentation.<ref name="OCA2" /> A specific mutation within the ''HERC2'' gene, a gene that regulates ''OCA2'' expression, is partly responsible for blue eyes.<ref name="Kayser">{{cite journal |title=Three genome-wide association studies and a linkage analysis identify HERC2 as a human iris color gene |journal=Am. J. Hum. Genet. |volume=82 |issue=2 |pages=411–23 |year=2008 |pmid=18252221 |pmc=2427174 |doi=10.1016/j.ajhg.2007.10.003 |last1=Kayser |first1=Manfred |last2=Liu |first2=Fan |last3=Janssens |first3=A. Cecile J.W. |last4=Rivadeneira |first4=Fernando |last5=Lao |first5=Oscar |last6=Van Duijn |first6=Kate |last7=Vermeulen |first7=Mark |last8=Arp |first8=Pascal |last9=Jhamai |first9=Mila M. |last10=Van Ijcken |first10=Wilfred F.J. |last11=Den Dunnen |first11=Johan T. |last12=Heath |first12=Simon |last13=Zelenika |first13=Diana |last14=Despriet |first14=Dominiek D.G. |last15=Klaver |first15=Caroline C.W. |last16=Vingerling |first16=Johannes R. |last17=De Jong |first17=Paulus T.V.M. |last18=Hofman |first18=Albert |last19=Aulchenko |first19=Yurii S. |last20=Uitterlinden |first20=Andre G. |last21=Oostra |first21=Ben A. |last22=Van Duijn |first22=Cornelia M. |display-authors=8 }}</ref> Other genes implicated in eye color variation are [[SLC24A4]]<ref name="Sulem">{{cite journal |title=Genetic determinants of hair, eye and skin pigmentation in Europeans |journal=Nat. Genet. |volume=39 |issue=12 |pages=1443–52 |year=2007 |pmid=17952075 |doi=10.1038/ng.2007.13 |last1=Sulem |first1=Patrick |last2=Gudbjartsson |first2=Daniel F |last3=Stacey |first3=Simon N |last4=Helgason |first4=Agnar |last5=Rafnar |first5=Thorunn |last6=Magnusson |first6=Kristinn P |last7=Manolescu |first7=Andrei |last8=Karason |first8=Ari |last9=Palsson |first9=Arnar |last10=Thorleifsson |first10=Gudmar |last11=Jakobsdottir |first11=Margret |last12=Steinberg |first12=Stacy |last13=Pálsson |first13=Snæbjörn |last14=Jonasson |first14=Fridbert |last15=Sigurgeirsson |first15=Bardur |last16=Thorisdottir |first16=Kristin |last17=Ragnarsson |first17=Rafn |last18=Benediktsdottir |first18=Kristrun R |last19=Aben |first19=Katja K |last20=Kiemeney |first20=Lambertus A |last21=Olafsson |first21=Jon H |last22=Gulcher |first22=Jeffrey |last23=Kong |first23=Augie |last24=Thorsteinsdottir |first24=Unnur |last25=Stefansson |first25=Kari |s2cid=19313549 |display-authors=8 }}</ref> and [[Tyrosinase|TYR]].<ref name="Sulem" /> A 2010 study of eye color variation in hue and saturation values using high-resolution digital full-eye photographs found three new loci for a total of ten genes, allowing the explanation of about 50% of eye color variation.<ref>{{Cite journal |doi=10.1371/journal.pgen.1000934|pmid = 20463881|pmc = 2865509|title = Digital Quantification of Human Eye Color Highlights Genetic Association of Three New Loci|journal = PLOS Genetics|volume = 6|issue = 5|pages = e1000934|year = 2010|last1 = Liu|first1 = Fan|last2 = Wollstein|first2 = Andreas|last3 = Hysi|first3 = Pirro G.|last4 = Ankra-Badu|first4 = Georgina A.|last5 = Spector|first5 = Timothy D.|last6 = Park|first6 = Daniel|last7 = Zhu|first7 = Gu|last8 = Larsson|first8 = Mats|last9 = Duffy|first9 = David L.|last10 = Montgomery|first10 = Grant W.|last11 = MacKey|first11 = David A.|last12 = Walsh|first12 = Susan|last13 = Lao|first13 = Oscar|last14 = Hofman|first14 = Albert|last15 = Rivadeneira|first15 = Fernando|last16 = Vingerling|first16 = Johannes R.|last17 = Uitterlinden|first17 = André G.|last18 = Martin|first18 = Nicholas G.|last19 = Hammond|first19 = Christopher J.|last20 = Kayser|first20 = Manfred | doi-access=free }}</ref>
 
{| class="wikitable"
|-
! Gene name
! Effect on eye color
|-
| [[OCA2]]
| Associated with [[melanin]] producing cells. Central importance to eye color.
|-
| [[HERC2]]
| Affects function of OCA2, with a specific mutation strongly linked to blue eyes.
|-
|[[SLC24A4]]
| Associated with differences between blue and green eyes.<ref name="Sulem" />
|-
|[[Tyrosinase|TYR]]
| Associated with differences between blue and green eyes.<ref name="Sulem" />
|}

Blue eyes with a brown spot, green eyes, and gray eyes are caused by an entirely different part of the genome.

==Changes in eye color==
A 1997 study of [[White American]]s found that eye color may be subject to change in infancy, and from adolescence to adulthood.<ref>{{cite journal|pmid=9152135|title=Eye Color Changes Past Early Childhood|year=1997|last1=Bito|first1=LZ|volume=115|issue=5|pages=659–63|journal=Archives of Ophthalmology|doi=10.1001/archopht.1997.01100150661017}}</ref> Seventeen percent of children experienced a change of eye color by adulthood. Of those children, 50% of developed lighter eyes as they got older. The other 50% developed darker eyes.{{sfn|Bito|1997|p=660}}  

Generally, children with hazel and light brown eyes tended to experience a lightening of their eye color by adulthood.{{sfn|Bito|1997|p=660}} Children with green eyes often experienced a darkening of their eye color.{{sfn|Bito|1997|p=660}}  It was also found that 11% of the children's [[mothers]] experienced an eye color change during the same period, with most developing lighter eyes, relative to their original color at the time of their child's birth.{{sfn|Bito|1997|p=661-662}}

==Eye color range==
As is the case with [[hair color]] and [[skin color]], the melanin that is usually produced in the bodies of almost all mammals is either [[eumelanin]] or [[pheomelanin]], resulting in a high proportion of black, brown, red, and yellow coloring in the biological features that make use of it. As such, most mammals generally have a predominantly orange-based color palette, which generally varies from light orange colors like [[Peach (color)|peach]] to dark orange colors like [[brown]]; sometimes even more varied colors like [[maroon]] are exhibited.

===Brown===
{{Redirect|Brown eyes}}
[[File:MyStrangeIris.JPG|thumb|A brown iris]]
Almost all mammals have brown or darkly-pigmented irises.<ref>Brenda J Bradley, Anja Pedersen, Nicholas I Mundy: [https://pubmed.ncbi.nlm.nih.gov/19278018/ Blue eyes in lemurs and humans: same phenotype, different genetic mechanism]  Am J Phys Anthropol. 2009</ref> In humans, brown is by far the most common eye color, with approximately 79% of people in the world having it.<ref name=":0">{{Cite web|last=Debrowski|first=Adam|title=Which Eye Colors Are the Rarest?|url=https://www.allaboutvision.com/eye-anatomy/rarest-eye-color/|access-date=2021-02-04|website=All About Vision|language=en-us}}</ref> Brown eyes result from a relatively high concentration of melanin in the stroma of the iris, which causes light of both shorter and longer wavelengths to be absorbed.<ref name="Fox">{{cite book |last=Fox |first=Denis Llewellyn |title=Biochromy: Natural Coloration of Living Things |publisher=University of California Press |year=1979 |page=9 |url=https://books.google.com/books?id=c2xyxwlm2UkC&pg=PA9|isbn=978-0-520-03699-4}}</ref>

[[File:Human_eye,_anterior_view.jpg|right|thumb|A light brown iris with limbal ring]]

In many parts of the world, it is nearly the only iris color present.<ref>{{OMIM|227220|SKIN/HAIR/EYE PIGMENTATION, VARIATION IN, 1; SHEP1}}</ref> Brown eyes are common in [[Europe]], [[East Asia]], [[Southeast Asia]], [[Central Asia]], [[South Asia]], [[West Asia]], [[Oceania]], [[West Africa]] and the [[Americas]].<ref name="Sulem"/> Light or medium-pigmented brown eyes can also be commonly found in [[Europe]], among the [[Americas]], and parts of [[Central Asia]], [[West Asia]], [[South Asia]], and [[East Africa]]. Light brown eyes bordering amber and hazel coloration are more common in [[Europe]], but can also be observed in [[East Asia]], [[Southeast Asia]], [[North Africa]] and [[East Africa]].
<!--- [[File:225 of 'The Races of Great Britain. A contribution to the anthropology of Western Europe. (With plates and maps.)' (11176801375).jpg|thumb|189x240px|Percentage of dark eyes among British military recruits in the 19th century by region]] --->
===Amber===
[[File:Ambereye.jpg|thumb|Amber eye]]
Amber eyes are a solid color with a strong yellowish/golden or russet/coppery tint, which may be due to a yellow pigment called [[lipochrome]]  (also found in green eyes).<ref>[http://www.hhmi.org/cgi-bin/askascientist/highlight.pl?kw=&file=answers%2Fgenetics%2Fans_044.html Howard Hughes Medical Institute: Ask A Scientist] {{webarchive|url=https://web.archive.org/web/20100901150309/http://www.hhmi.org/cgi-bin/askascientist/highlight.pl?kw=&file=answers%2Fgenetics%2Fans_044.html |date=1 September 2010 }}. Hhmi.org. Retrieved on 23 December 2011.</ref><ref>Larry Bickford [http://www.eyecarecontacts.com/eyecolor.html Eye Color] {{webarchive|url=https://web.archive.org/web/20101023191830/http://www.eyecarecontacts.com/eyecolor.html |date=23 October 2010 }}. Eyecarecontacts.com. Retrieved on 23 December 2011.</ref> Amber eyes should not be confused with hazel eyes. Although hazel eyes may contain specks of amber or gold, they usually tend to have many other colors, including green, brown, and orange. Also, hazel eyes may appear to shift in color and consist of flecks and ripples, while amber eyes are of a solid gold hue. Even though amber is similar to gold, some people have russet- or copper-colored amber eyes that are mistaken for hazel, though hazel tends to be duller and contains green with red/gold flecks, as mentioned above. Amber eyes may also contain amounts of very light gold-ish gray.

The eyes of some pigeons contain yellow fluorescing pigments known as [[pteridine]]s.<ref>{{cite journal |author=Oliphant LW |title=Observations on the pigmentation of the pigeon iris |journal=Pigment Cell Res. |volume=1 |issue=3 |pages=202–8 |year=1987 |pmid=3508278 |doi=10.1111/j.1600-0749.1987.tb00414.x }}</ref> The bright yellow eyes of the [[great horned owl]] are thought to be due to the presence of the pteridine pigment [[xanthopterin]] within certain [[chromatophore]]s (called xanthophores) located in the iris stroma.<ref>{{cite journal |author=Oliphant LW |title=Crystalline pteridines in the stromal pigment cells of the iris of the great horned owl |journal=Cell Tissue Res. |volume=217 |issue=2 |pages=387–95 |year=1981 |pmid=7237534 |doi=10.1007/BF00233588 |s2cid=8061493 }}</ref> In humans, yellowish specks or patches are thought to be due to the pigment [[lipofuscin]], also known as lipochrome.<ref name=Lefohn/> Many animals such as canines, domestic cats, owls, eagles, pigeons, and fish have amber eyes, whereas in humans this color occurs less frequently. Amber is the third-rarest natural eye color after green and gray, occurring in 5% of the world's population.<ref>{{Cite web|title=Amber eyes|url=https://www.allaboutvision.com/eye-care/eye-anatomy/amber-eyes/|access-date=2021-05-09|website=All About Vision|language=en-us}}</ref> People with amber-colored eyes are found in [[Europe]], and in fewer numbers in the [[Middle East]], [[North Africa]], and [[South America]].<ref>{{Cite journal |last1=Ruiz-Linares |first1=Andrés |last2=Adhikari |first2=Kaustubh |last3=Acuña-Alonzo |first3=Victor |last4=Quinto-Sanchez |first4=Mirsha |last5=Jaramillo |first5=Claudia |last6=Arias |first6=William |last7=Fuentes |first7=Macarena |last8=Pizarro |first8=María |last9=Everardo |first9=Paola |last10=de Avila |first10=Francisco |last11=Gómez-Valdés |first11=Jorge |last12=León-Mimila |first12=Paola |last13=Hunemeier |first13=Tábita |last14=Ramallo |first14=Virginia |last15=Silva de Cerqueira |first15=Caio C. |date=2014-09-25 |title=Admixture in Latin America: Geographic Structure, Phenotypic Diversity and Self-Perception of Ancestry Based on 7,342 Individuals |journal=PLOS Genetics |volume=10 |issue=9 |pages=e1004572 |doi=10.1371/journal.pgen.1004572 |issn=1553-7390 |pmc=4177621 |pmid=25254375 |doi-access=free |bibcode=2014PLOSG..10.4572R }}</ref>

===Hazel===
[[File:Extreme close-up shot of the eye of Cuban news anchor Armando Ramirez Diaz.jpg|thumb|Hazel eye]]
[[File:Hazel Eye HD.JPG|thumb|Hazel eye]]
The hazel color of eyes is caused by a combination of [[Rayleigh scattering]] and a moderate amount of melanin in the iris' anterior border layer.<ref name=Wang/><ref name=Lefohn>{{cite journal |vauthors=Lefohn A, Budge B, Shirley P, Caruso R, Reinhard E |title=An Ocularist's Approach to Human Iris Synthesis |journal=IEEE Comput. Graph. Appl. |volume=23 |issue=6 |pages=70–5 |year=2003 |doi=10.1109/MCG.2003.1242384 |s2cid=537404 |url=https://escholarship.org/uc/item/8d88d3qc }}</ref> Hazel eyes often appear to shift in color from a brown to a green. Although hazel mostly consists of brown and green, the dominant color in the eye can either be brown/gold or green. This is why hazel eyes can be mistaken as amber, and why amber is often counted as hazel in studies, and vice versa.<ref name=Zhu>{{cite journal |title=A genome scan for eye color in 502 twin families: most variation is due to a QTL on chromosome 15q |journal=Twin Res |volume=7 |issue=2 |pages=197–210 |year=2004 |pmid=15169604 |doi=10.1375/136905204323016186 |last1=Zhu |first1=Gu |last2=Evans |first2=David M. |last3=Duffy |first3=David L. |last4=Montgomery |first4=Grant W. |last5=Medland |first5=Sarah E.|author-link5=Sarah Medland |last6=Gillespie |first6=Nathan A. |last7=Ewen |first7=Kelly R. |last8=Jewell |first8=Mary |last9=Liew |first9=Yew Wah |first10=Nicholas K. |last10=Hayward |first11=Richard A. |last11=Sturma |first12=Jeffrey M. |last12=Trenta |first13=Nicholas G. |last13=Martina|doi-access=free }}</ref><ref>{{cite journal |last1=Albert |first1=Daniel M |last2=Green |first2=W Richard |last3=Zimbric |first3=Michele L |last4=Lo |first4=Cecilia |last5=Gangnon |first5=Ronald E |last6=Hope |first6=Kirsten L |last7=Gleiser |first7=Joel |title=Iris melanocyte numbers in Asian, African American, and Caucasian irides. |journal=Transactions of the American Ophthalmological Society |date=2003 |volume=101 |pages=217–222 |pmid=14971580 |pmc=1358991 }}</ref><ref name=Mitchell>{{cite journal |vauthors=Mitchell R, Rochtchina E, Lee A, Wang JJ, Mitchell P |title=Iris color and intraocular pressure: the Blue Mountains Eye Study |journal=Am. J. Ophthalmol. |volume=135 |issue=3 |pages=384–6 |year=2003 |pmid=12614760 |doi=10.1016/S0002-9394(02)01967-0 |url=https://ur.booksc.eu/dl/17418414/f57a1f }}</ref><ref>{{cite journal |vauthors=Lindsey JD, Jones HL, Hewitt EG, Angert M, Weinreb RN |title=Induction of tyrosinase gene transcription in human iris organ cultures exposed to latanoprost |journal=Arch. Ophthalmol. |volume=119 |issue=6 |pages=853–60 |year=2001|pmid=11405836 |doi=10.1001/archopht.119.6.853|doi-access=free }}</ref><ref name=Frank>{{cite journal |vauthors=Frank RN, Puklin JE, Stock C, Canter LA |title=Race, iris color, and age-related macular degeneration |journal=Trans Am Ophthalmol Soc |volume=98 |pages=109–15; discussion 115–7 |year=2000 |pmid=11190014 |pmc=1298217 }}</ref><ref name=Regan>{{cite journal |vauthors=Regan S, Judge HE, Gragoudas ES, Egan KM |title=Iris color as a prognostic factor in ocular melanoma |journal=Arch. Ophthalmol. |volume=117 |issue=6 |pages=811–4 |year=1999 |pmid=10369595 |doi=10.1001/archopht.117.6.811|doi-access=free }}</ref><ref>{{cite journal |vauthors=Hawkins TA, Stewart WC, McMillan TA, Gwynn DR |title=Analysis of diode, argon, and Nd: YAG peripheral iridectomy in cadaver eyes |journal=Doc Ophthalmol |volume=87 |issue=4 |pages=367–76 |year=1994 |pmid=7851220 |doi=10.1007/BF01203345 |s2cid=30893783 }}</ref> The combination can sometimes produce a multicolored iris, i.e. an eye that is light brown/amber near the pupil and charcoal or dark green on the outer part of the iris (or vice versa) when observed in sunlight.

Definitions of the eye color "hazel" vary: it is sometimes considered to be synonymous with light brown or gold, as in the color of a [[hazelnut]] shell.<ref name=Zhu /><ref name=Mitchell /><ref name=Regan /><ref name=Hammond>{{cite journal |vauthors=Hammond BR, Fuld K, Snodderly DM |title=Iris color and macular pigment optical density |journal=Exp. Eye Res. |volume=62 |issue=3 |pages=293–7 |year=1996 |pmid=8690039 |doi=10.1006/exer.1996.0035}}</ref>

Around 18% of the US population and 5% of the world population have hazel eyes.<ref name=":0" /> 55.2% of Spanish subjects in a series of 221 photographs were judged to have hazel eyes.<ref name="pubmed"/> Hazel eyes are found in [[Europe]], most commonly in the [[Netherlands]] and the [[United Kingdom]],<ref>{{cite web | url=https://www.coalitionbrewing.com/where-are-hazel-eyes-most-common/ | title=Where are hazel eyes most common? | date=26 February 2023 }}</ref> and have also been observed to be very common among the [[Low Saxon]]-speaking populations of northern Germany.<ref>{{cite book |last1=Beddoe |first1=John |title=The Races of Britain. A contribution to the anthropology of Western Europe |date=1971 |publisher=Michigan State University |isbn=9780091013707 |page=39 |url=https://books.google.com/books?id=7z06AQAAMAAJ |language=en |quote=the Saxons...being very often hazel-eyed}}</ref>

===Green===
{{misleading|section|date=August 2023}}
[[File:Greeneyes.jpg|thumb|Green eyes]]
Green eyes probably result from the interaction of multiple allelic variants of ''[[OCA2]]'' and other genes. They may have been present in southern [[Siberia]] during the [[Bronze Age]].<ref name=bronzeage>{{cite journal |last1=Keyser |first1=Christine |last2=Bouakaze |first2=Caroline |last3=Crubézy |first3=Eric |last4=Nikolaev |first4=Valery G. |last5=Montagnon |first5=Daniel |last6=Reis |first6=Tatiana |last7=Ludes |first7=Bertrand |year=2009 |title=Ancient DNA Provides New Insights into the History of South Siberian Kurgan People |journal=Human Genetics |volume=126 |issue=3 |pages=395–410 |doi=10.1007/s00439-009-0683-0 |pmid=19449030 |s2cid=21347353 |quote=Indeed, among the SNPs tested was rs12913832, a single DNA variation within a regulatory element of HERC2 gene which is associated to blue eye color in humans. This polymorphism, together with the diplotypes obtained from variations of the OCA2 locus (major contributor to the human eye color variation) showed that at least 60% of the ancient Siberian specimens under study had blue (or green) eyes.}}</ref>

Green eyes are most common in [[Northern Europe|Northern]], [[Western Europe|Western]], and [[Central Europe]].<ref>[http://www.eyedoctorguide.com/eye_general/eye_color.html Blue Eyes Versus Brown Eyes: A Primer on Eye Color] {{Webarchive|url=https://web.archive.org/web/20081017164118/http://www.eyedoctorguide.com/eye_general/eye_color.html |date=17 October 2008 }}. Eyedoctorguide.com. Retrieved on 23 December 2011.</ref><ref>[http://cogweb.ucla.edu/ep/Frost_06.html Why Do Europeans Have So Many Hair and Eye Colors?]. Cogweb.ucla.edu. Retrieved on 23 December 2011.</ref> Around 8–10% of men and 18–21% of women in [[Iceland]] and 6% of men and 17% of women in the [[Netherlands]] have green eyes.<ref name="sulem">{{cite journal |last1=Sulem|first1=P.|last2= Gudbjartsson|first2=D. |display-authors=et al.|title=Genetic determinants of hair, eye and skin pigmentation in Europeans |journal=Nature Genetics |year=2007 |volume=39 |issue=12 |pages=1443–1452 |doi=10.1038/ng.2007.13 |pmid=17952075 |s2cid=19313549 |url=https://www.researchgate.net/publication/5893814 |access-date=21 February 2022}}</ref> Among [[European Americans]], green eyes are most common among those of recent [[Celts (modern)|Celtic]] and [[Germanic peoples|Germanic]] ancestry, occurring in about 16% of people with those backgrounds.<ref name=sulem/>

The green color is caused by the combination of: 1) an amber or light brown pigmentation in the stroma of the iris (which has a low or moderate concentration of melanin), and 2) a blue shade created by the Rayleigh scattering of reflected light.<ref name="Fox"/> Green eyes contain the yellowish pigment [[lipochrome]].<ref>[http://allaboutgenes.weebly.com/oca2.html "OCA2: The Gene for Color"] {{Webarchive|url=https://web.archive.org/web/20161006151409/http://allaboutgenes.weebly.com/oca2.html |date=6 October 2016 }}. allaboutgenes.weebly.com. Retrieved on 8 September 2016.</ref>
<!---[[File:Chinchilla Persian cat with sea-green eyes.jpg|thumb|A [[Chinchilla Persian]] with sea-green eyes]]--->

===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>
--->
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>

===Gray===
[[File:Gray eye, Northern Italy.jpg|thumb|Gray eye]]
Like blue eyes, gray eyes have a dark epithelium at the back of the iris and a relatively clear stroma at the front. One possible explanation for the difference in appearance between gray and blue eyes is that gray eyes have larger deposits of [[collagen]] in the stroma, so that the light that is reflected from the epithelium undergoes [[Mie scattering]] (which is not strongly frequency-dependent) rather than Rayleigh scattering (in which shorter wavelengths of light are scattered more). This would be analogous to the change in the color of the sky, from the blue given by the Rayleigh scattering of sunlight by small gas molecules when the sky is clear, to the gray caused by Mie scattering of large water droplets when the sky is cloudy.<ref name="Southworth">{{cite web |url=http://www.thetech.org/genetics/ask.php?id=232 |title=Are gray eyes the same as blue in terms of genetics? |work=Understanding Genetics: Human Health and the Genome |publisher=Stanford School of Medicine |author=Lucy Southworth |access-date=19 October 2011 |archive-url=https://web.archive.org/web/20110927085904/http://www.thetech.org/genetics/ask.php?id=232 |archive-date=27 September 2011 }}</ref> Alternatively, it has been suggested that gray and blue eyes might differ in the concentration of melanin at the front of the stroma.<ref name="Southworth" />

Gray eyes can also be found among the [[Algeria]]n [[Shawia people]]<ref>{{in lang|fr}}  [https://books.google.com/books?id=7rwBAAAAYAAJ&pg=PA272 Provincia: bulletin trimestriel de la Société de Statistique ...], Volumes 16–17 By Société de statistique, d'histoire et d'archéologie de Marseille et de Provence p. 273 ''l'iris gris est celui des chaouias...''</ref> of the [[Aurès Mountains]] in Northwest Africa, in the [[Middle East]]/[[West Asia]], [[Central Asia]], and [[South Asia]]. In the ''[[Iliad]]'', the Greek goddess [[Athene]] is said to have gray eyes (γλαυκῶπις).<ref>''[[Iliad]]'' 1:206 http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.01.0133%3Abook%3D1%3Acard%3D206</ref> Under magnification, gray eyes exhibit small amounts of yellow and brown color in the iris.

Gray is the second-rarest natural eye color after green, with 3% of the world's population having it.<ref>{{Cite web|title=Gray eyes|url=https://www.allaboutvision.com/eye-care/eye-anatomy/gray-eyes/|access-date=2021-05-09|website=All About Vision|language=en-us}}</ref>

=== Special cases ===
==== Two different colors ====
[[File:Heterochromia_Iridis.jpg|thumb|Two different eye colors are known as [[heterochromia iridum]]]]
As a result of [[heterochromia iridum]], it is also possible to have two different eye colors. This occurs in humans and certain breeds of domesticated animals and affects less than 1 percent of the world's population.<ref name=":3" />

====Red and violet====
[[File:OCA1 Auge.jpg|thumb|Red-appearing albino eyes]]
The eyes of people with severe forms of [[albinism in humans|albinism]] may appear red under certain lighting conditions owing to the extremely low quantities of [[melanin]],<ref name="albinism">[http://www.albinism.org/publications/what_is_albinism.html NOAH – What is Albinism?] {{webarchive|url=https://web.archive.org/web/20120514003907/http://albinism.org/publications/what_is_albinism.html |date=14 May 2012 }}. Albinism.org. Retrieved on 23 December 2011.</ref> allowing the blood vessels to show through. In addition, [[flash photography]] can sometimes cause a "[[red-eye effect]]", in which the very bright light from a flash reflects off the retina, which is abundantly vascular, causing the pupil to appear red in the photograph.<ref>
{{cite news |title=HOW TO: Avoid the red eye effect |author=Dave Johnson |newspaper=[[PC World (magazine)#Countries|New Zealand PC World]] |date=16 January 2009 |url=http://pcworld.co.nz/pcworld/pcw.nsf/digi/D193D7DD8DD78628CC257540000F4A4F |access-date=9 January 2010 |archive-url=https://web.archive.org/web/20100224065704/http://pcworld.co.nz/pcworld/pcw.nsf/digi/D193D7DD8DD78628CC257540000F4A4F |archive-date=24 February 2010 }}
</ref>

Although the deep blue eyes of some people such as [[Elizabeth Taylor]] can appear purple or violet at certain times, "true" violet-colored eyes occur only due to albinism.<ref name="palmer20110325">{{cite web |url=http://www.slate.com/blogs/blogs/browbeat/archive/2011/03/25/elizabeth-taylor-beautiful-mutant.aspx |archive-url=https://web.archive.org/web/20110327113617/http://www.slate.com/blogs/blogs/browbeat/archive/2011/03/25/elizabeth-taylor-beautiful-mutant.aspx |archive-date=27 March 2011 |title=Elizabeth Taylor: Beautiful Mutant |access-date=26 March 2011 |author=Palmer, Roxanne |date=25 March 2005 |work=Slate}}</ref><ref name="Albinism">{{cite encyclopedia |last1=Fertl |first1=Dagmar |last2=Rosel |first2=Patricia |title=Albinism |encyclopedia=Encyclopedia of Marine Mammals |edition=Second |entry=Albinism |publisher=Academic Press |date=2009 |pages=24–26 |doi=10.1016/b978-0-12-373553-9.00006-7 |isbn=978-0-12-373553-9 |entry-url=https://www.sciencedirect.com/science/article/pii/B9780123735539000067 |access-date=11 October 2022}}</ref><ref>{{cite journal |last1=White |first1=Désirée |last2=Rabago-Smith |first2=Montserrat |title=Genotype–phenotype associations and human eye color |journal=Journal of Human Genetics |date=2010 |volume=56 |issue=1 |pages=5–7 |doi=10.1038/jhg.2010.126 |pmid=20944644 |s2cid=33808164 |doi-access=free }}</ref> Eyes that appear red or violet under certain conditions due to albinism occur in less than 1 percent of the world's population.<ref name=":3">{{Cite web|title=The World's Population By Eye Color|url=https://www.worldatlas.com/articles/which-eye-color-is-the-most-common-in-the-world.html|access-date=2021-05-10|website=WorldAtlas|date=6 October 2020|language=en-US}}</ref>

==Medical implications==
The most important role of melanin in the iris is to protect the eyes from the sun's harmful rays.<ref>{{cite journal|title=Melanins: Skin Pigments and Much More—Types, Structural Models, Biological Functions, and Formation Routes |year=2014 | publisher =hindawi.com |doi=10.1155/2014/498276 |doi-access=free |last1=Solano |first1=F. |journal=New Journal of Science |volume=2014 |pages=1–28 }} Eye color change and the affecting factors.</ref> People with lighter eye colors, such as blue or green, have lessened protection from the sun, and so need greater protection from the sun's rays than those with darker eye colors.{{citation needed|date=February 2022}}

Those with lighter iris color have been found to have a higher prevalence of [[macular degeneration|age-related macular degeneration]] (ARMD) than those with darker iris color;<ref name=Frank /> lighter eye color is also associated with an increased risk of ARMD progression.<ref name=Nicolas>{{cite journal |title=Iris colour, ethnic origin and progression of age-related macular degeneration |journal=Clin. Experiment. Ophthalmol. |volume=31 |issue=6 |pages=465–9 |year=2003|pmid=14641151 |doi=10.1046/j.1442-9071.2003.00711.x |last1=Nicolas |first1=Caroline M |last2=Robman |first2=Luba D |last3=Tikellis |first3=Gabriella |last4=Dimitrov |first4=Peter N |last5=Dowrick |first5=Adam |last6=Guymer |first6=Robyn H |author-link6=Robyn Guymer|last7=McCarty |first7=Catherine A|s2cid=25878963 }}</ref> A gray iris may indicate the presence of a [[uveitis]], and an increased risk of [[uveal melanoma]] has been found in those with blue, green or gray eyes.<ref name="Rafnsson">{{cite journal |vauthors=Rafnsson V, Hrafnkelsson J, Tulinius H, Sigurgeirsson B, Olafsson JH |title=Risk factors for malignant melanoma in an Icelandic population sample |journal=Prev Med |volume=39 |issue=2 |pages=247–52 |year=2004|pmid=15226032 |doi=10.1016/j.ypmed.2004.03.027 }}</ref><ref name=Stang>{{cite journal |vauthors=Stang A, Ahrens W, Anastassiou G, Jöckel KH|title=Phenotypical characteristics, lifestyle, social class and uveal melanoma |journal=Ophthalmic Epidemiol |volume=10 |issue=5 |pages=293–302 |year=2003 |pmid=14566630 |doi=10.1076/opep.10.5.293.17319 |s2cid=1592701 }}</ref> However, a study in 2000 suggests that people with dark brown eyes are at increased risk of developing [[cataract]]s and therefore should protect their eyes from direct exposure to sunlight.<ref name=Cumming>{{cite journal |vauthors=Cumming RG, Mitchell P, Lim R|title=Iris color and cataract: The Blue Mountains Eye Study |journal=American Journal of Ophthalmology |volume=130 |issue=2 |pages=237–238 |year=2000 |doi=10.1016/S0002-9394(00)00479-7 |pmid=11004303 }}</ref>

===Wilson's disease===
[[File:Kayser-Fleischer ring.jpg|thumb|right|A [[Kayser–Fleischer ring]] in a patient with [[Wilson's disease]]]]
[[Wilson's disease]] involves a mutation of the gene coding for the enzyme [[Wilson disease protein|ATPase 7B]], which prevents [[copper]] within the liver from entering the [[Golgi apparatus]] in cells. Instead, the copper accumulates in the liver and in other tissues, including the iris of the eye. This results in the formation of [[Kayser–Fleischer ring]]s, which are dark rings that encircle the periphery of the iris.<ref name=McDonnell>{{cite journal |vauthors=McDonnell G, Esmonde T|title=A homesick student |journal=Postgrad Med J |volume=75 |issue=884 |pages=375–8 |year=1999 |pmid=10435182|pmc=1741256 | doi = 10.1136/pgmj.75.884.375 }}</ref>

===Coloration of the sclera===
Eye color outside of the iris may also be symptomatic of disease. Yellowing of the [[sclera]] (the "whites of the eyes") is associated with [[jaundice]],<ref name="Sclera">{{cite book |doi=10.1007/978-1-4419-6502-8_8 |chapter=Noninflammatory Diseases of the Sclera |title=The Sclera |year=2012 |last1=de la Maza |first1=Maite Sainz |last2=Tauber |first2=Joseph |last3=Foster |first3=C. Stephen |pages=277–297 |isbn=978-1-4419-6501-1 }}</ref> and may be symptomatic of liver diseases such as [[cirrhosis]] or [[hepatitis]].<ref>{{cite book |doi=10.1017/CBO9780511545740.013 |chapter=Upper gastrointestinal surgery |title=Fundamentals of Surgical Practice |year=2006 |last1=Wheatley |first1=TJ |pages=230–248 |isbn=978-0-511-54574-0 |editor2-first=Aljafri A |editor2-last=Majid |editor1-first=Andrew N |editor1-last=Kingsnorth }}</ref> A blue coloration of the sclera may also be symptomatic of disease.<ref name="Sclera"/>

===Aniridia===
{{Main|Aniridia}}
[[Aniridia]] is a congenital condition characterized by an extremely underdeveloped iris, which appears absent on superficial examination.<ref>{{EMedicine|article|1208379|Aniridia}}</ref>

===Ocular albinism and eye color===
Normally, there is a thick layer of melanin on the back of the iris. Even people with the lightest blue eyes, with no melanin on the front of the iris at all, have dark brown coloration on the back of it, to prevent light from scattering around inside the eye. In those with milder forms of [[albinism]], the color of the iris is typically blue but can vary from blue to brown. In severe forms of albinism, there is no pigment on the back of the iris, and light from inside the eye can pass through the iris to the front. In these cases, the only color seen is the red from the hemoglobin of the blood in the capillaries of the iris. Such albinos have pink eyes, as do albino rabbits, mice, or any other animal with a total lack of melanin. [[Transillumination]] defects can almost always be observed during an [[eye examination]] due to lack of iridial pigmentation.<ref>{{EMedicine|OPH|260|Ocular Manifestations of Albinism}}</ref> The ocular albino also lacks normal amounts of melanin in the retina as well, which allows more light than normal to reflect off the retina and out of the eye. Because of this, the [[Pupillary light reflex|pupillary reflex]] is much more pronounced in albino individuals, and this can emphasize the [[red eye effect]] in photographs.

===Heterochromia===
{{Main|Heterochromia iridum}}
{{more citations needed section|date=November 2012}}
[[File:Heterochromia.jpg|thumb|An example of complete heterochromia. The subject has one brown eye and one hazel eye.]]
[[File:HeterochromiaEye.jpg|thumb|left|An example of sectoral heterochromia. The subject has a blue iris with a brown section.]]

Heterochromia (''[[heterochromia iridum]]'' or ''heterochromia iridis'') is an eye condition in which one iris is a different color from the other (complete heterochromia), or where a part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia). It is a result of the relative excess or lack of [[pigment]] within an iris or part of an iris, which may be [[Biological inheritance|inherited]] or acquired by [[disease]] or [[injury]].<ref name=Imesch>{{cite journal |vauthors=Imesch PD, Wallow IH, Albert DM|title=The color of the human eye: a review of morphologic correlates and of some conditions that affect iridial pigmentation |journal=Surv Ophthalmol |volume=41 |issue=Suppl 2 |pages=S117–23 |year=1997 |pmid=9154287 |doi=10.1016/S0039-6257(97)80018-5 }}</ref> This uncommon condition usually results due to uneven [[melanin]] content. A number of causes are responsible, including genetic, such as [[Chimera (genetics)|chimerism]], [[Horner's syndrome]] and [[Waardenburg syndrome]].

A [[Chimera (genetics)|chimera]] can have two different colored eyes just like any two siblings can—because each cell has different eye color genes. A [[Mosaic (genetics)|mosaic]] can have two different colored eyes if the DNA difference happens to be in an eye-color gene.

There are many other possible reasons for having two different-colored eyes. For example, the film actor [[Lee Van Cleef]] was born with one blue eye and one green eye, a trait that reportedly was common in his family, suggesting that it was a genetic trait. This anomaly, which film producers thought would be disturbing to film audiences, was "corrected" by having Van Cleef wear brown contact lenses.<ref>{{IMDb name|1812|section=bio|Lee Van Cleef}}</ref> [[David Bowie]], on the other hand, had the appearance of different eye colors due to an injury that caused one pupil to be permanently dilated.

Another hypothesis about heterochromia is that it can result from a viral infection ''in utero'' affecting the development of one eye, possibly through some sort of genetic mutation. Occasionally, heterochromia can be a sign of a serious medical condition.

A common cause in females with heterochromia is [[X-inactivation]], which can result in a number of heterochromatic traits, such as [[calico cats]]. Trauma and certain medications, such as some [[prostaglandin analogue]]s, can also cause increased pigmentation in one eye.<ref>{{cite journal |vauthors=Hejkal TW, Camras CB|title=Prostaglandin analogs in the treatment of glaucoma |journal=Seminars in Ophthalmology |volume=14 |issue=3 |pages=114–23 |year=1999 |pmid=10790575 |doi=10.3109/08820539909061464 }}</ref> On occasion, a difference in eye color is caused by blood staining the iris after injury.

== Limbal ring ==
{{Main|Limbal ring}}
[[File:Human eye close up, anterior view.jpg|thumb|Light brown iris with a dark limbal ring]]

The limbal ring is also a feature of the iris contributing to eye color, appearing as a darkened, occasionally black region encircling the iris resulting from a manifestation of the optical properties of the [[corneal limbus]]. Limbal rings are not present in all individuals, and their thickness and prominence may correlate with health or youthfulness, and contributes to [[Physical attractiveness|facial attractiveness]].<ref name=":02">{{cite journal |author=Darren Peshek |author2=Negar Semmaknejad |author3=Donald Hoffman |author4=Pete Foley |year=2011 |title=Preliminary evidence that the limbal ring influences facial attractiveness |url=https://escholarship.org/content/qt0964610k/qt0964610k.pdf?t=nfa9wu |journal=Evolutionary Psychology |volume=9 |issue=2 |pages=137–146 |doi=10.1177/147470491100900201 |pmid=22947961 |doi-access=free|pmc=10519137 }}</ref><ref name="pmid289782502">{{cite journal |author=Brown M, Sacco DF |year=2018 |title=Put a (Limbal) Ring on It: Women Perceive Men's Limbal Rings as a Health Cue in Short-Term Mating Domains. |journal=Pers Soc Psychol Bull |volume=44 |issue=1 |pages=80–91 |doi=10.1177/0146167217733072 |pmid=28978250 |doi-access=free}}</ref>

== Impact on vision ==
Although people with lighter eye color are generally more sensitive to light because they have less pigment in the iris to protect them from sunlight, there is little to no evidence that eye color has a direct impact on vision qualities such as [[visual acuity]].<ref name=":1">{{Cite news|last=O'Connor|first=Anahad|date=2009-01-19|title=The Claim: Eye Color Can Have an Effect on Vision|language=en-US|work=The New York Times|url=https://www.nytimes.com/2009/01/20/health/20real.html|access-date=2021-05-05|issn=0362-4331}}</ref> However, there is a study that found that dark-eyed people perform better at "reactive-type tasks", which suggests they may have better reaction times.<ref name=":2">{{Cite journal|last1=Miller|first1=L. K.|last2=Rowe|first2=P. J.|last3=Lund|first3=J.|date=August 1992|title=Correlation of eye color on self-paced and reactive motor performance |journal=Perceptual and Motor Skills|volume=75|issue=1|pages=91–95|doi=10.2466/pms.1992.75.1.91|issn=0031-5125|pmid=1528697|s2cid=31398375}}</ref> People with light-colored eyes, however, performed better at so-called "self-paced tasks", which include activities like hitting a golf ball or throwing baseballs.<ref name=":2" /> In another study, people with darker eyes performed better at hitting racquetballs.<ref>{{Cite journal|last1=Rowe|first1=P. J.|last2=Evans|first2=P.|date=August 1994|title=Ball color, eye color, and a reactive motor skill |journal=Perceptual and Motor Skills|volume=79|issue=1 Pt 2|pages=671–674|doi=10.2466/pms.1994.79.1.671|issn=0031-5125|pmid=7808908|s2cid=6743916}}</ref> There are also other studies that challenge these findings,<ref>{{Cite journal|last1=Crowe|first1=M.|last2=O'Connor|first2=D.|date=October 2001|title=Eye colour and reaction time to visual stimuli in rugby league players |journal=Perceptual and Motor Skills|volume=93|issue=2|pages=455–460|doi=10.2466/pms.2001.93.2.455|issn=0031-5125|pmid=11769902|s2cid=22785194}}</ref> and more study is needed to verify these results.<ref name=":1" />

==Classification of color==
Iris color can provide a large amount of information about a person, and a classification of colors may be useful in documenting pathological changes or determining how a person may respond to ocular pharmaceuticals.<ref name=German>{{cite journal |vauthors=German EJ, Hurst MA, Wood D, Gilchrist J|title=A novel system for the objective classification of iris colour and its correlation with response to 1% tropicamide |journal=Ophthalmic Physiol Opt |volume=18 |issue=2 |pages=103–10 |year=1998|pmid=9692029 |doi=10.1016/S0275-5408(97)00070-7}}</ref> Classification systems have ranged from a basic light or dark description to detailed gradings employing photographic standards for comparison.<ref name=German/> Others have attempted to set objective standards of color comparison.<ref name=Fan>Fan S, Dyer CR, Hubbard L. [http://www.cs.wisc.edu/~shaohua/research/iris/IrisColor_files/IrisTr1495.pdf Quantification and Correction of Iris Color."] Technical report 1495, University of Wisconsin–Madison, Dec 2003.</ref>

The [[Martin–Schultz scale]], developed from the [[Martin scale]], is one standard [[color scale]] used in [[physical anthropology]] to establish the eye color of an individual. It was created by the anthropologists [[Rudolf Martin (anthropologist)|Rudolf Martin]] and [[Bruno K Schultz]] in the first half of the 20th century. The scale consists of 20 colors<ref>{{Cite web|url=http://antropologia-fizyczna.pl/images/antropometria/somatoskopia/martin-schulz.jpg |title=Martin-Schulz Eye Color Chart |access-date=2017-01-13 |archive-url=https://web.archive.org/web/20160802080400/http://antropologia-fizyczna.pl/images/antropometria/somatoskopia/martin-schulz.jpg |archive-date=2016-08-02 }}</ref> ranging from light blue to dark brown-black, corresponding to natural eye colors caused by the amount of melanin in the iris:<ref>Piquet-Thepot M.-M. - Bulletins et Mémoires de la Société d'anthropologie de Paris, XII° Série, tome 3 fascicule 3, pg. 207,208 - (1968)</ref><ref>{{cite journal|url=http://www.persee.fr/web/revues/home/prescript/article/bmsap_0037-8984_1968_num_3_3_1417|title=Contribution à l'anthropologie des Corses: Anthropologie de la tête (suite)|first=M.-M.|last=Piquet-Thepot|date=26 March 1968|journal=Bulletins et Mémoires de la Société d'anthropologie de Paris|volume=3|issue=3|pages=183–218|doi=10.3406/bmsap.1968.1417}}</ref>

Normal eye colors range from the darkest shades of brown to the lightest tints of blue.<ref name=Sturm/> To meet the need for standardized classification, at once simple yet detailed enough for research purposes, Seddon et al. developed a graded system based on the predominant iris color and the amount of brown or yellow pigment present.<ref name=Seddon>{{cite journal |title=Evaluation of an iris color classification system |journal=[[Investigative Ophthalmology & Visual Science]] |last=Seddon |first=JM |author2=CR Sahagian |author3=RJ Glynn |author4=RD Sperduto |author5=ES Gragoudas |volume=31 |pages=1592–8 |date=1 August 1990|url=http://www.iovs.org/content/31/8/1592.abstract |access-date=19 October 2011 |pmid=2201662 |issue=8 }}</ref> There are three pigment colors that determine, depending on their proportion, the outward appearance of the iris, along with [[structural color]]. Green irises, for example, have some yellow and the blue structural color. Brown irises contain more or less melanin. Some eyes have a dark ring around the iris, called a [[limbal ring]].

Eye color in non-human animals is regulated differently. For example, instead of blue as in humans, [[Autosome|autosomal]] [[recessive]] eye color in the skink species ''[[Corucia zebrata]]'' is black, and the autosomal [[Dominance (genetics)|dominant]] color is yellow-green.<ref>{{cite journal|author1=Jones, S.L. |author2=Schnirel, B.L. |title=Subspecies comparison of the Genus: Corucia |journal=Polyphemos |year=2006 |volume=4 |pages=1–25 |url=http://www.biology-online.org/biology-forum/about7587.html |issue=1 |archive-url=https://web.archive.org/web/20090202023446/http://www.biology-online.org/biology-forum/about7587.html |archive-date=2 February 2009 }}</ref>{{Clear}}

As the [[color vision|perception of color]] depends on viewing conditions (e.g., the amount and kind of illumination, as well as the hue of the surrounding environment), so does the perception of eye color.<ref>[http://www.edromanguitars.com/tech/color.htm Color Perception] {{webarchive|url=https://web.archive.org/web/20061020182804/http://www.edromanguitars.com/tech/color.htm |date=20 October 2006 }}. Edromanguitars.com. Retrieved on 23 December 2011.</ref><!--A better citation or link may be available for this assertion.-->

==See also==
* [[Hair color]]
* [[Human skin color]]
* [[Iridology]]
* [[Xanthophore]]

==References==
{{Reflist}}

==External links==
{{Commons category|Eyes by color}}
* [https://www.thetech.org/ask-a-geneticist/articles/2011/ask424/ Ask a Geneticist: If both parents have blue eyes, how could they have a child with brown eyes?], [[The Tech Interactive]]
* [https://longevity-science.blogspot.com/2007/02/eye-color.html Eye Color and Human Diseases]
{{Authority control}}

{{DEFAULTSORT:Eye Color}}
[[Category:Eye color| ]]
[[Category:Eye]]
[[Category:Facial features]]
[[Category:Color]]