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| '''Eye color''' is a ] ] and is determined by two distinct factors: the ]ation of the ]'s ]<ref name=Wielgus>{{cite journal |author=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 }}</ref><ref name=Prota>{{cite journal |author=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 }}</ref> and the frequency-dependence of the ] of light by the ] medium in the ].<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=http://books.google.com/books?id=c2xyxwlm2UkC&pg=PA9 |isbn= 0-520-03699-9}}</ref> | |||
| In humans, the pigmentation of the iris varies from light brown to black, depending on the concentration of ] in the ] (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 journal |author=Huiqiong Wang, Stephen Lin, Xiaopei Liu, Sing Bing Kang |title=Separating Reflections in Human Iris Images for Illumination Estimation |journal=Tenth IEEE International Conference on Computer Vision |volume=2 |pages=1691–1698 |year=2005 |url=http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.87.418 | doi=10.1109/ICCV.2005.215 |isbn=0-7695-2334-X}}</ref> The appearance of blue, green, as well as hazel eyes results from the ] of light in the stroma, a phenomenon similar to that which accounts for the blueness of the sky. Neither blue nor green pigments are ever present in the human iris or ocular fluid.<ref name="Fox" /><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> Eye color is thus an instance of ] and varies depending 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 ]s, ]s, and ]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 ] variations in eye color.<ref name="Morris">Morris, PJ. Athro Limited website. Retrieved May 10, 2006.</ref> The genetics of eye color are complicated, and color is determined by multiple genes. So far, as many as 15 different genes have been associated with eye color inheritance. Some of the eye-color genes include ''OCA2'' and ''HERC2''.<ref name="nature.com">, ''Journal of Human Genetics'' January 2011. {{cite journal |title=Genotype-phenotype associations and human eye color |journal=] |volume=56 |issue=1 |pages=5–7 |year=2011 |doi=10.1038/jhg.2010.126 |pmid=20944644 |last1=White |first1=Désirée |last2=Rabago-Smith |first2=Montserrat}}</ref> The once-held view that blue eye color is a simple ] has been shown to be incorrect. The genetics of eye color are so complex that almost any parent-child combination of eye colors can occur.<ref>. Sciencedaily.com (2007-02-22). Retrieved on 2011-12-23.</ref><ref>{{cite web|author=October 19, 2011 |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=2003-10-29 |accessdate=2011-10-19}}</ref> However, ] gene polymorphism, close to proximal ] regulatory region, explains most human eye-color variation.<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. }}</ref> | |||
| == Genetic determination of eye color == | |||
| {{See also|Human genetic clustering}} | |||
| Eye colors can range from the most common color, brown, to the least common, green. Eye color is an inherited trait influenced by more than one ].<ref name=Sturm>{{cite journal |author=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}}</ref><ref name=Grant>{{cite journal |author=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 }}</ref> These genes are being sought using associations to small changes in the genes themselves and in neighboring genes. These changes are known as ] or SNPs. The actual number of genes that contribute to eye color is currently unknown, but there are a few likely candidates. A study in ] (2009) found that it was possible to predict the color of eyes with more than 90% accuracy for brown and blue, using just six SNPs.<ref>, ''New Scientist'' 14 March 2009. | |||
| {{cite journal |title=Eye color and the prediction of complex phenotypes from genotypes |journal=] |volume=19 |issue=5 |pages=R192–R193 |year=2009 |doi=10.1016/j.cub.2009.01.027 |pmid=19278628 |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. |last7=Kayser |first7=Manfred}}</ref> Finally, there is evidence that as much 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'', both localized in Chromosome 15.<ref name="nature.com"/> | |||
| The gene ''OCA2'' ({{OMIM3|203200}}), when in a variant form, causes the pink eye color and ] common in human ]. (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 ], ] counts, ] and ]. The polymorphisms may be in an ''OCA2'' ], 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. }}</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 }}</ref> and ''TYR''.<ref name=Sulem/> | |||
| Blue eyes with a brown spot, green eyes, and gray eyes are caused by an entirely different part of the genome. As Eiberg said: "The SNP rs12913832 is found to be associated with the brown and blue eye color, but this single DNA variation cannot explain all the brown eye color variation from dark brown over hazel to blue eyes with brown spots." {{citation}} | |||
| == Classification of color == | |||
| Iris color can provide a large amount of information about a person, and a classification of various colors may be useful in documenting pathological changes or determining how a person may respond to various ocular pharmaceuticals.<ref name=German>{{cite journal |author=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> Various 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. Technical report 1495, University of Wisconsin–Madison, Dec, 2003.</ref> | |||
| As the ] 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>. Edromanguitars.com. Retrieved on 2011-12-23.</ref><!--a better citation or link may be available for this assertion--> | |||
| 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=] |last=Seddon |first=J.M. |coauthors=CR Sahagian, RJ Glynn, RD Sperduto and ES Gragoudas |volume=31 |pages=1592–8 |publisher=Association for Research in Vision and Ophthalmology |date=1 August 1990|url=http://www.iovs.org/content/31/8/1592.abstract |accessdate=2011-10-19 |pmid=2201662 |issue=8 }}</ref> There are three pigment colors that determine, depending on their proportion, the outward appearance of the iris: brown, yellow, and blue. Green irides, for example, have blue and some yellow. Brown irides contain mostly brown. Eye color in animals other than '']'' is differently regulated. For example, instead of blue as in humans, ] ] eye color in the skink species '']'' is black, and the autosomal ] color is yellow-green.<ref>{{cite journal|author=Jones, S.L., Schnirel, B.L.|title=Subspecies comparison of the Genus: Corucia|journal=Polyphemos|year=2006|volume =4|pages= 1–25|url=http://web.archive.org/web/20090202023446/http://www.biology-online.org/biology-forum/about7587.html|issue=1}}</ref> | |||
| {{-}} | |||
| ==Changes in eye color== | |||
| Most babies who have European ancestry have light-colored eyes before the age of one. As the child develops, melanocytes (cells found within the iris of human eyes, as well as skin and hair follicles) slowly begin to produce ]. Because ] continually produce pigment, in theory eye color can be changed. Most eye changes happen when the infant is around one year old, although it can happen up to three years of age.<ref>, retrieved 1 June 2009.</ref> Observing the iris of an infant from the side using only transmitted light with no reflection from the back of the iris, it is possible to detect the presence or absence of low levels of melanin. An iris that appears blue under this method of observation is more likely to remain blue as the infant ages. An iris that appears golden contains some melanin even at this early age and is likely to turn green or brown as the infant ages. | |||
| Changes (lightening or darkening) of eye colors during puberty, early childhood, pregnancy, and sometimes after serious trauma (like ]) do represent cause for plausible argument to state that some eyes can or do change, based on chemical reactions and hormonal changes within the body. | |||
| Studies on Caucasian twins, both fraternal and identical, have shown that eye color over time can be subject to change, and major demelanization of the iris may also be genetically determined. Most eye-color changes have been observed or reported in the Caucasian population with hazel eyes.<ref>{{cite journal|pmid=9152135|title=Eye Color Changes Past Early Childhood|year=1997|last1=Bito|first1=LZ|last2=Matheny|first2=A|last3=Cruickshanks|first3=KJ|last4=Nondahl|first4=DM|last5=Carino|first5=OB|volume=115|issue=5|pages=659–63|journal=Archives of ophthalmology}}</ref> | |||
| == Eye color chart (Martin–Schultz scale) == | |||
| ] created this chart by the ] often used in ]. | |||
| <ol style="list-style-type:upper-roman"> | |||
| <li>Light eyes</li> | |||
| Eyes light and light mixed are 16–12 in Martin scale. | |||
| ;Light: 15: Blue, Gray only. 16: Blue exclusive; 16-1a: pure light blue. | |||
| ;Light-mixed: 14–12 in Martin scale. | |||
| <ol style="list-style-type:lower-latin"> | |||
| <li>Very light-mixed (blue with gray or green or green with gray)</li> | |||
| <li>Light-mixed (light or very light-mixed with small admixture of brown pigment)</li> | |||
| </ol> | |||
| <li>Mixed eyes</li> | |||
| ;Mixed: 12–6 in Martin scale. Mixture of light eyes (blue, gray or green) with brown pigment when light and brown pigment are the same level. | |||
| <li>Dark eyes</li> | |||
| ;Dark-mixed: 6–4 in Martin scale. Brown with small admixture of light pigment. | |||
| ;Dark: 4–1 in Martin scale. Brown (light brown and dark brown) and very dark brown (almost black). | |||
| </ol> | |||
| {{-}} | |||
| ===Amber=== | |||
| ] | |||
| Amber eyes are of a solid color and have a strong yellowish/golden and russet/coppery tint. This might be due to the deposition of the yellow pigment called ] in the iris (which is also found in green eyes).<ref>. Hhmi.org. Retrieved on 2011-12-23.</ref><ref>Larry Bickford . Eyecarecontacts.com. Retrieved on 2011-12-23.</ref> Amber eyes should not be confused with hazel eyes; although hazel eyes may contain specks of amber or gold, they usually tend to comprise 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 considered to be like gold, some people have russet or copper colored amber eyes which many people mistake 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 ]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 ] are thought to be due to the presence of the pteridine pigment ] within certain ]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 }}</ref> In humans, yellowish specks or patches are thought to be due to the pigment ], also known as lipochrome.<ref name=Lefohn/> Many animals such as canines, domestic cats, owls, eagles, pigeons and fish have amber eyes as a common color, whereas in humans this color occurs less frequently. | |||
| ===Blue=== | |||
| ] | |||
| There is no blue pigmentation either in the iris or in the ocular fluid. Dissection reveals that the iris pigment epithelium is brownish black due to the presence of ].<ref name=Menon>{{cite journal |author=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 ] 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 ] in the ] medium of the stroma.<ref name="Wang"/> This is the same frequency-dependence of scattering that accounts for the blue appearance of the sky.<ref name="Fox">Denis Llewellyn Fox, , (Berkeley: University of California Press, 1979), p. 9.</ref><ref name="Mason" /> The result is a "] blue" ] that varies with external lighting conditions. | |||
| -martyna | |||
| ====Distribution==== | |||
| ] | |||
| Blue eyes are most common in the ] area and ],<ref>{{cite news| url=http://www.nytimes.com/2006/10/18/world/americas/18iht-web.1018eyes.3199975.html?_r=1 | work=New York Times | title=Blue eyes are increasingly rare in America – Americas – International Herald Tribune | date=2006-10-18 | accessdate=2012-03-07 | first1=Douglas | last1=Belkin}}</ref> and are also found in ], ], and ]. Blue eyes are also found in parts of the ], most notably in ], ], ] and ],<ref name="altervista1"></ref><ref name="toqonline5">{{cite journal|title=In Quest of Our Linguistic Ancestors: The Elusive Origins of the Indo-Europeans|journal=The Occidental Quarterly|year= 2002|first=John V.|last=Day|volume=2|issue=3|pages=5–20|url=http://www.toqonline.com/archives/v2n3/TOQv2n3Day.pdf |format=PDF|accessdate=2010-05-08 }}</ref><ref name="worldisround1">{{cite web|author=Showcase IRAN (by various photographers) |url=http://www.worldisround.com/articles/73022/photo653.html |title=Amazing IRAN – A small peasant girl – Khorasan, Iran – Worldisround photo |publisher=Worldisround.com |accessdate=2011-10-19}}</ref> as well as in the ], especially amongst the Jewish population of ]. Many modern Israeli Jews are of European ] origin, among whom this trait is common (53.7% of ] have blue eyes<ref>{{cite book|author=Maurice Fishberg|title=Jews, race & environment|url=http://books.google.com/books?id=CkRV_HaCAxEC|accessdate=23 December 2011|year=1911|publisher=Transaction Publishers|isbn=978-1-4128-0574-2}}</ref>). | |||
| Y-Chromosome DNA testing performed on ancient Scythian skeletons dating to the ] and ]s in the ] ] region found that all but one of 11 subjects carried Y-DNA ], with blue or green eye color and light hair common, suggesting mostly European origin of that particular population.<ref>{{cite journal | |||
| |title=Ancient DNA provides new insights into the history of south Siberian Kurgan people | |||
| |publisher=Human Genetics | |||
| |year= 2009|doi=10.1007/s00439-009-0683-0 | |||
| |last=Keyser | |||
| |first=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 | |||
| |journal=Human Genetics | |||
| |volume=126 | |||
| |issue=3 | |||
| |pages=395–410 | |||
| |pmid=19449030 | |||
| }}</ref> | |||
| In ], 99% of people have blue eyes, stated Hans Eiberg from the Department of Cellular and Molecular Medicine at the ].<ref name="More than">Weise, Elizabeth. (2008-02-05) . Usatoday.com. Retrieved on 2011-12-23.</ref> In ] 30 years ago, only 8% of the population had brown eyes, though through immigration, today that number is about 11%. In ], about 75% have blue eyes.<ref name="More than"/> | |||
| A 2002 study found that the prevalence of blue eye color among Caucasians in the United States to be 33.8 percent for those born from 1936 through 1951 compared with 57.4 percent for those born from 1899 through 1905.<ref name=Grant/> Blue eyes are continuing to become less common among American children, with only one out of every six or 16.6% of the total population, and 22.3% of the ] (including Hispanics) having blue eyes.<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=October 17, 2006|author=Douglas Belkin }}</ref><ref>{{cite news|last=Belkin |first=Douglas |url=http://www.nytimes.com/2006/10/18/world/americas/18iht-web.1018eyes.3199975.html |title=Blue eyes are increasingly rare in America – Americas|work=International Herald Tribune – The New York Times|date=2006-10-18 |accessdate=2011-10-19}}</ref> | |||
| Worldwide, only 150 million people (some 2.2%) have blue eyes.<ref name="People of blue eyes descended from a single ancestor some 6000 years ago">{{cite web|url =http://www.elmundo.es/elmundo/2008/02/01/ciencia/1201889800.html |title =People of blue eyes descended from a single ancestor some 6000 years ago |language =Spanish|date=2 February 2002 }}</ref> | |||
| ====Genetics==== | |||
| In humans, the inheritance pattern followed by blue eyes is considered similar to that of a ] trait (in general, eye color inheritance is considered a ], meaning that it is controlled by the interactions of several genes, not just one).<ref name=Grant/> In 2008, new research suggested that people with blue eyes have a single common ancestor. Scientists tracked down a genetic mutation that leads to blue eyes. "Originally, we all had brown eyes," said Eiberg.<ref name="msnbc.msn.com">{{cite news|first=Jeanna|last=Bryner|title= Genetic mutation makes those brown eyes blue|publisher=]|date=2008-01-31|accessdate=2009-10-19|url=http://www.msnbc.msn.com/id/22934464/}}</ref> Eiberg and colleagues showed in a study published in ''Human Genetics'' that a mutation in the 86th ] of the '']'' gene, which is hypothesized to interact with the ''OCA2'' gene ], reduced expression of ''OCA2'' with subsequent reduction in melanin production.<ref name="blue mutation">{{cite journal |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 |year=2008 |pmid=18172690 |doi=10.1007/s00439-007-0460-x |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 }}</ref> The authors concluded that the mutation may have arisen in a single individual probably living in the northwestern part of the ] region (around modern ]) 6,000–10,000 years ago during the ].<ref name=msnbc.msn.com/><ref name="blue mutation"/><ref name="telegraph.co.uk">{{cite news| url=http://www.telegraph.co.uk/science/science-news/3323607/Blue-eyes-result-of-ancient-genetic-mutation.html | location=London | work=The Daily Telegraph | title=Blue eyes result of ancient genetic 'mutation' | first=Roger | last=Highfield | date=2008-01-30|accessdate=2011-10-19}}</ref> Eiberg stated, "A genetic mutation affecting the OCA2 gene in our chromosomes resulted in the creation of a 'switch,' which literally 'turned off' the ability to produce brown eyes."<blockquote>The genetic switch is located in the gene adjacent to OCA2 and rather than completely turning off the gene, the switch limits its action, which reduces the production of melanin in the iris. In effect, the turned-down switch diluted brown eyes to blue. If the OCA2 gene had been completely shut down, our hair, eyes and skin would be melanin-less, a condition known as ].<ref name=msnbc.msn.com/></blockquote> | |||
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| Blue eyes are rare in mammals; one example is the quite recently-discovered ] (''Spilocuscus wilsoni''). The trait is hitherto known only from a single ] other than humans – ] (''Eulemur flavifrons'') of Madagascar. While some ]s and ]s have blue eyes, this is usually due to another mutation which is ]. But in cats alone, there are three known mutations which produce blue eyes – the one found in ]s does not have adverse health effects, while the others – that of blue-eyed white cats and the ] breed – are associated with ] ]s. | |||
| ===Brown=== | |||
| {{Redirect|Brown eyes}} | |||
| ], ], ].]] | |||
| ], ] and among the ].]] | |||
| Dark brown eyes are dominant in humans<ref>{{cite journal |author=Eiberg H, Mohr J |title=Assignment of genes coding for brown eye colour (BEY2) and brown hair colour (HCL3) on chromosome 15q |journal=Eur. J. Hum. Genet. |volume=4 |issue=4 |pages=237–41 |year=1996 |pmid=8875191 }}</ref> and 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> Dark pigment of brown eyes is most common in ], ], ], ], ], ], ], etc. as well as parts of ] and ].<ref name="Sulem"/><ref>http://en.wikipedia.org/File:Westernparadigm_blue_eye_color_map.jpg</ref> | |||
| Dark brown is also found in the ] seen in a fair percentage among ] and ] populations in the ]. | |||
| Light or medium-pigmented brown eyes are common in ], ], ] and ], as well as some parts of the ]. Light-pigmented brown eyes are sometimes referred to as "honey eyes". | |||
| In humans, 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" /> | |||
| ===Gray=== | |||
| ] | |||
| ] | |||
| 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 the appearance of gray and blue eyes is that gray eyes have larger deposits of ] in the stroma, so that the light that is reflected from the epithelium undergoes ] (which is not strongly frequency-dependent) rather than ] (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 off 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 |accessdate=2011-10-19}}</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 are most common in Northern and Eastern Europe.<ref name="Herbert Risley 1999">Herbert Risley, William Crooke, The People of India, (1999)</ref> Gray eyes can also be found in parts of North West Africa (]) among the ]n ]<ref>{{fr}} , 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> and in the ], most notably in ], ] (the ] ]), and ] (the ], another Indo-Iranian people).<ref name="altervista1"/><ref name="toqonline5"/><ref name="worldisround1"/> Under magnification, gray eyes exhibit small amounts of yellow and brown color in the iris. | |||
| A gray iris may indicate the presence of a ]. However, other visual signs make a uveitis obvious. Gray iris color, as well as blue, are at increased risk of ].<ref name=Stang/> | |||
| ===Green=== | |||
| ] | |||
| ] | |||
| Green is the least common eye color and as in the case of blue eyes, the color of green eyes does not result simply from the pigmentation of the iris. Rather, their appearance is caused by the combination of an amber or light brown pigmentation of the stroma, given by a low or moderate concentration of melanin, with the blue tone imparted by the Rayleigh scattering of the reflected light.<ref name="Fox" /> | |||
| Green eyes probably result from the interaction of multiple variants within the '']'' and other genes. They were present in south ] during the ].<ref name=bronzeage>{{cite journal|title=Ancient DNA provides new insights into the history of south Siberian Kurgan people|doi=10.1007/s00439-009-0683-0|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.|year=2009|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|journal=Human Genetics|volume=126|issue=3|pages=395–410|pmid=19449030}}</ref> They are most common in ] and ].<ref>. Eyedoctorguide.com. Retrieved on 2011-12-23.</ref><ref>. Cogweb.ucla.edu. Retrieved on 2011-12-23.</ref> They can also be found in ]<ref name="Herbert Risley 1999"/> and parts of ] (], northern ] and northwestern ]), ] and ]. In ], 89% of women and 87% of men have either blue or green eye color.<ref name=Rafnsson>{{cite journal |author=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> A study of Icelandic and Dutch adults found green eyes to be much more prevalent in women than in men. {{Citation needed|date=March 2012}} Among ], green eyes are most common among those of recent ] and ] ancestry, about 16%.<ref>{{cite web |url=http://www.gnxp.com/blog/2008/12/nlsy-blogging-eye-and-hair-color-of.php |title=Gene Expression: NLSY blogging: Eye and hair color of Americans }}</ref> | |||
| ===Hazel=== | |||
| Hazel eyes are due to a combination of ] and a moderate amount of melanin in the iris' anterior border layer.<ref name=Wang/><ref name=Lefohn>{{cite journal |author=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 }}</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 how many people mistake hazel eyes to be amber 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. |last6=Gillespie |first6=Nathan A. |last7=Ewen |first7=Kelly R. |last8=Jewell |first8=Mary |last9=Liew |first9=Yew Wah }}</ref><ref>{{cite journal|title=Iris melanocyte numbers in Asian, African American, and Caucasian irides |journal=Trans Am Ophthalmol Soc |volume=101 |pages=217–21; discussion 221–2 |year=2003 |pmid=14971580 |pmc=1358991 |url=http://www.aosonline.org/xactions/2003/1545-6110_v101_p217.pdf|last1=Albert|first1=DM|last2=Green|first2=WR|last3=Zimbric|first3=ML|last4=Lo|first4=C|last5=Gangnon|first5=RE|last6=Hope|first6=KL|last7=Gleiser|first7=J}}</ref><ref name=Mitchell>{{cite journal |author=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}}</ref><ref>{{cite journal |author=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 }}</ref><ref name=Frank>{{cite journal |author=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 |author=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 }}</ref><ref>{{cite journal |author=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 }}</ref> This 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 ] shell.<ref name=Zhu /><ref name=Mitchell /><ref name=Regan /><ref name=Hammond>{{cite journal |author=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> | |||
| ===Red and violet=== | |||
| ] | |||
| The eyes of people with severe forms of ] may appear red under certain lighting conditions owing to the extremely low quantities of ],<ref name=autogenerated1>. Albinism.org. Retrieved on 2011-12-23.</ref> allowing the blood vessels to show through. In addition, ] can sometimes cause a "]", in which the very bright light from a flash reflects off the back of the eyeball, 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=] |date=2009-01-16 |url=http://pcworld.co.nz/pcworld/pcw.nsf/digi/D193D7DD8DD78628CC257540000F4A4F |accessdate=2010-01-09 }} | |||
| </ref> | |||
| Although the deep blue eyes of some people such as ] can appear 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 | title=Elizabeth Taylor: Beautiful Mutant | accessdate=March 26, 2011 | author=Palmer, Roxanne | date=2005-03-25 | work=Slate}}</ref> | |||
| ==Spectrum of eye color== | |||
| <gallery> | |||
| File:Human Iris JD052007.jpg | |||
| <!-- RESTORE THIS IF IMAGE IS NOT PRESENT IN ANOTHER PART OF THE ARTICLE: File:Blueye.JPG --> | |||
| <!-- RESTORE THIS IF IMAGE IS NOT PRESENT IN ANOTHER PART OF THE ARTICLE: File:Mybluishgrayeye.JPG --> | |||
| File:Eye Central Heterochromia crop and lighter.jpg | |||
| File:eye55.jpg | |||
| File:Humaniris.jpg | |||
| File:Greeneye.jpg | |||
| File:Green Eye.jpg | |||
| File:Amber Brown Green.jpg | |||
| File:Eye big.JPG | |||
| File:Menschliches auge.jpg | |||
| File:MyStrangeIris.JPG | |||
| File:Indianeye.png | |||
| File:Lens5.jpg | |||
| File:Blue-Green-Grey eye.JPG | |||
| </gallery> | |||
| ==Medical implications== | |||
| Those with lighter iris color have been found to have a higher prevalence of ] (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 |last7=McCarty |first7=Catherine A}}</ref> An increased risk of ] has been found in those with blue, green or gray iris color.<ref name=Stang>{{cite journal |author=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 }}</ref><ref name=Rafnsson/> However, a study in 2000 suggests that people with dark brown eyes are at increased risk of developing cataracts and therefore should protect their eyes from direct exposure to sunlight.<ref name=Cumming>{{cite journal |author=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> | |||
| Eye color may also be symptomatic of disease. Aside from the iris, yellowing of the ] of the eyes is associated with ] and symptomatic of liver disease, including ], ] and ]. Yellowing of the whites of the eyes in people with darker pigmented skin is often due to melanin being present in the whites of the eyes. However, any sudden changes in the color of the whites of the eyes should be addressed by a medical professional. | |||
| ===Wilson's disease=== | |||
| ] in a patient with ]]] | |||
| ] involves a mutation of the gene coding for the enzyme ]7B, which prevents ] within the liver from entering the ] 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 ]s, which are dark rings that encircle the periphery of the iris.<ref name=McDonnell>{{cite journal |author=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}}</ref> | |||
| ==Anomalous conditions== | |||
| ===Aniridia=== | |||
| {{Main|Aniridia}} | |||
| ] is a congenital condition characterized by an extremely underdeveloped iris, which appears absent on superficial examination.<ref>{{EMedicine|oph|43|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 ], 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. ] defects can almost always be observed during an ] 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 ] is much more pronounced in albino individuals, and this can emphasize the ] in photographs. | |||
| ===Heterochromia=== | |||
| ] | |||
| ] | |||
| {{Main|Heterochromia}} | |||
| Heterochromia (also known as a ''heterochromia iridis'' or ''heterochromia iridum'') is an ocular condition in which one iris is a different color from the other iris (complete heterochromia), or where the 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 ] within an iris or part of an iris, which may be ] or acquired by ] or ].<ref name=Imesch>{{cite journal |author=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 ] content. A number of causes are responsible, including genetic, such as ], ] and ]. | |||
| ] | |||
| A ] can have two different colored eyes just like any two siblings can—because each cell has different eye color genes. A ] 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, ] has the appearance of different eye colors due to an injury that caused one pupil to be permanently dilated. Another idea about how this can happen is if an early viral infection while in the womb turns an eye color gene on or off in just one eye. Occasionally it can be a sign of a serious disease. | |||
| A common cause in females with heterochromia is ], which can result in a number of heterochromatic traits, such as ]. Trauma and certain medications, such as some ]s, can also cause increased pigmentation in one eye.<ref>{{cite journal |author=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, the condition of having two different-colored eyes is caused by blood staining the iris after sustaining injury. | |||
| ==See also== | |||
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| ==References== | |||
| {{Reflist|35em}} | |||
| ==External links== | |||
| {{Commons category|Eyes by color}} | |||
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