Eye color

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Eye color is a polygenic trait and is determined primarily by the amount and type of pigments in the eye's iris.<ref name="Wielgus">Wielgus AR, Sarna T. "Melanin in human irides of different color and age of donors." Pigment Cell Res. 2005 Dec;18(6):454-64. PMID 16280011.</ref><ref name="Prota">Prota G, Hu DN, Vincensi MR, McCormick SA, Napolitano A. "Characterization of melanins in human irides and cultured uveal melanocytes from eyes of different colors." Exp Eye Res. 1998 Sep;67(3):293-9. PMID 9778410.</ref> Humans and animals have many phenotypic variations in eye color.<ref name="Morris">Morris, PJ. "Phenotypes and Genotypes for human eye colors." Athro Limited website. Retrieved May 10, 2006.</ref> In humans, these variations in color are attributed to varying ratios of eumelanin produced by melanocytes in the iris.<ref name="Prota"/> The brightly colored eyes of many bird species are largely determined by other pigments, such as pteridines, purines, and carotenoids.<ref name="Oliphant 1">Oliphant LW. "Pteridines and purines as major pigments of the avian iris." Pigment Cell Res. 1987;1(2):129-31. PMID 3507666.</ref>

Three main elements within the iris contribute to its color: the melanin content of the iris pigment epithelium, the melanin content within the iris stroma, and the cellular density of the iris stroma.<ref name="Wang">Huiqiong Wang, Stephen Lin, Xiaopei Liu, Sing Bing Kang. "Separating Reflections in Human Iris Images for Illumination Estimation." Proc. IEEE International Conference on Computer Vision, 2005.</ref> In eyes of all colors, the iris pigment epithelium contains the black pigment, eumelanin.<ref name="Prota"/><ref name="Wang"/> Color variations among different irises are typically attributed to the melanin content within the iris stroma.<ref name="Wang"/> The density of cells within the stroma affects how much light is absorbed by the underlying pigment epithelium.<ref name="Wang"/>

Determination of eye color

Eye color is an inherited trait influenced by more than one gene.<ref name="Sturm">Sturm RA, Frudakis TN. "Eye color: portals into pigmentation genes and ancestry." Trends Genet. 2004 Aug;20(Image:Cool.gif:327-32. PMID: 15262401.</ref><ref name="Grant">Grant MD, Lauderdale DS. "Cohort effects in a genetically determined trait: eye color among US whites." Ann Hum Biol. 2002 Nov-Dec;29(6):657-66. PMID 12573082.</ref> There are two major genes and other minor ones that account for the tremendous variation of human eye color.<ref>Eye color mocks easy rules</ref> In humans, three genes associated with eye color are currently known: EYCL1, EYCL2, and EYCL3.<ref>Rebbeck TR, Kanetsky PA, Walker AH, Holmes R, Halpern AC, Schuchter LM, Elder DE, Guerry D. "P gene as an inherited biomarker of human eye color." Cancer Epidemiol Biomarkers Prev. 2002 Aug;11(Image:Cool.gif:782-4. PMID 12163334.</ref><ref>"Eye color is more complex than two genes." Athro, Limited. Retrieved September 1, 2006.</ref> These genes account for three phenotypic eye colors (brown, green, and blue) in humans.<ref name="Morris"/> Eye color usually stabilizes when an infant is around 6 months old.<ref>http://chemistry.about.com/cs/howthingswork/f/eyecolor.htm</ref>

In 2006, the molecular basis of the EYCL3 locus was resolved.<ref name=BBC>Paul Rincon, Genetics of eye color unlocked, BBC News, 20 December, 2006. </ref> In a study of 3839 people, researchers reported that 74% of total variation in eye color was explained by a number of single nucleotide polymorphisms (SNPs) near the OCA2 gene (Modèle:OMIM3). OCA2 was previously known because, when mutated, the gene can result in a type of albinism. The recent study showed that different SNPs strongly associate with blue and green eyes as well as variations in freckling, mole counts, hair and skin tone. The authors speculate that the SNPs may be in an OCA2 regulatory sequence and thus influence the expression of the gene product, which in turn affects pigmentation.<ref name=OCA2>Duffy DL, Montgomery GW, Chen W, Zhao ZZ, Le L, James MR, Hayward NK, Martin NG, and Sturm, RA "A Three–Single-Nucleotide Polymorphism Haplotype in Intron 1 of OCA2 Explains Most Human Eye-color Variation" Am. J. Hum. Genet. 2002 80:000, 2007.</ref>

Classification of colors

Iris color can provide a large amount of information about an individual, 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">German EJ, Hurst MA, Wood D, Gilchrist J. "A novel system for the objective classification of iris color and its correlation with response to 1% tropicamide." Ophthalmic Physiol Opt. 1998 Mar;18(2):103-10. PMID 9692029.</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. Quantification and Correction of Iris Color." Technical report 1495, University of Wisconsin-Madison, Dec, 2003.</ref>

As the perception of color is dependent on viewing conditions (e.g. the amount and type of illumination, as well as the hue of the surrounding environment), so is the perception of eye color.<ref>http://www.edromanguitars.com/tech/color.htm</ref>

Eye color exists on a continuum from the darkest shades of brown to the lightest shades of blue.<ref name="Sturm"/> Seeing the need for a standardized classification system that was simple, yet detailed enough for research purposes, Seddon et. al developed a graded one based on the predominant iris color (brown, light brown, green, gray, and blue) and the amount of brown or yellow pigment present. There are 3 true colors in the eyes that determine the outward appearance; brown, yellow, and gray. How much of each color you have determines the appearance of the eye color. The color the eyes in turn depends on how much of these colors are present. For example, green eyes have yellow and some brown, making them appear green. Blue eyes have a little yellow and little to no brown, making them appear blue. Gray eyes appear gray because they have a little yellow and no brown in them. Brown eyes appear brown because most of the eye contains the brown color. Brown is the most common, blue is second, and green is rarest.<ref name="Seddon">Seddon JM, Sahagian CR, Glynn RJ, Sperduto RD, Gragoudas ES. "Evaluation of an iris color classification system." The Eye Disorders Case-Control Study Group. Invest Ophthalmol Vis Sci. 1990 Aug;31(Image:Cool.gif:1592-8. PMID: 2201662.</ref> The above is true for the species Homo sapiens. The iris color can vary in the animal world. Instead of blue in humans, autosomal recessive color in the species Corucia zebrata is black. whereas, the autosomal dominant color is yellow - green {52} ( Subspecies comparison of the Genus: Corucia - Leeway Corucia Research Center - LCRC, Polyphemos, {2006} Volume 4, Issue 1. pp. 1-25.).

Brown

In humans, brown eyes contain large amounts of melanin (eumelanin) within the iris stroma which serves to absorb light, particularly at the shorter wavelengths.<ref name="Wang"/><ref name="Menon">Menon IA, Basu PK, Persad S, Avaria M, Felix CC, Kalyanaraman B. "Is there any difference in the photobiological properties of melanins isolated from human blue and brown eyes?" Br J Ophthalmol. 1987 Jul;71(7):549-52. PMID 2820463.</ref> Very dark brown irises may appear to be black.<ref name="Hammond">Hammond BR Jr, Fuld K, Snodderly DM. "Iris color and macular pigment optical density." Exp Eye Res. 1996 Mar;62(3):293-7. PMID 8690039.</ref><ref>Prieto JG. "Eye color in skin cancer." Int J Dermatol. 1977 Jun;16(5):406-7. PMID 873674.</ref>

Brown is very predominant<ref>Eiberg H, Mohr J. "Assignment of genes coding for brown eye colour (BEY2) and brown hair colour (HCL3) on chromosome 15q." Eur J Hum Genet. 1996;4(4):237-41. PMID 8875191.</ref> and, in many populations, it is (with few exceptions) the only iris color present.<ref>http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=227220</ref> It is least common in the countries around the Baltic Sea.

Those of mixed European and non-European ancestry generally have dark eyes, but with more variability in eye color within their families than those from more homogenous origins.<ref>Frudakis T, Thomas M, Gaskin Z, Venkateswarlu K, Chandra KS, Ginjupalli S, Gunturi S, Natrajan S, Ponnuswamy VK, Ponnuswamy KN. Sequences associated with human iris pigmentation." Genetics. 2003 Dec;165(4):2071-83. PMID 14704187.</ref> Inhabitants indigenous to Africa, Asia and the Americas generally have brown eyes. Brown eyes are equally found in Europe and Oceania, though within some European populations they are not predominant to the same extent.

Hazel

Hazel eyes are due to a combination of a Rayleigh scattering and a moderate amount of melanin in the iris' anterior border layer.<ref name="Lefohn">Lefohn, A., Budge, B., Shirley, P., Caruso, R., and Reinhard, E. 2003. An Ocularist's Approach to Human Iris Synthesis. IEEE Comput. Graph. Appl. 23, 6 (Nov. 2003), 70-75. DOI= http://dx.doi.org/10.1109/MCG.2003.1242384</ref><ref name="Wang"/> A number of studies using three-point scales have assigned "hazel" to be the medium-color between light brown and dark green.<ref name="Zhu">Zhu G, Evans DM, Duffy DL, Montgomery GW, Medland SE, Gillespie NA, Ewen KR, Jewell M, Liew YW, Hayward NK, Sturm RA, Trent JM, Martin NG. "A genome scan for eye color in 502 twin families: most variation is due to a QTL on chromosome 15q." 1: Twin Res. 2004 Apr;7(2):197-210. PMID 15169604.</ref><ref>Albert DM, Green WR, Zimbric ML, Lo C, Gangnon RE, Hope KL, Gleiser J. Hazel eyes often appear to shift in color from light brown to a medium or dark olive green. "Iris melanocyte numbers in Asian, African American, and Caucasian irides." Trans Am Ophthalmol Soc. 2003;101:217-21; discussion 221-2. PMID 14971580.</ref><ref name="Mitchell">Mitchell R, Rochtchina E, Lee A, Wang JJ, Mitchell P; Blue Mountains Eye Study. "Iris color and intraocular pressure: the Blue Mountains Eye Study." Am J Ophthalmol. 2003 Mar;135(3):384-6. PMID 12614760.</ref><ref>Lindsey JD, Jones HL, Hewitt EG, Angert M, Weinreb RN. "Induction of tyrosinase gene transcription in human iris organ cultures exposed to latanoprost." Arch Ophthalmol. 2001 Jun;119(6):853-60. PMID 11405836.</ref><ref name="Frank">Frank RN, Puklin JE, Stock C, Canter LA. "Race, iris color, and age-related macular degeneration." Trans Am Ophthalmol Soc. 2000;98:109-15; discussion 115-7. PMID 11190014.</ref><ref name="Regan">Regan S, Judge HE, Gragoudas ES, Egan KM. "Iris color as a prognostic factor in ocular melanoma." Arch Ophthalmol. 1999 Jun;117(6):811-4. PMID 10369595.</ref><ref>Hawkins TA, Stewart WC, McMillan TA, Gwynn DR. "Analysis of diode, argon, and Nd: YAG peripheral iridectomy in cadaver eyes." Doc Ophthalmol. 1994;87(4):367-76. PMID 7851220.</ref> This can sometimes produce a multicolored iris, i.e., an eye that is light brown near the pupil and charcoal or amber/dark green on the outer part of the iris when it is open to the elements of the sun/shined in the sunlight. Hazel is mostly found in some regions of the Americas and Europe and certain populations of Asia including Iran and Turkey. Rarely, hazel eyes can be found in people with African or Asian descent.

There is some difficulty in defining the eye color "hazel" as it is sometimes considered to be synonymous with light-brown and other times with dark green.<ref>http://www.thetech.org/genetics/ask.php?id=126</ref><ref name="Hammond"/><ref name="Zhu"/><ref name="Mitchell"/><ref name="Regan"/><ref>Naldi L, Altieri A, Imberti GL, Giordano L, Gallus S, La Vecchia C; Oncology Study Group of the Italian Group for Epidemiologic Research in Dermatology (GISED). "Cutaneous malignant melanoma in women. Phenotypic characteristics, sun exposure, and hormonal factors: a case-control study from Italy." Ann Epidemiol. 2005 Aug;15(7):545-50. PMID 16029848.</ref> They have been described as dark green or yellowish brown,<ref>April Holladay. "Funny — you can't hide those lightening eyes." USATODAY.com. October 8, 2004. Retrieved September 17, 2006.</ref> or as a lighter shade of brown.<ref>English JS, Swerdlow AJ, MacKie RM, O'Doherty CJ, Hunter JA, Clark J, Hole DJ. "Relation between phenotype and banal melanocytic naevi." Br Med J (Clin Res Ed). 1987 Jan 17;294(6565):152-4. PMID 3109545.</ref> Hazel eyes have also been described as being equivalent to a dark green or amber color eyes/light brown.<ref>Hara T. "[Increased iris pigmentation after use of latanoprost in Japanese brown eyes.]" Nippon Ganka Gakkai Zasshi. 2001 May;105(5):314-21. PMID 11406947.</ref> In North America, "hazel" is often used to describe eyes that appear to change color, ranging from light brown to green and even gray, depending on current lighting in the environment.

Amber

Amber colored 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 "lipochrome" in the iris (which is also found in green and violet eyes).<ref>Howard Hughes Medical Institute: Ask A Scientist</ref><ref>http://www.eyecarecontacts.com/eyecolor.html</ref>^{[citation needed]} They are also nicknamed "cat eyes".

The yellow eyes of some pigeons contain yellow fluorescing pigments known as pteridines.<ref>Oliphant LW. "Observations on the pigmentation of the pigeon iris." Pigment Cell Res. 1987;1(3):202-8. PMID 3508278.</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 chromatophores (called xanthophores) located in the iris stroma.<ref>Oliphant LW. "Crystalline pteridines in the stromal pigment cells of the iris of the great horned owl." Cell Tissue Res. 1981;217(2):387-95. PMID 7237534.</ref> In humans, yellow specks or patches are thought to be due to the pigment lipofuscin, also known as lipochrome.<ref name="Lefohn"/>

Green

Green eyes are the product of moderate amounts of melanin. They are most often found among people of North and Eastern European descent, and to a slightly lesser extent, in Southern Europe, and parts of Central and South Asia.[1][2][3][4].Green eyes are the rarest eye color; only 1-2% of human population possesses this trait. Hungary is the country with the highest percentage of green eyed people, about 20%.^{[citation needed]}.

In Iceland 88% of the population have green or blue eyes.[5]

Blue

Blue eyes contain low amounts of melanin within the iris stroma; longer wavelengths of light tend to be absorbed by the underlying iris pigment epithelium and shorter wavelengths are reflected and undergo Rayleigh scattering.<ref name="Wang"/> The type of melanin present is eumelanin.<ref name="Menon"/> The inheritance pattern followed by blue eyes is considered similar to that of a recessive trait.<ref name="Grant"/>

Blue eyes are relatively common throughout Europe and other areas with populations of European descent, such as the Americas and Oceania. While most common among people of Finno-Ugric descent, there are also rare populations in West Asia, Central and South Asia with fairly uncommon natural occurrences of blue eyes.

Blue eyes are common throughout northern Europe. One survey estimated that nearly 90% of Icelanders have blue or green eyes,<ref>Rafnsson V, Hrafnkelsson J, Tulinius H, Sigurgeirsson B, Olafsson JH. "Risk factors for malignant melanoma in an Icelandic population sample." Prev Med. 2004 Aug;39(2):247-52. PMID 15226032</ref> while another reported 50-80%, in contrast to less than 20% of Spaniards and Italians of the north.<ref>http://cogweb.ucla.edu/ep/Frost_06.html</ref> A 2002 study found the prevalence of blue eye color among whites in the United States to be 33.8% for those born between 1936 and 1951 compared to 57.4% for those born between 1899 and 1905.<ref name="Grant"/>

As melanin production generally increases during the first few years of life (especially if exposed to the sun), the blue eyes of some babies may darken as they get older.^{[citation needed]}

Gray

Gray eyes are a variant of blue eyes and are sometimes very hard to tell apart, but are most common in Finland and the Baltic States. Usually, gray eyes are considered a darker shade of blue (like blue-green), where in fact they are lighter. Under magnification, gray eyes exhibit small amounts of yellow and brown color in the iris.

A gray iris may indicate the presence of a uveitis. However, other visual signs make a uveitis obvious.

The Greek goddess Athena was renowned for having "owl-gray" (in Greek, γλαυκῶπις – glaukōpis) or "sea-gray" eyes.<ref>The Perseus Digital Library: Homeric Hymns</ref>

Violet and red

The appearance of violet eyes is thought to occur from the mixing of red and blue reflections.<ref>http://www.bbc.co.uk/dna/h2g2/A734933</ref> Some albinos have eyes that appear violet.<ref>http://www.albinism.org/publications/what_is_albinism.html</ref>

'Red' eyes are also found in an extremely small (<0.001%) percentage of the world's population. This is believed to be a result of either large quantities of the normally scarce red areas in the eye, or a small leakage of blood into the iris; however the number of people on record as having red eyes is believed to not exceed 20 (Recording of this condition started in 1964). This should be noted as being very different from the 'red-eye' condition found in photography.

Anomalous conditions

Aniridia

Aniridia is a congenital condition characterized by an extremely underdeveloped iris which appears absent on superficial examination.<ref>http://www.emedicine.com/oph/topic43.htm</ref> This can also cause blindness.

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 irises 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 total lack of melanin. Transillumination defects can almost always be observed during an eye examination due to lack of iridial pigmentation.<ref>http://www.emedicine.com/OPH/topic260.htm</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 reflex is much brighter in the albino, and this can increase the red eye effect in photographs. Edgar Winter's eyes are an example of this trait.

Heterochromia

Main article: Heterochromia

Heterochromia (also known as a heterochromia iridis or heterochromia iridium) 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 pigment within an iris or part of an iris, which may be inherited or acquired by disease or injury.<ref name="Imesch">Imesch PD, Wallow IH, Albert DM. "The color of the human eye: a review of morphologic correlates and of some conditions that affect iridial pigmentation." Surv Ophthalmol. 1997 Feb;41 Suppl 2:S117-23. PMID 9154287.</ref> This uncommon condition usually results due to uneven melanin content. A number of causes are responsible, including genetics such as chimerism and Waardenburg syndrome. Trauma and certain medications, such as latanoprost can also cause increased or decreased pigmentation in one eye. On occasion the condition of having two different colored eyes is caused by blood staining the iris after sustaining injury. David Bowie (Jones) is a famous person often wrongly attributed with heterochromia. His apparent condition is due to a teenage injury. American actress Kate Bosworth has sectoral heterochromia, resulting in a hazel section at the bottom of her right blue eye, while the left is completely blue. The lead vocalist of American band Rise Against, Tim McIlrath, has heterochromia; his left eye is blue while his right is brown. American actress Mila Kunis also has heterochromia, resulting in one green eye and one brown eye.

Eye color change

Often, paler newborns have blue eyes, which change to green, hazel, light brown or dark brown, as they grow older. This is probably the origin of the idiom "being blue-eyed" (= naïve, gullible).

It is thought that exposure to light after birth triggers the production of melanin in the iris of the eye. By three years of age, the eyes produce and store enough melanin to indicate their natural shade. While changes in eye color of infants are more common, even in adults, eye color changes are seen, most often as a result of exposure to the sun. Sunlight triggers melanin production in the eye, as it does to the skin.

Eyedrops containing a prostaglandin analogue (such as latanoprost) may result in a permanently darkened iris; these eyedrops are commonly used to treat open-angle glaucoma.<ref>Modèle:Cite journal</ref>

Medical implications

Those with lighter iris color have been found to have a higher prevalence of 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">Nicolas CM, Robman LD, Tikellis G, Dimitrov PN, Dowrick A, Guymer RH, McCarty CA. "Iris colour, ethnic origin and progression of age-related macular degeneration." Clin Experiment Ophthalmol. 2003 Dec;31(6):465-9. PMID 14641151.</ref> An increased risk of uveal melanoma has been found in those with blue or gray iris color.<ref name="Stang">Stang A, Ahrens W, Anastassiou G, Jockel KH. "Phenotypical characteristics, lifestyle, social class and uveal melanoma." Ophthalmic Epidemiol. 2003 Dec;10(5):293-302. PMID 14566630.</ref>

Eye color can also be symptomatic of disease. In particular, yellow eyes are associated with jaundice and symptomatic of liver disease, including cirrhosis, hepatitis and malaria.

References

• 52. Jones, S.L., Schnirel, B.L., " Subspecies Comparison of the Genus: Corucia" Polyphemos, (2006). pp 1-25.

See also

• Iridology
• Hair color
• Human skin color
• Xanthophore
• List of Mendelian traits in humans

External links

• Genetics of eye color
• Eye Color and Human Diseases
• Eye colour: portals into pigmentation genes and ancestry
• Various links on human eye colorda:Øjenfarve

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