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Eye color is a produced by Melanocytes in the iris. The brightly colored eyes of many bird species are largely determined by other pigments, such as Pteridine s, Purine s, and Carotenoid s.Oliphant LW. "Pteridines and purines as major pigments of the avian iris." ''Pigment Cell Res.'' 1987;1(2):129-31. PMID 3507666. 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.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. In eyes of all colors, the iris pigment epithelium contains the black pigment, eumelanin. Color variations among different irises are typically attributed to the melanin content within the iris stroma. The density of cells within the stroma affects how much light is absorbed by the underlying pigment epithelium. DETERMINATION OF EYE COLOR Eye color is an inherited trait influenced by more than one gene.Sturm RA, Frudakis TN. color: portals into pigmentation genes and ancestry." ''Trends Genet.'' 2004 Aug;20(8):327-32. PMID: 15262401.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. There are two major genes and other minor ones that account for the tremendous variation of human eye color. Eye color mocks easy rules In humans, three eye colors (brown, green, and blue) in humans. Eye color usually stabilizes when an infant is around 6 months old.http://chemistry.about.com/cs/howthingswork/f/eyecolor.htm In 2006, the molecular basis of the ''EYCL3'' locus was resolved.Paul Rincon, Genetics of eye color unlocked , ''BBC News'', 20 December, 2006. In a study of 3839 people, researchers reported that 74% of total variation in eye color was explained by a number of Single Nucleotide Polymorphism s (SNPs) near the ''OCA2'' gene (). ''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.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. 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.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. Various classification systems have ranged from a basic "light" or "dark" description to detailed gradings employing photographic standards for comparison. Others have attempted to set objective standards of color comparison.Fan S, Dyer CR, Hubbard L. Quantification and Correction of Iris Color." Technical report 1495, University of Wisconsin-Madison, Dec, 2003. As the Perception Of Color is dependent upon the conditions in which color is viewed (e.g. the amount and type of illumination, as well as the hue of the surrounding environment), so is the perception of eye color.http://www.edromanguitars.com/tech/color.htm Eye color exists on a continuum from the darkest shades of brown to the lightest shades of blue. 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 your eyes turn depends on how much of these colors are present. For example, green eyes have a lot of 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.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(8):1592-8. PMID: 2201662. 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.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. Very dark brown irises may appear to be black.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.Prieto JG. "Eye color in skin cancer." ''Int J Dermatol.'' 1977 Jun;16(5):406-7. PMID 873674. People with light brown or golden irises are sometimes said to have "whiskey eyes". Brown is very predominantEiberg 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. and, in many populations, it is (with few exceptions) the only iris color present.http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=227220 It is least common in the countries around the Baltic Sea . Those with both European and non-European ancestry generally have dark eyes and more variability in eye color within their families.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. Inhabitants [indigenous] of Africa , Asia , and the Americas have brown eyes. Brown eyes are equally found in Europe , Oceania , and the Americas , though within some European populations they are not predominant to the same extent. Brown is considered to be the most dominant eye color in the world. Hazel Hazel eyes are due to a combination of a Rayleigh Scattering and a moderate amount of melanin in the iris' anterior border layer.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 A number of studies using three-point scales have assigned "hazel" to be the medium-color between light brown and dark green.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.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.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.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.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.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.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. 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 the regions of Southern and Eastern Europe, Britain, and the Middle East. There is some difficulty in defining the eye color "hazel" as it is sometimes considered to be synonymous with lightbrown and other times with dark green.http://www.thetech.org/genetics/ask.php?id=126Naldi 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. They have been described as dark green or yellowish brown,April Holladay. "Funny — you can't hide those lightening eyes." USATODAY.com. October 8, 2004. Retrieved September 17, 2006. or as a lighter shade of brown.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. Hazel eyes have also been described as being equivalent to a dark green or amber color eyes/light brown.Hara T. " iris pigmentation after use of latanoprost in Japanese brown eyes. " ''Nippon Ganka Gakkai Zasshi.'' 2001 May;105(5):314-21. PMID 11406947. In North America, "hazel" is often used to describe eyes that appear to change color, ranging from light brown to green and even blue, 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).http://www.hhmi.org/cgi-bin/askascientist/highlight.pl?kw=&file=answers%2Fgenetics%2Fans_044.htmlhttp://www.eyecarecontacts.com/eyecolor.html Amber eyes are much more common in other animals than they are in humans. They are also nicknamed "cat eyes", or, as in the case of light golden brown eyes, "whiskey". The yellow eyes of some , also known as ''lipochrome''. Green Green eyes are the product of moderate amounts of melanin. Green eyes are the rarest eye color. They are most often found in people of Celtic, Germanic and Slavic descent, and to a slightly lesser extent, in Southern Europe as well as Southwest Asia . Deities in both Chinese and Greek mythology had green eyes, possibly because of their rarity. Green eyes are most often depicted with red and brown hair. 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 . The type of melanin present is eumelanin. The inheritance pattern followed by blue eyes is thought to be similar to that of a Recessive trait. Blue eyes are relatively common throughout Europe and other areas with populations of European descent, such as Canada, the United States, Australia and some parts of South West Asia. There are also populations in northern India, Pakistan, Central Asia ( Kazakhs notably] and the Mongols in Mongolia that have rare natural occurances of blue eyes. Throughout the rest of the world they are relatively rare. Blue eyes are most commonly depicted with blonde and red haired people. Finland and Lithuania have the highest proportions of blue-eyed people; they are also very common in other countries around the Baltic Sea , such as Poland and Germany. One survey estimated that nearly 90% of Icelander s have blue or green eyes,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 in contrast to 20% of Spaniards and Italians of the north.http://cogweb.ucla.edu/ep/Frost_06.html 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. 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. Gray Gray eyes are a variant of blue eyes and are sometimes very hard to tell apart. 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. Athena , in Greek Mythology , was renowned for having "sea-gray" eyes.http://www.sacred-texts.com/cla/mgr/mgr06.htm Violet The appearance of violet eyes is thought to occur due to the mixing of red and blue reflections.http://www.bbc.co.uk/dna/h2g2/A734933 Some albinos have eyes that appear to be violet.http://www.albinism.org/publications/what_is_albinism.html Elizabeth Taylor is said to have "Violet Eyes" and this is sometimes referred to as a unique or distinguishing characteristic (or "trademark") of hers. ANOMALOUS CONDITIONS Aniridia Aniridia is a congenital condition characterized by an extremely underdeveloped iris which appears absent on superficial examination.http://www.emedicine.com/oph/topic43.htm 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 in photographs. Edgar Winter 's eyes are an example of this trait. Heterochromia See Also: 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 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. Actress Kate Bosworth has one blue eye and one hazel eye. Lead singer Tim McIlrath of Rise Against has one brown eye and one blue eye. Actress Jane Seymour has one brown eye and one green eye. Actress Demi Moore has one green eye and one hazel eye. Contrary to popular belief, David Bowie does not have heterochromia. When he was young, Bowie was punched in the left eye by a classmate wearing a ring. This injury caused the pupil of his eye to remain dilated, commonly being mistaken for a differing iris color. Eye color change Often, newborns have blue eyes, which change to green, hazel, light brown or dark brown, as they grow older. 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. People with green eyes in the winter might develop darker, more hazel eyes if they spend more time outdoors. 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 .1 MEDICAL IMPLICATIONS Those with lighter iris color have been found to have a higher prevalence of has been found in those with blue or gray iris color.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. REFERENCES SEE ALSO EXTERNAL LINKS
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