Eye color

Eye color is a polygenic trait and is determined by the amount and type of pigments in the eye's iris.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.] 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.] Humans and animals have many phenotypic variations in eye color.Morris, PJ. [http://www.athro.com/evo/gen/eyecols.html "Phenotypes and Genotypes for human eye colors."] Athro Limited website. Retrieved May 10, 2006.] In human eyes, these variations in color are attributed to varying ratios of eumelanin produced by melanocytes in the iris. The brightly colored eyes of many bird species are largely determined by other pigments, such as pteridines, purines, and carotenoids.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. [http://research.microsoft.com/~stevelin/iris.pdf "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.

Genetic determination of eye color

Eye colors can range from the most common color, brown, to the least common, green. Rare genetic specialties can even lead to unusual eye colors: black, red, and violet. Eye color is an inherited trait influenced by more than one gene.Sturm RA, Frudakis TN. [http://www.biosci.ohio-state.edu/~pfuerst/courses/eeobmg640/reading1eyecolor.pdf"Eye 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.] These genes are being sought using associations to small changes in the genes themselves and in neighboring genes. These changes are known as single nucleotide polymorphisms or SNPs. The actual number of genes that contribute to eye color is unknown at present, but there are a few likely candidates.

The gene "OCA2" (OMIM3|203200), when in a variant form the gene causes the pink eye color and hypopigmentation common in human 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 brown and green eyes as well as variations in freckling, mole counts, hair and 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.Duffy DL, Montgomery GW, Chen W, Zhao ZZ, Le L, James MR, Hayward NK, Martin NG, and Sturm, RA [http://www.journals.uchicago.edu/AJHG/journal/issues/v80n2/44159/brief/44159.abstract.html "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.] A specific mutation within the "HERC2" gene, a gene that regulates OCA2 expression, is partly responsible for blue eyes. Kayser M, Liu F, Janssens AC, Rivadeneira F, Lao O, van Duijn K, Vermeulen M, Arp P, Jhamai MM, van Ijcken WF, den Dunnen JT, Heath S, Zelenika D, Despriet DD, Klaver CC, Vingerling JR, de Jong PT, Hofman A, Aulchenko YS, Uitterlinden AG, Oostra BA, van Duijn CM. "Am J Hum Genet." 2008 Feb; 82(2):411-23.] Other genes implicated in eye color variation are: "SLC24A4"Sulem P, Gudbjartsson DF, Stacey SN, Helgason A, Rafnar T, Magnusson KP, Manolescu A, Karason A, Palsson A, Thorleifsson G, Jakobsdottir M, Steinberg S, Pálsson S, Jonasson F, Sigurgeirsson B, Thorisdottir K, Ragnarsson R, Benediktsdottir KR, Aben KK, Kiemeney LA, Olafsson JH, Gulcher J, Kong A, Thorsteinsdottir U, Stefansson K. "Nature Genetics" 2007 Dec; 39(12):1443-52.PMID: 17952075] , "TYR" .

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" [of the"Herc2" gene] " 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"."

Eye color usually stabilizes when an infant is around 6 months old. [ [http://chemistry.about.com/cs/howthingswork/f/eyecolor.htm Why Babies Are Born with Blue Eyes] ]

ynopsis

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. [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.]

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. [ [http://www.edromanguitars.com/tech/color.htm Color Perception] ]

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 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 blue. How much of each color one has determines the appearance of the eye color. The color of the eyes in turn depends on how much of these colors are present. For example, green eyes have yellow and some blue, making them appear green. Brown eyes appear brown because most of the eye contains the brown color. The above is true for 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. [Jones, S.L., Schnirel, B.L., "Subspecies comparison of the Genus: Corucia", Leeway Corucia Research Center - LCRC, Polyphemos, {2006} Volume 4, Issue 1. pp. 1-25.]

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 "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.html Howard Hughes Medical Institute: Ask A Scientist] ] [ [http://www.eyecarecontacts.com/eyecolor.html Eye Color ] ] They are nicknamed "Wolf eyes" due to the high rate of the amber eye color in wolves. Amber eyes should not be confused with hazel eyes; although hazel eyes may contain specks of amber or gold, they usually tend to comprise of 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.

The eyes of some pigeons contain yellow fluorescing pigments known as pteridines. [Oliphant LW. "Observations on the pigmentation of the pidgeon iris." "Pigment Cell Res." 1987; 1(3):202-8. PMID 3508278.] 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. [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.] In humans, yellowish specks or patches are thought to be due to the pigment lipofuscin, also known as "lipochrome".

Blue

Blue eyes are the second most common eye color in the world, although they are very recessive. 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 considered similar to that of a recessive trait, however it is a polygenic trait (meaning that it is controlled by the interactions of several genes, not just one). Eiberg and colleagues showed 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, reduced expression of OCA2 with subsequent reduction in melanin production.Eiberg H, Troelsen J, Nielsen M, Mikkelsen A, Mengel-From J, Kjaer KW, Hansen L. " [http://www.springerlink.com/content/2045q6234h66p744/fulltext.html Blue eye color in human beings may be caused by a perfectly associated founder mutation in a regulatory element located within the HERC2 gene inhibiting OCA2 expression] " "Human Genetics" 2008 Jan 3. PMID 18172690]

The authors concluded that the mutation may have arisen in a single individual around the Black Sea region 6,000-10,000 years ago, perhaps suggesting that all people with true blue eyes are more closely related. However, blue eyes with brown spots around the pupil are not related to this mutation.

Blue eyes are most common in Ireland [ [http://carnby.altervista.org/troe/10-02.htm Ireland ] ] , Poland [http://carnby.altervista.org/troe/12-09.htm Poland and Russia ] ] , [ [http://carnby.altervista.org/troe/12-05.htm Germany ] ] , Netherlands [ [http://carnby.altervista.org/troe/12-04.htm The Netherlands and Frisia ] ] , Iceland [ [http://carnby.altervista.org/troe/09-05.htm Iceland ] ] , Austria [ [http://carnby.altervista.org/troe/12-06.htm Switzerland and Austria ] ] , Sweden [ [http://carnby.altervista.org/troe/09-06.htm Sweden ] ] , Norway [ [http://carnby.altervista.org/troe/09-04.htm Scandinavia: Norway ] ] , Denmark [ [http://carnby.altervista.org/troe/09-07.htm Denmark ] ] , Russia, Finland [ [http://carnby.altervista.org/troe/09-11.htm The Baltic Finns: Finland ] ] , Germany [ [http://de.wikipedia.org/wiki/Bild:Augenfarbe.svg] ] ,France [ [http://carnby.altervista.org/troe/09-16.htm] ] ,
Estonia and Latvia, [ [http://carnby.altervista.org/troe/09-10.htm The Baltic Finns: Livs and Esths ] ] , and the United Kingdom [ [http://carnby.altervista.org/troe/10-03.htm Great Britain, General Survey ] ] [ [http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WPG-4CBW58K2&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=e178ee28135d39d955ba1c6436b3d082] 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] They are also present in Southern Europe, Spain [ [http://carnby.altervista.org/troe/11-15.htm The Iberian Peninsula ] ] , Portugal [ [http://carnby.altervista.org/troe/11-15.htm The Iberian Peninsula ] ] ,Italy [ [http://carnby.altervista.org/troe/12-07.htm Italy ] ] and the Balkans [ [http://carnby.altervista.org/troe/12-12.htm Serbs, Croats, and Slovenes ] ] , the Middle East [ [http://carnby.altervista.org/troe/08-05.htm Pigmentation, the Pilous System, and Morphology of the Soft Parts ] ] , and are also found in Afghanistan, Iran and north of India. A 2002 study found the prevalence of blue eye color among Whites 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.

Brown

Brown eyes are predominant in humans [Eiberg H, Mohr J. "Assignment of genes coding for brown eye color (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 OMIM - SKIN/HAIR/EYE PIGMENTATION, VARIATION IN, 1; SHEP1 ] ] It is less common in countries around the Baltic Sea, such as Finland, Estonia and Latvia.

In humans, brown eyes contain large amounts of melanin (melanin) within the iris stroma, which serves to absorb light, particularly at the shorter wavelengths. Brown eyes are the most common eye color, with over half of the world population having them. They are also the most dominant eye color gene. 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 at a glance 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.]

Gray

Gray eyes have less melanin than blue eyes, even though they are considered a darker shade of blue (like blue-green). They are most common in European Russia, Finland and the Baltic States. 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.

Visually, gray eyes often tend to appear to change between the shades of blue, green and gray. This is said to be influenced by the lighting and the surroundings (such as clothes, makeup, etc.).

The Greek goddess Athena was renowned for having "owl-gray" or "sea-gray" eyes (in Greek, "γλαυκῶπις" – "glaukōpis"). [ [http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus:text:1999.01.0137:hymn=28:line=1 The Perseus Digital Library: Homeric Hymns] ]

Green

Green eyes are the product of light to moderate amounts of melanin. According to some researchers, green eyes are the result of mutations that change the melanin structure. [ [http://news.softpedia.com/news/How-Blue-or-Green-Eyes-Appeared-82575.shtml How Blue or Green Eyes Appeared - Some researches - Softpedia ] ] Green eyes are the rarest natural eye color found in humans. Only 1-2% of the world population has green eyes. Green eyes are most common in northern Europe, Scandinavia, Ireland, and sometimes can be found in France, Spain, Italy, to a lesser extent in the Middle East, as well as Northern parts of India, Pakistan and Afghanistan. It is also found within parts of South America, including Colombia. Much of the Icelandic population has either green or blue eye color.

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] Hazel eyes often appear to shift in color from a light brown to a medium golden-green. A number of studies using three-point scales have assigned "hazel" to be the medium-color between the lightest shade of blue and darkest shade of brown.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. [http://www.pubmedcentral.nih.gov/picrender.fcgi?artinstid=1358991&blobtype=pdf "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. [http://www.pubmedcentral.gov/articlerender.fcgi?tool=pubmed&pubmedid=11190014 "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 (and vice versa) when observed in sunlight. Hazel is commonly found in some regions of the Middle East, Europe, North America, parts of Central Asia, parts of North India, Northern Pakistan, Iran and in Afghanistan. Rarely, hazel eyes can be found in people of sub-saharan African, Native American and East Asian descent.

Definitions of the eye color "hazel" vary: it is sometimes considered to be synonymous with light-brown and gold. [ [http://www.thetech.org/genetics/ask.php?id=126 Understanding Genetics: Human Health and the Genome ] ] [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.] [April Holladay. [http://www.usatoday.com/tech/columnist/aprilholladay/2004-10-08-wonderquest_x.htm "Funny — you can't hide those lightening eyes."] USATODAY.com. October 8, 2004. Retrieved September 17, 2006.] [English JS, Swerdlow AJ, MacKie RM, O'Doherty CJ, Hunter JA, Clark J, Hole DJ. [http://www.pubmedcentral.nih.gov/picrender.fcgi?artinstid=1245163&blobtype=pdf "Relation between phenotype and banal melanocytic naevi."] "Br Med J (Clin Res Ed)." 1987 Jan 17; 294(6565):152-4. PMID 3109545.] [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.] 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 lighting in the environment.

Red

The eyes of a person with albinism may appear red under certain lighting conditions due to the very low quantities of melanin. "True" red eyes also exist in albinistic populations, but are very rare. [http://www.albinism.org/publications/what_is_albinism.html NOAH — What is Albinism? ] ]

The red-eye effect in flash photographs makes the pupils (rather than the irises) appear red. It is not related to eye color.

Violet

The appearance of "violet" eyes is thought to occur from the mixing of red and blue reflections. [ [http://www.bbc.co.uk/dna/h2g2/A734933 BBC - h2g2 - Determination of Eye Colour ] ] Some albinos have eyes that appear violet. Violet eyes are a form of blue eyes, and are just a reflection, pigment, or variant of blue.

Kaleidoscope

This color pattern resembles an image that might be generated by a kaleidoscope. Most listings of eye coloring found in documents meant to describe a person's physical characteristics usually do not include this as a choice of eye color, so 'hazel' is often picked, even though that is not entirely accurate. Possibly this omission is due to the relative scarcity of this pattern, as it is expressed by a recessive-recessive gene combination. In addition, the pattern varies from person to person in color combinations and the details of particular shadings and sub-patterns, so it cannot be definitively represented by just one instance. The key factor distinguishing this pattern from hazel coloring is that hazel eyes tend to display a distinctive pattern of coloring emanating in a circular fashion from the pupil, forming concentric rings of color. Kaleidoscopic coloring includes emanations of varying colors from the pupil out into the other colored areas (which may also be concentric) surrounding the pupil, generally forming a starburst-like pattern.

'Kaleidoscope eyes' is mentioned in the well-known song by The Beatles entitled Lucy in the Sky with Diamonds.

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; lighter eye color is also associated with an increased risk of ARMD progression.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.] An increased risk of uveal melanoma has been found in those with blue, green 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.] [http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=15226032 Risk factors for malignant melanoma in an Icelandi... [Prev Med. 2004 - PubMed Result ] ]

Eye color may also be symptomatic of disease. Aside from the iris, yellowing of the whites of the eyes is associated with jaundice and symptomatic of liver disease, including cirrhosis, hepatitis and malaria.

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 eMedicine - Aniridia : Article by Daljit Singh ] ]

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. [ [http://www.emedicine.com/OPH/topic260.htm eMedicine - Ocular Manifestations of Albinism : Article by Mohammed O Peracha, MD ] ] 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.

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.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.] 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 some prostaglandin analogues 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.

British singer David Bowie is a famous person often wrongly attributed with heterochromia. His apparent condition is due to a teenage injury. (One eye appears darker because the pupil is permanently dilated.) 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. American Actress Elizabeth Berkley has sectoral heterochromia; her right eye is half green and half brown, and her entire left eye is green. So does actor Anthony Head - he has a patch of hazel in his left eye where both eyes are blue-green overall. 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 blue eye and one brown-green eye. American Actress Demi Moore also has heterochromia, by having one green eye, and the other hazel.The English Actress Jane Seymour also has heterochromia; her left eye is green whereas her right eye is brown.

Eye color change

Often, Caucasian newborns have blue eyes, which change to green, hazel, light brown or dark brown. This is possibly the origin of the idiom "being blue-eyed" (i. e. 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. [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=10.153/SOPH01400114 |doi_brokendate=2008-06-21]

Eye color and red-eye effect

The photographic red-eye effect is more prominent in people with pale (blue, green, or grey) eyes, and a similar effect (possibly the same) is observed in the eyeshine of blue-eyed cats and dogs.

References

ee also

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

External links

* [http://www.howtofriends.com/eyes/ Genotypes For Human Eye Colors]
* [http://www.thetech.org/genetics/ask.php?id=29 Genetics of eye color]
* [http://longevity-science.blogspot.com/2007/02/eye-color.html Eye Color and Human Diseases]
* [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15262401 Eye colour: portals into pigmentation genes and ancestry]
* [http://www.kumc.edu/gec/support/eyecolor.html Various links on human eye color]