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Eye color

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Eye color or eye colour is a polygenic trait and is determined by the amount and type of pigments, primarily melanin, present in the eye's iris[1][2]. Although there seem to be three genotypic eye colors (brown, green, and blue), humans and other animals have many phenotypic variations in eye color[3]. These variations in color are attributed to varying ratios of eumelanin and pheomelanin produced by melanocytes in the iris.[2].

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.[4] In eyes of all colors, the iris pigment epithelium contains the black pigment, eumelanin.[2][4] Color variations among different irises are typically attributed to the melanin content within the iris stroma.[4] The density of cells within the stroma affects how much light is absorbed by the underlying pigment epithelium.[4]

Classification of colors

Iris color can provide a large amount of 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.[5] Various classfication systems have ranged from a basic "light" or "dark" description to detailed gradings employing photographic standards for comparison.[5] Others have attempted to set objective standards of color comparison.[6]

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.[7]

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.[8]

Brown

File:Browneyez.jpg
A woman with light brown eyes.

Brown eyes contain large amounts of melanin within the iris stroma which serves to absorb light, particularly at the shorter wavelengths.[4] Very dark brown irises may appear to be black.[9][10]

The vast majority of the world's people have dark eyes, ranging from brown to nearly black. Light brown eyes are also present in many people, but to a lesser extent.[citation needed] Most of the original inhabitants of Africa, Asia, and the Americas have brown eyes. Brown eyes are also found in Europe, Oceania and North America, though within European populations they are not predominant to the same extent. Brown had been considered to be the most dominant eye color in any gene, but new studies have revealed that this is not always true[citation needed].

Hazel

File:Eye S1NN3R.jpg
A hazel eye.

Hazel eyes are the product of moderate amounts of melanin.[4]

Hazel has been subjectively classified as a lighter shade of brown.[11] A number of studies using three-point scales have assigned "hazel" to be the medium-color between brown and blue.[12][13][14][15][16][17][18] Hazel is sometimes grouped with green.[9][12][14][17][19]

Hazel eyes have also been described as being equivalent to green, blue/gray, and yellow combined with brown.[20]

Hazel eyes often change color depending on what the person is wearing (i.e. if the person is wearing green or purple, the eyes appear mostly green).

Hazel eyes are not associated with a particular geographical region or population, and can occur in people of all descents, but are especially common amongst Europeans and often arise from having one blue eyed parent and a hazel eyed parent.[citation needed] In this way, hazel eyes are an example of an incomplete dominance relationship, and people with hazel eyes commonly carry the recessive genes for blue eyes.[citation needed] Thus, a person with hazel eyes who has children with a blue eyed person has a fifty percent chance of producing a blue eyed child and equally, fifty percent chance of producing a hazel eyed child.[citation needed]

Amber

File:Amber Eyes.JPG
Amber eyes displaying the milky greenish yellow and russet/coppery tint.

Amber eyes are similar to hazel. They are, however, of a solid color and have a much stronger yellowish/golden and russet/coppery tint than hazel eyes. The golden tint appears at its brightest in direct lighting or sunlight, but turns into a golden green-brown in the shade or a coppery brown in candlelight. Amber colored eyes are predominantly found in other animals than humans. Amber eyes are also nicknamed "cat eyes."

Green

Light Green eyes.
Dark Green eyes

Green eyes are also the product of moderate amounts of melanin.[4]

Green eyes are most often found in people of Celtic, Germanic, and Slavic descent. Hungarians have the highest percentage of green eyes of any population, close to 20%.[citation needed] Green eyes are also found, though in far lower proportions, among other Caucasian populations in the Middle East and South Asia. They are so common among Pashtuns that in Pakistan, Pashtuns are often called "Hare Ankheian Vaale": those with green eyes (Urdu translation).[citation needed]. They may also be found in many areas of Afghanistan (the native land of the Pashtuns) and parts of India - most common in the north, with the highest percentages respectively in Punjab, U.P, Gujurat, and Rajasthan.

One of the most famous photographs ever published by National Geographic was a close-up of Sharbat Gula, a Pashtun girl with startling green eyes, taken in western Afghanistan by Steve McCurry in 1984. Details of her irises captured by the photograph were used to confirm her identity after she was relocated in 2002.

Gray

Grey eyes.

Gray eyes are a (often lighter) variant of blue eyes. A wide variety of shades of grey exist, from the almost white (light grey) to dark.

Steel Blue-Gray eyes.

The underlying gray color may be tinted with various other colors, as in the greenish-gray eyes in the picture. "Steel blue" eyes are also found, ranging from a slightly desaturated blue to light blue-gray.

A yellow-, amber- or copper-colored ring is commonly seen around the pupil. This is a normal part of the iris, and should not be confused with Kayser-Fleischer rings. As with other colors, gray eyes are often seen to change color depending on the surrounding colors.

Blue

File:My blue blue eyes.JPG
Blue eyes are relatively common throughout Europe, especially in Northern Europe, including the northern Baltics and in East Central Europe.
File:Tamil-blue.jpg
An Indian boy from Tamil Nadu with startling blue eyes

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.[4]

Blue eyes are found mainly in people of northern European and eastern European descent, and to a lesser extent, in people of southern Europe[citation needed], the Middle East and Central Asia, most commonly in people who live at higher elevations. South Asians may also have blue eyes, but this is uncommon, except amongst Pathans in Afghanistan, Pakistan, and India. It also occurs in other Indians and Pakistanis, generally in highest percentages amoung Punjabiis - though even then it is not exactly common. Finland and Lithuania have the highest proportions of blue-eyed people, with at least 80% in both countries respectively. Ireland and Great Britain also have high proportions of blue eyes, with estimates of around 70% for Ireland and about 65% for Britain. Many Caucasian babies are born with blue eyes, though their eyes darken, or change color, with time. Most infants' eye color will set within a couple of days to a couple of weeks, though some people's will continue to change for a number of years.

Blue is the color of the indole monomer that when polymerised forms melanin[citation needed]. If both alleles for brown eyes (a polymerase gene) are absent or damaged, the blue color remains. Hence blue eye color is a recessive trait.

A 2002 study found the prevalence of blue eye color among non-Hispanic whites in the United States to be 57.4% for those born between 1899 and 1905 compared to 33.8% for those born between 1936 and 1951[21].

Blue-green eyes

Blue-green eyes: These are the same eyes, however depending on the light and surrounding hues, the eye color can appear quite different. Notice the copper ring around the pupil.

Blue-green eyes are relatively uncommon and usually consist of an iris with a predominantly blue color, usually darker blue, with green streaks or stippling caused by a yellow- or copper-colored overlay.[citation needed] Yellow-, amber-, or copper-colored rings are often present around the pupil.[citation needed] Blue-green eyes are considered a blue variant and to some extent most common in Sweden.[citation needed]

Violet eyes

Violet eyes are extremely rare and are a variation of blue eyes. They are believed to be caused by eyes having so little pigmentation that the red and blue blood vessels permeate through, causing a violet color of the iris. A noteworthy subject is Elizabeth Taylor, whose physical trademark is her violet eyes.

Anomalous conditions

Aniridia (Black eyes)

Aniridia is a congenital condition characterized by an extremely underdeveloped iris which appears absent on superficial examination.[1]

Ocular albinism (Red eyes)

In those with albinism, the color of the irises is typically blue (but can vary from blue to brown); transillumination defects can almost always be observed during an eye examination due to lack of iridial pigmentation.[2]

Because of this lack of pigment, the blood vessels underneath may lend a reddish color to the eye enhancing the red eye effect in photographs. Edgar Winter's eyes are an example of this trait.

Heterochromia

File:Heterochromiadeux.jpg
An example of heterochromia iridis. The subject has a brown and green eye.
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[22]. This uncommon condition usually results due to uneven melanin content. A number of causes are responsible, including genetics 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 and Christopher Walken's eyes are a well known example of a trauma-induced heterochromia.[citation needed]

Genetics

Three gene pairs coding for human eye color are currently known.[23]

This model can partially explain how two brown-eyed parents can give birth to a blue-eyed child. Also Caucasian babies are often born with blue eyes which turn brown as they get older. However this may not always occur because of genetic damage or environmental factors. If such a person has a child with another blue-eyed person then that child may inherit the gene for brown eyes.

Eye color typically stabilizes by 6 years of age[24].

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[16]; lighter eye color is also associated with an increased risk of ARMD progression[25]. An increased risk of uveal melanoma has been found in those with blue or grey iris color[26]. Darker iris colors have been found to have slightly higher intraocular pressures than lighter iris colors [14]. An increased incidence of age-related cataracts has been found in those with dark brown irises[27][9].

References

  1. ^ 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.
  2. ^ a b c 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.
  3. ^ Morris, PJ. "Phenotypes and Genotypes for human eye colors." Athro Limited website. Retrieved May 10, 2006.
  4. ^ a b c d e f g h 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.
  5. ^ a b German EJ, Hurst MA, Wood D, Gilchrist J. "A novel system for the objective classification of iris colour and its correlation with response to 1% tropicamide." Ophthalmic Physiol Opt. 1998 Mar;18(2):103-10. PMID 9692029.
  6. ^ Fan S, Dyer CR, Hubbard L. Quantification and Correction of Iris Color." Technical report 1495, University of Wisconsin-Madison, Dec, 2003.
  7. ^ http://www.edromanguitars.com/tech/color.htm
  8. ^ 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.
  9. ^ a b c 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. Cite error: The named reference "Hammond" was defined multiple times with different content (see the help page).
  10. ^ Prieto JG. "Eye color in skin cancer." Int J Dermatol. 1977 Jun;16(5):406-7. PMID 873674.
  11. ^ 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.
  12. ^ a b 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.
  13. ^ 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.
  14. ^ a b c 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.
  15. ^ 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.
  16. ^ a b 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.
  17. ^ a b 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.
  18. ^ 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.
  19. ^ 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.
  20. ^ 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.
  21. ^ Grant MD, Lauderdale DS. "Cohort effects in a genetically determined trait: eye colour among US whites." Ann Hum Biol. 2002 Nov-Dec;29(6):657-66. PMID 12573082.
  22. ^ 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.
  23. ^ "Eye color is more complex than two genes." Athro, Limited. Retrieved September 1, 2006.
  24. ^ Bito LZ, Matheny A, Cruickshanks KJ, Nondahl DM, Carino OB. "Eye color changes past early childhood. The Louisville Twin Study." Arch Ophthalmol. 1997 May;115(5):659-63. PMID 9152135.
  25. ^ 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.
  26. ^ 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.
  27. ^ Younan C, Mitchell P, Cumming RG, Rochtchina E, Wang JJ. "Iris color and incident cataract and cataract surgery: the Blue Mountains Eye Study." Am J Ophthalmol. 2002 Aug;134(2):273-4. PMID 12140040.

See also

External links

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