Eye color

Jump to: navigation, search

Eye color is a polygenic trait and is determined primarily by the amount and type of pigments present in the eye's iris.[1][2] Humans and animals have many phenotypic variations in eye color.[3] In humans, these variations in color are attributed to varying ratios of eumelanin produced by melanocytes in the iris.[2] The brightly colored eyes of many bird species are largely determined by other pigments, such as pteridines, purines, and carotenoids.[4]

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.[5] In eyes of all colors, the iris pigment epithelium contains the black pigment, eumelanin.[2][5] Color variations among different irises are typically attributed to the melanin content within the iris stroma.[5] The density of cells within the stroma affects how much light is absorbed by the underlying pigment epithelium.[5]

Determination of eye color

Eye color is an inherited trait influenced by more than one gene.[6][7] There are two major genes and other minor ones that account for the tremendous variation of human eye color.[8] In humans, three loci associated with eye color are currently known: EYCL1, EYCL2, and EYCL3.[9][10] These genes account for three phenotypic eye colors (brown, green, and blue) in humans.[3] Eye color usually stabilizes when an infant is around 6 months old.[11]

In 2006, the molecular basis of the EYCL3 locus was resolved.[12] 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 (OMIM: 203200). 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.[13]

Classification of colors

File:BlueGreen.jpg
The perception of color depends upon various factors. These are the same eyes; however, depending on the light and surrounding hues, the eye color can appear quite different.

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.[14] Various classification systems have ranged from a basic "light" or "dark" description to detailed gradings employing photographic standards for comparison.[14] Others have attempted to set objective standards of color comparison.[15]

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.[16]

Eye color exists on a continuum from the darkest shades of brown to the lightest shades of blue.[6] 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.[17]

Brown

File:Brunoeil.JPG
Brown human iris

In humans, brown eyes contain large amounts of melanin (eumelanin) within the iris stroma which serves to absorb light, particularly at the shorter wavelengths.[5][18] Very dark brown irises may appear to be black.[19][20] People with light brown or golden irises are sometimes said to have "whiskey eyes".

File:Menschliches Auge.jpg
Dark brown human iris

Brown is very predominant[21] and, in many populations, it is (with few exceptions) the only iris color present.[22] 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.[23] Inhabitants indigenous to Africa, Asia, and the Americas generally 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

File:Movie Eyes.jpg
Some eye colors are too mixed to identify properly, and are called hazel for simplicity's sake. This eye is dark gray with orange toward the center and green towards the edge.

Hazel eyes are due to a combination of a Rayleigh scattering and a moderate amount of melanin in the iris' anterior border layer.[24][5] A number of studies using three-point scales have assigned "hazel" to be the medium-color between light brown and dark green.[25][26][27][28][29][30][31] 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.

File:IradjEye2.JPG
This eye shows an outer green olive color, and towards the middle a brown.

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.[32][19][25][27][30][33] They have been described as dark green or yellowish brown,[34] or as a lighter shade of brown.[35] Hazel eyes have also been described as being equivalent to a dark green or amber color eyes/light brown.[36] 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

File:Amber Eyes.jpg
Human amber eyes displaying the greenish yellow and russet/coppery tint

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).[37][38] Amber eyes are much more common in other animals than they are in humans.[citation needed] They are also nicknamed "cat eyes", or, as in the case of light golden brown eyes, "whiskey".[citation needed]

The yellow eyes of some pigeons contain yellow fluorescing pigments known as pteridines.[39] 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.[40] In humans, yellow specks or patches are thought to be due to the pigment lipofuscin, also known as lipochrome.[24]


Green

Green eyes are the product of moderate amounts of melanin.

File:Greeneyes.jpg
girl with green eyes

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.[citation needed] Deities in both Chinese and Greek mythology had green eyes, possibly because of their rarity.[citation needed] Green eyes are most often depicted with red and brown hair.

Blue

File:Blueye.JPG
A blue eye

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.[5] The type of melanin present is eumelanin.[18] The inheritance pattern followed by blue eyes is thought to be similar to that of a recessive trait.[7]

Blue eyes are relatively common throughout Europe and other areas with populations of European descent, such as Argentina, Australia, Brazil, Canada, the United States, and some parts of West Asia. There are also populations in South and Central Asia that have rare natural occurances of blue eyes. Throughout the rest of the world they are rare. Blue eyes are most commonly depicted with blonde and red haired people.

File:Oeil better.jpg
A blue-green human eye

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 Icelanders have blue or green eyes,[41] in contrast to 20% of Spaniards and Italians of the north.[42] 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.[7]

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

File:Greyeyes.jpg
Female gray eyes

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.

File:Mybluishgrayeye.JPG
A steel blue-gray eye

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.[43]


Violet

The appearance of violet eyes is thought to occur due to the mixing of red and blue reflections.[44] Some albinos have eyes that appear to be violet.[45] 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: Eyes wherein the irises are not present; the eyes appear to be two large pupils.

Aniridia is a congenital condition characterized by an extremely underdeveloped iris which appears absent on superficial examination.[46] 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.[47] 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

File:Heterochromia.jpg
An example of heterochromia. The subject has one brown and one hazel eye.

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.[48] 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.

Actress Kate Bosworth has one blue eye and one brown 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. Actress Mila Kunis has one blue eye and the other has a tint of green.


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.[citation needed]

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.[49]

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;[29] lighter eye color is also associated with an increased risk of ARMD progression.[50] An increased risk of uveal melanoma has been found in those with blue or gray iris color.[51]

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. 2.0 2.1 2.2 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. 3.0 3.1 Morris, PJ. "Phenotypes and Genotypes for human eye colors." Athro Limited website. Retrieved May 10, 2006.
  4. Oliphant LW. "Pteridines and purines as major pigments of the avian iris." Pigment Cell Res. 1987;1(2):129-31. PMID 3507666.
  5. 5.0 5.1 5.2 5.3 5.4 5.5 5.6 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.
  6. 6.0 6.1 Sturm RA, Frudakis TN. "Eye color: portals into pigmentation genes and ancestry." Trends Genet. 2004 Aug;20(8):327-32. PMID: 15262401.
  7. 7.0 7.1 7.2 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.
  8. Eye color mocks easy rules
  9. 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(8):782-4. PMID 12163334.
  10. "Eye color is more complex than two genes." Athro, Limited. Retrieved September 1, 2006.
  11. http://chemistry.about.com/cs/howthingswork/f/eyecolor.htm
  12. Paul Rincon, Genetics of eye color unlocked, BBC News, 20 December, 2006.
  13. 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.
  14. 14.0 14.1 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.
  15. Fan S, Dyer CR, Hubbard L. Quantification and Correction of Iris Color." Technical report 1495, University of Wisconsin-Madison, Dec, 2003.
  16. http://www.edromanguitars.com/tech/color.htm
  17. 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.
  18. 18.0 18.1 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.
  19. 19.0 19.1 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.
  20. Prieto JG. "Eye color in skin cancer." Int J Dermatol. 1977 Jun;16(5):406-7. PMID 873674.
  21. 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.
  22. http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=227220
  23. 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.
  24. 24.0 24.1 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
  25. 25.0 25.1 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.
  26. 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.
  27. 27.0 27.1 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.
  28. 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.
  29. 29.0 29.1 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.
  30. 30.0 30.1 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.
  31. 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.
  32. http://www.thetech.org/genetics/ask.php?id=126
  33. 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.
  34. April Holladay. "Funny — you can't hide those lightening eyes." USATODAY.com. October 8, 2004. Retrieved September 17, 2006.
  35. 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.
  36. 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.
  37. http://www.hhmi.org/cgi-bin/askascientist/highlight.pl?kw=&file=answers%2Fgenetics%2Fans_044.html
  38. http://www.eyecarecontacts.com/eyecolor.html
  39. Oliphant LW. "Observations on the pigmentation of the pigeon iris." Pigment Cell Res. 1987;1(3):202-8. PMID 3508278.
  40. 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.
  41. 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
  42. http://cogweb.ucla.edu/ep/Frost_06.html
  43. http://www.sacred-texts.com/cla/mgr/mgr06.htm
  44. http://www.bbc.co.uk/dna/h2g2/A734933
  45. http://www.albinism.org/publications/what_is_albinism.html
  46. http://www.emedicine.com/oph/topic43.htm
  47. http://www.emedicine.com/OPH/topic260.htm
  48. 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.
  49. Hejkal TW, Camras CB (1999). "Prostaglandin analogs in the treatment of glaucoma". Seminars in ophthalmology. 14 (3): 114–23. doi:10.153/SOPH01400114. PMID 10790575.
  50. 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.
  51. 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.

See also

External links

da:Øjenfarve de:Augenfarbe fa:رنگ چشم gd:Dath nan sùilean fi:Silmien väri sv:Ögonfärg


Linked-in.jpg