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Wikipedia

Eye color

April 30, 2023

"Iris color" redirects here. For the shade of purple, see Iris (color).

Eye color is a polygenic phenotypic trait determined by two factors: the pigmentation of the eye's iris[1][2] and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris.[3]: 9 

Close up of a gray human iris

In humans, the pigmentation of the iris varies from light brown to black, depending on the concentration of melanin in the iris pigment epithelium (located on the back of the iris), the melanin content within the iris stroma (located at the front of the iris), and the cellular density of the stroma.[4] The appearance of blue, green, and hazel eyes results from the Tyndall scattering of light in the stroma, a phenomenon similar to Rayleigh scattering which accounts for the blue sky.[5] Neither blue nor green pigments are present in the human iris or vitreous humour.[3][6] This is an example of structural color, which depends on the lighting conditions, especially for lighter-colored eyes.

The brightly colored eyes of many bird species result from the presence of other pigments, such as pteridines, purines, and carotenoids.[7] Humans and other animals have many phenotypic variations in eye color.[8]

The genetics and inheritance of eye color in humans is complicated. As of 2010, as many as 16 genes have been associated with eye color inheritance. Some of the eye-color genes include OCA2 and HERC2.[9] The earlier belief that blue eye color is a simple recessive trait has been shown to be incorrect. The genetics of eye color are so complex that almost any parent-child combination of eye colors can occur.[10][11] However, OCA2 gene polymorphism, close to proximal 5' regulatory region, explains most human eye-color variation.[12]

Genetic determination

Eye color is an inherited trait determined by multiple genes.[13][14] These genes are sought by studying small changes in the genes themselves and in neighboring genes, called single-nucleotide polymorphisms or SNPs. The total number of genes that contribute to eye color is unknown, but there are a few likely candidates. A study in Rotterdam (2009) found that it was possible to predict eye color with more than 90% accuracy for brown and blue using just six SNPs.[15][16]

People of European descent show the greatest variety in eye color of any population worldwide. Recent advances in ancient DNA technology have revealed some of the history of eye color in Europe. Through the analysis of ancient DNA, a 2020 study published in Experimental Dermatology suggested that the common gene for blue eye color likely originated in the Near East and arrived in Europe around 42,000 years ago, after the exodus out of Africa.[17]

There is evidence that as many as 16 different genes could be responsible for eye color in humans; however, the main two genes associated with eye color variation are OCA2 and HERC2, and both are localized in Chromosome 15.[9]

The gene OCA2 (OMIM: 203200), when in a variant form, 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 blue 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.[12] A specific mutation within the HERC2 gene, a gene that regulates OCA2 expression, is partly responsible for blue eyes.[18] Other genes implicated in eye color variation are SLC24A4[19] and TYR.[19] A 2010 study of eye color variation in hue and saturation values using high-resolution digital full-eye photographs found three new loci for a total of ten genes, allowing the explanation of about 50% of eye color variation.[20]

Gene name Effect on eye color
OCA2 Associated with melanin producing cells. Central importance to eye color.
HERC2 Affects function of OCA2, with a specific mutation strongly linked to blue eyes.
SLC24A4 Associated with differences between blue and green eyes.[19]
TYR Associated with differences between blue and green eyes.[19]

Blue eyes with a brown spot, green eyes, and gray eyes are caused by an entirely different part of the genome.

Changes in eye color

 
Percentage of light eyes in and near Europe according to anthropologist Peter Frost.[21]
  80+
  50–79
  20–49
  1–19

Most newborn babies who have European ancestry have light-colored eyes. As the child develops, melanocytes (cells found within the iris of human eyes, as well as skin and hair follicles) slowly begin to produce melanin. Because melanocyte cells continually produce pigment, in theory eye color can be changed. Adult eye color is usually established between 3 and 6 months of age, though this can be later.[22] Observing the iris of an infant from the side using only transmitted light with no reflection from the back of the iris, it is possible to detect the presence or absence of low levels of melanin. An iris that appears blue under this method of observation is more likely to remain blue as the infant ages. An iris that appears golden contains some melanin even at this early age and is likely to turn from blue to green or brown as the infant ages.

Changes (lightening or darkening) of eye color during early childhood, puberty, pregnancy, and sometimes after serious trauma (like heterochromia) suggest that eyes sometimes change color under certain circumstances, due to chemical reactions or hormonal changes within the body.

Studies on Caucasian twins, both fraternal and identical, have shown that eye color over time can be subject to change, and major demelanization of the iris may also be genetically determined. Most eye color changes have been observed or reported in the Caucasian population with hazel and amber eyes.[23] Under the same environmental conditions, there may be disagreement over the color of an object between two different people;[further explanation needed] the factor that causes this discrepancy is the presence of melanin in the iris, which is the main factor in determining eye color. The higher the amount of melanin in the iris and the denser the texture of the melanin, the darker the color of a person's eyes; the same melanin concentration also depends on many factors such as hereditary and environmental ones.

The human eye consists of two types of light and color receptors in the retina. Cylindrical cells are the photoreceptors of the eye that have a black and white vision and, depending on the amount of light received from the environment, determine the amount of darkness and brightness of objects. The number of cylindrical cells is more than the number of color receptors and reaches about 120 million; cone cells, which are smaller in number than light receptors, have color vision and are divided into three distinct categories, each of which recognizes one of the colors blue, red, and green, allowing the individual to distinguish colors.[24]

Eye color range

Brown

"Brown eyes" redirects here. For other uses, see Brown eyes (disambiguation).
 
Dark brown iris is common in Europe, Americas, Africa, West Asia, East Asia and Southeast Asia.
 
Light brown iris can be found in Europe, West Asia, South Asia, Central Asia and among the Americas.

Almost all mammals have brown or darkly-pigmented irises.[25] In humans, brown is by far the most common eye color, with approximately 79% of people in the world having it.[26] Brown eyes result from a relatively high concentration of melanin in the stroma of the iris, which causes light of both shorter and longer wavelengths to be absorbed.[27]

Dark brown eyes are dominant in humans.[28] In many parts of the world, it is nearly the only iris color present.[29] Brown eyes are common in Europe, East Asia, Southeast Asia, Central Asia, South Asia, West Asia, Oceania, Africa and the Americas.[19] Light or medium-pigmented brown eyes can also be commonly found in South Europe, among the Americas, and parts of Central Asia, West Asia and South Asia.

Amber

 
Amber eye

Amber eyes are a solid color with a strong yellowish/golden and russet/coppery tint, which may be due to the yellow pigment called lipochrome (also found in green eyes).[30][31] Amber eyes should not be confused with hazel eyes. Although hazel eyes may contain specks of amber or gold, they usually tend to have 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. Even though amber is similar to gold, some people have russet or copper colored amber eyes that are mistaken for hazel, though hazel tends to be duller and contains green with red/gold flecks, as mentioned above. Amber eyes may also contain amounts of very light gold-ish gray.

The eyes of some pigeons contain yellow fluorescing pigments known as pteridines.[32] 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.[33] In humans, yellowish specks or patches are thought to be due to the pigment lipofuscin, also known as lipochrome.[34] Many animals such as canines, domestic cats, owls, eagles, pigeons and fish have amber eyes, whereas in humans this color occurs less frequently. Amber is the third-rarest natural eye color after green and gray, in 5% of the world's population.[35] People with that eye color are found in Spain, Balkan region, Hungary, Italy, Southern Cone, Colombia, Pakistan and Middle East.[36]

Hazel

 
Hazel eyes
 
Hazel eye

Hazel eyes are due to a combination of Rayleigh scattering and a moderate amount of melanin in the iris' anterior border layer.[4][34] Hazel eyes often appear to shift in color from a brown to a green. Although hazel mostly consists of brown and green, the dominant color in the eye can either be brown/gold or green. This is why hazel eyes can be mistaken as amber and vice versa.[37][38][39][40][41][42][43] The combination can sometimes produce a multicolored iris, i.e., an eye that is light brown/amber near the pupil and charcoal or dark green on the outer part of the iris (or vice versa) when observed in sunlight.

Definitions of the eye color hazel vary: it is sometimes considered to be synonymous with light brown or gold, as in the color of a hazelnut shell.[37][39][42][44]

Around 18% of the US population and 5% of the world population have hazel eyes.[26] Hazel eyes are more common in Spaniards, North Africa, the Middle East, and Brazil.[45]

Green

 
Green eyes
 
A Chinchilla Persian with sea-green eyes

Green eyes probably result from the interaction of multiple variants within the OCA2 and other genes. They were present in south Siberia during the Bronze Age.[46]

 
Green-hazel eyes

Green eyes are most common in Northern, Western and Central Europe.[47][48] In Scotland, 29% of people have green eyes.[49] Around 8–10% of men and 18–21% of women in Iceland and 6% of men and 17% of women in the Netherlands, have green eyes.[50] Among European Americans, green eyes are most common among those of recent Celtic and Germanic ancestry with about 16%.[51]

The green color is caused by the combination of: 1) an amber or light brown pigmentation in the stroma of the iris (which has a low or moderate concentration of melanin) with: 2) a blue shade created by the Rayleigh scattering of reflected light.[27] Green eyes contain the yellowish pigment lipochrome.[52]

Blue

"Blue eyes" redirects here. For other uses, see Blue eyes (disambiguation).
 
A light blue iris with limbal ring

There is no blue pigmentation either in the iris or in the vitreous body. Dissection reveals that the iris pigment epithelium is brownish black due to the presence of melanin.[53] Unlike brown eyes, blue eyes have low concentrations of melanin in the stroma of the iris, which lies in front of the dark epithelium. Longer wavelengths of light tend to be absorbed by the dark underlying epithelium, while shorter wavelengths are reflected and undergo Rayleigh scattering in the turbid medium of the stroma.[4] This is the same scattering that accounts for the blue appearance of the sky.[3]: 9 [6] The result is a "Tyndall blue" structural color that varies with external lighting conditions.

The inheritance pattern followed by blue eyes was previously assumed to be a mendelian recessive trait, however, eye color inheritance is now recognized as a polygenic trait, meaning that it is controlled by the interactions of several genes.[54]

In 2008 team of researchers from the University of Copenhagen have located a single mutation that causes the phenomenon of blue eyes. The research was published in the Journal of Human Genetics. The same DNA sequence of the OCA2 gene among blue-eyed people suggests they may have a single common ancestor. The researchers hypothesized that the OCA2 mutation responsible for blue eyes arose in an individual who lived in the northwestern part of the Black Sea region in Europe sometime between 6,000 and 10,000 years ago during the Neolithic period.[55][56] However, more recent ancient DNA research has identified human remains much older than the Neolithic period with the OCA2 mutation for blue eyes. It is now believed that the OCA2 allele dates back to the recent migration of modern humans out of Africa roughly 50,000 years ago, and entered Europe from western Asia.[17]

Eiberg and colleagues suggested 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.[55][57][58]

Blue eyes are predominant in northern and eastern Europe, particularly around the Baltic Sea. Blue eyes are also found in southern Europe, Central Asia, South Asia, North Africa and West Asia.[59][60]

 
Actor Daniel Craig has the most common eye color in the U.K. as of 2014: (blue: 48%, green: 30%, brown: 22%).[62]


Approximately 8% to 10% of the global population have blue eyes.[26] A 2002 study found that the prevalence of blue eye color among the white population 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.[14] As of 2006, one out of every six Americans, or 16.6% of the total US population, has blue eyes,[63] including 22.3% of whites. The incidence of blue eyes continues to decline among American children.[64] Of Slovenes, 56% have blue/green eyes.[65]

 
Sclater's lemur, also known as the blue-eyed black lemur

Gray

 
Gray eyes

Like blue eyes, gray eyes have a dark epithelium at the back of the iris and a relatively clear stroma at the front. One possible explanation for the difference in the appearance of gray and blue eyes is that gray eyes have larger deposits of collagen in the stroma, so that the light that is reflected from the epithelium undergoes Mie scattering (which is not strongly frequency-dependent) rather than Rayleigh scattering (in which shorter wavelengths of light are scattered more). This would be analogous to the change in the color of the sky, from the blue given by the Rayleigh scattering of sunlight by small gas molecules when the sky is clear, to the gray caused by Mie scattering of large water droplets when the sky is cloudy.[66] Alternatively, it has been suggested that gray and blue eyes might differ in the concentration of melanin at the front of the stroma.[66]

Gray eyes can also be found among the Algerian Shawia people[67] of the Aurès Mountains in Northwest Africa, in the Middle East/West Asia, Central Asia, and South Asia. The Greek goddess Athene appears with gray eyes (γλαυκῶπις).[68] Under magnification, gray eyes exhibit small amounts of yellow and brown color in the iris.

Gray is the second-rarest natural eye color after green, with 3% of the world's population having it.[69]

Special cases

Red and violet

 
Red-appearing albino eyes

The eyes of people with severe forms of albinism may appear red under certain lighting conditions owing to the extremely low quantities of melanin,[70] allowing the blood vessels to show through. In addition, flash photography can sometimes cause a "red-eye effect", in which the very bright light from a flash reflects off the retina, which is abundantly vascular, causing the pupil to appear red in the photograph.[71]

Although the deep blue eyes of some people such as Elizabeth Taylor can appear purple or violet at certain times, "true" violet-colored eyes occur only due to albinism.[72][73][74] Eyes that appear red or violet under certain conditions due to albinism are less than 1 percent of the world's population.[75]

Two different colors

 
Two different eye colors are known as heterochromia iridum

As a result of heterochromia iridum, it is also possible to have two different eye colors. This occurs in humans and certain breeds of domesticated animals and affects less than 1 percent of the world's population.[75]

Medical implications

The most important role of melanin in the iris is to protect the eyes from the sun's harmful rays.[76] People with lighter eye colors, such as blue or green, have lessened protection from the sun, and so need greater protection from the sun's rays than those with darker eye colors.[citation needed]

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;[41] lighter eye color is also associated with an increased risk of ARMD progression.[77] A gray iris may indicate the presence of a uveitis, and an increased risk of uveal melanoma has been found in those with blue, green or gray eyes.[78][79] However, a study in 2000 suggests that people with dark brown eyes are at increased risk of developing cataracts and therefore should protect their eyes from direct exposure to sunlight.[80]

Wilson's disease

Wilson's disease involves a mutation of the gene coding for the enzyme ATPase 7B, which prevents copper within the liver from entering the Golgi apparatus in cells. Instead, the copper accumulates in the liver and in other tissues, including the iris of the eye. This results in the formation of Kayser–Fleischer rings, which are dark rings that encircle the periphery of the iris.[81]

Coloration of the sclera

Eye color outside of the iris may also be symptomatic of disease. Yellowing of the sclera (the "whites of the eyes") is associated with jaundice,[82] and may be symptomatic of liver diseases such as cirrhosis or hepatitis.[83] A blue coloration of the sclera may also be symptomatic of disease.[82]

Aniridia

Main article: Aniridia

Aniridia is a congenital condition characterized by an extremely underdeveloped iris, which appears absent on superficial examination.[84]

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 iris 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 a total lack of melanin. Transillumination defects can almost always be observed during an eye examination due to lack of iridial pigmentation.[85] 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 more pronounced in albino individuals, and this can emphasize the red eye effect in photographs.

Heterochromia

Main article: Heterochromia iridum
 
An example of complete heterochromia. The subject has one brown eye and one hazel eye.
 
An example of sectoral heterochromia. The subject has a blue iris with a brown section.

Heterochromia (heterochromia iridum or heterochromia iridis) is an eye condition in which one iris is a different color from the other (complete heterochromia), or where a 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.[86] This uncommon condition usually results due to uneven melanin content. A number of causes are responsible, including genetic, such as chimerism, Horner's syndrome and Waardenburg syndrome.

A chimera can have two different colored eyes just like any two siblings can—because each cell has different eye color genes. A mosaic can have two different colored eyes if the DNA difference happens to be in an eye-color gene.

There are many other possible reasons for having two different-colored eyes. For example, the film actor Lee Van Cleef was born with one blue eye and one green eye, a trait that reportedly was common in his family, suggesting that it was a genetic trait. This anomaly, which film producers thought would be disturbing to film audiences, was "corrected" by having Van Cleef wear brown contact lenses.[87] David Bowie, on the other hand, had the appearance of different eye colors due to an injury that caused one pupil to be permanently dilated.

Another hypothesis about heterochromia is that it can result from a viral infection in utero affecting the development of one eye, possibly through some sort of genetic mutation. Occasionally, heterochromia can be a sign of a serious medical condition.

A common cause in females with heterochromia is X-inactivation, which can result in a number of heterochromatic traits, such as calico cats. Trauma and certain medications, such as some prostaglandin analogues, can also cause increased pigmentation in one eye.[88] On occasion, a difference in eye color is caused by blood staining the iris after injury.

Impact on vision

Although people with lighter eye color are generally more sensitive to light because they have less pigment in the iris to protect them from sunlight, there is little to no evidence that eye color has a direct impact on vision qualities such as visual acuity.[89] However, there is a study that found that dark-eyed people perform better at "reactive-type tasks", which suggests they may have better reaction times.[90] People with light-colored eyes, however, performed better at so-called "self-paced tasks", which include activities like hitting a golf ball or throwing baseballs.[90] In another study, people with darker eyes performed better at hitting racquetballs.[91] There are also other studies that challenge these findings.[92] According to scientists, more study is needed to verify these results.[89]

Classification of color

Iris color can provide a large amount of information about a person, and a classification of colors may be useful in documenting pathological changes or determining how a person may respond to ocular pharmaceuticals.[93] Classification systems have ranged from a basic light or dark description to detailed gradings employing photographic standards for comparison.[93] Others have attempted to set objective standards of color comparison.[94]

The Martin–Schultz scale, developed from the Martin scale, is one standard color scale used in physical anthropology to establish the eye color of an individual. It was created by the anthropologists Rudolf Martin and Bruno K Schultz in the first half of the 20th century. The scale consists of 20 colors[95] ranging from light blue to dark brown-black, corresponding to natural eye colors caused by the amount of melanin in the iris:[96][97]

Normal eye colors range from the darkest shades of brown to the lightest tints of blue.[13] To meet the need for standardized classification, at once simple yet detailed enough for research purposes, Seddon et al. developed a graded system based on the predominant iris color and the amount of brown or yellow pigment present.[98] There are three pigment colors that determine, depending on their proportion, the outward appearance of the iris, along with structural color. Green irises, for example, have some yellow and the blue structural color. Brown irises contain more or less melanin. Some eyes have a dark ring around the iris, called a limbal ring.

Eye color in non-human animals is regulated differently. For example, instead of blue as in humans, autosomal recessive eye color in the skink species Corucia zebrata is black, and the autosomal dominant color is yellow-green.[99]

As the perception of color depends on viewing conditions (e.g., the amount and kind of illumination, as well as the hue of the surrounding environment), so does the perception of eye color.[100]

See also

References

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April 30, 2023
color, iris, color, redirects, here, shade, purple, iris, color, polygenic, phenotypic, trait, determined, factors, pigmentation, iris, frequency, dependence, scattering, light, turbid, medium, stroma, iris, close, gray, human, iris, humans, pigmentation, iris. Iris color redirects here For the shade of purple see Iris color Eye color is a polygenic phenotypic trait determined by two factors the pigmentation of the eye s iris 1 2 and the frequency dependence of the scattering of light by the turbid medium in the stroma of the iris 3 9 Close up of a gray human iris In humans the pigmentation of the iris varies from light brown to black depending on the concentration of melanin in the iris pigment epithelium located on the back of the iris the melanin content within the iris stroma located at the front of the iris and the cellular density of the stroma 4 The appearance of blue green and hazel eyes results from the Tyndall scattering of light in the stroma a phenomenon similar to Rayleigh scattering which accounts for the blue sky 5 Neither blue nor green pigments are present in the human iris or vitreous humour 3 6 This is an example of structural color which depends on the lighting conditions especially for lighter colored eyes The brightly colored eyes of many bird species result from the presence of other pigments such as pteridines purines and carotenoids 7 Humans and other animals have many phenotypic variations in eye color 8 The genetics and inheritance of eye color in humans is complicated As of 2010 update as many as 16 genes have been associated with eye color inheritance Some of the eye color genes include OCA2 and HERC2 9 The earlier belief that blue eye color is a simple recessive trait has been shown to be incorrect The genetics of eye color are so complex that almost any parent child combination of eye colors can occur 10 11 However OCA2 gene polymorphism close to proximal 5 regulatory region explains most human eye color variation 12 Contents 1 Genetic determination 2 Changes in eye color 3 Eye color range 3 1 Brown 3 2 Amber 3 3 Hazel 3 4 Green 3 5 Blue 3 6 Gray 3 7 Special cases 3 7 1 Red and violet 3 7 2 Two different colors 4 Medical implications 4 1 Wilson s disease 4 2 Coloration of the sclera 4 3 Aniridia 4 4 Ocular albinism and eye color 4 5 Heterochromia 5 Impact on vision 6 Classification of color 7 See also 8 References 9 External linksGenetic determinationEye color is an inherited trait determined by multiple genes 13 14 These genes are sought by studying small changes in the genes themselves and in neighboring genes called single nucleotide polymorphisms or SNPs The total number of genes that contribute to eye color is unknown but there are a few likely candidates A study in Rotterdam 2009 found that it was possible to predict eye color with more than 90 accuracy for brown and blue using just six SNPs 15 16 People of European descent show the greatest variety in eye color of any population worldwide Recent advances in ancient DNA technology have revealed some of the history of eye color in Europe Through the analysis of ancient DNA a 2020 study published in Experimental Dermatology suggested that the common gene for blue eye color likely originated in the Near East and arrived in Europe around 42 000 years ago after the exodus out of Africa 17 There is evidence that as many as 16 different genes could be responsible for eye color in humans however the main two genes associated with eye color variation are OCA2 and HERC2 and both are localized in Chromosome 15 9 The gene OCA2 OMIM 203200 when in a variant form 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 blue 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 12 A specific mutation within the HERC2 gene a gene that regulates OCA2 expression is partly responsible for blue eyes 18 Other genes implicated in eye color variation are SLC24A4 19 and TYR 19 A 2010 study of eye color variation in hue and saturation values using high resolution digital full eye photographs found three new loci for a total of ten genes allowing the explanation of about 50 of eye color variation 20 Gene name Effect on eye colorOCA2 Associated with melanin producing cells Central importance to eye color HERC2 Affects function of OCA2 with a specific mutation strongly linked to blue eyes SLC24A4 Associated with differences between blue and green eyes 19 TYR Associated with differences between blue and green eyes 19 Blue eyes with a brown spot green eyes and gray eyes are caused by an entirely different part of the genome Changes in eye color Percentage of light eyes in and near Europe according to anthropologist Peter Frost 21 80 50 79 20 49 1 19 Most newborn babies who have European ancestry have light colored eyes As the child develops melanocytes cells found within the iris of human eyes as well as skin and hair follicles slowly begin to produce melanin Because melanocyte cells continually produce pigment in theory eye color can be changed Adult eye color is usually established between 3 and 6 months of age though this can be later 22 Observing the iris of an infant from the side using only transmitted light with no reflection from the back of the iris it is possible to detect the presence or absence of low levels of melanin An iris that appears blue under this method of observation is more likely to remain blue as the infant ages An iris that appears golden contains some melanin even at this early age and is likely to turn from blue to green or brown as the infant ages Changes lightening or darkening of eye color during early childhood puberty pregnancy and sometimes after serious trauma like heterochromia suggest that eyes sometimes change color under certain circumstances due to chemical reactions or hormonal changes within the body Studies on Caucasian twins both fraternal and identical have shown that eye color over time can be subject to change and major demelanization of the iris may also be genetically determined Most eye color changes have been observed or reported in the Caucasian population with hazel and amber eyes 23 Under the same environmental conditions there may be disagreement over the color of an object between two different people further explanation needed the factor that causes this discrepancy is the presence of melanin in the iris which is the main factor in determining eye color The higher the amount of melanin in the iris and the denser the texture of the melanin the darker the color of a person s eyes the same melanin concentration also depends on many factors such as hereditary and environmental ones The human eye consists of two types of light and color receptors in the retina Cylindrical cells are the photoreceptors of the eye that have a black and white vision and depending on the amount of light received from the environment determine the amount of darkness and brightness of objects The number of cylindrical cells is more than the number of color receptors and reaches about 120 million cone cells which are smaller in number than light receptors have color vision and are divided into three distinct categories each of which recognizes one of the colors blue red and green allowing the individual to distinguish colors 24 Eye color rangeBrown Brown eyes redirects here For other uses see Brown eyes disambiguation Dark brown iris is common in Europe Americas Africa West Asia East Asia and Southeast Asia Light brown iris can be found in Europe West Asia South Asia Central Asia and among the Americas Almost all mammals have brown or darkly pigmented irises 25 In humans brown is by far the most common eye color with approximately 79 of people in the world having it 26 Brown eyes result from a relatively high concentration of melanin in the stroma of the iris which causes light of both shorter and longer wavelengths to be absorbed 27 Dark brown eyes are dominant in humans 28 In many parts of the world it is nearly the only iris color present 29 Brown eyes are common in Europe East Asia Southeast Asia Central Asia South Asia West Asia Oceania Africa and the Americas 19 Light or medium pigmented brown eyes can also be commonly found in South Europe among the Americas and parts of Central Asia West Asia and South Asia Amber Amber eye Amber eyes are a solid color with a strong yellowish golden and russet coppery tint which may be due to the yellow pigment called lipochrome also found in green eyes 30 31 Amber eyes should not be confused with hazel eyes Although hazel eyes may contain specks of amber or gold they usually tend to have 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 Even though amber is similar to gold some people have russet or copper colored amber eyes that are mistaken for hazel though hazel tends to be duller and contains green with red gold flecks as mentioned above Amber eyes may also contain amounts of very light gold ish gray The eyes of some pigeons contain yellow fluorescing pigments known as pteridines 32 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 33 In humans yellowish specks or patches are thought to be due to the pigment lipofuscin also known as lipochrome 34 Many animals such as canines domestic cats owls eagles pigeons and fish have amber eyes whereas in humans this color occurs less frequently Amber is the third rarest natural eye color after green and gray in 5 of the world s population 35 People with that eye color are found in Spain Balkan region Hungary Italy Southern Cone Colombia Pakistan and Middle East 36 Hazel Hazel eyes Hazel eye Hazel eyes are due to a combination of Rayleigh scattering and a moderate amount of melanin in the iris anterior border layer 4 34 Hazel eyes often appear to shift in color from a brown to a green Although hazel mostly consists of brown and green the dominant color in the eye can either be brown gold or green This is why hazel eyes can be mistaken as amber and vice versa 37 38 39 40 41 42 43 The combination can sometimes produce a multicolored iris i e an eye that is light brown amber near the pupil and charcoal or dark green on the outer part of the iris or vice versa when observed in sunlight Definitions of the eye color hazel vary it is sometimes considered to be synonymous with light brown or gold as in the color of a hazelnut shell 37 39 42 44 Around 18 of the US population and 5 of the world population have hazel eyes 26 Hazel eyes are more common in Spaniards North Africa the Middle East and Brazil 45 Green This section may contain misleading parts Please help clarify this article according to any suggestions provided on the talk page Green eyes A Chinchilla Persian with sea green eyes Green eyes probably result from the interaction of multiple variants within the OCA2 and other genes They were present in south Siberia during the Bronze Age 46 Green hazel eyes Green eyes are most common in Northern Western and Central Europe 47 48 In Scotland 29 of people have green eyes 49 Around 8 10 of men and 18 21 of women in Iceland and 6 of men and 17 of women in the Netherlands have green eyes 50 Among European Americans green eyes are most common among those of recent Celtic and Germanic ancestry with about 16 51 The green color is caused by the combination of 1 an amber or light brown pigmentation in the stroma of the iris which has a low or moderate concentration of melanin with 2 a blue shade created by the Rayleigh scattering of reflected light 27 Green eyes contain the yellowish pigment lipochrome 52 Blue Blue eyes redirects here For other uses see Blue eyes disambiguation A light blue iris with limbal ring There is no blue pigmentation either in the iris or in the vitreous body Dissection reveals that the iris pigment epithelium is brownish black due to the presence of melanin 53 Unlike brown eyes blue eyes have low concentrations of melanin in the stroma of the iris which lies in front of the dark epithelium Longer wavelengths of light tend to be absorbed by the dark underlying epithelium while shorter wavelengths are reflected and undergo Rayleigh scattering in the turbid medium of the stroma 4 This is the same scattering that accounts for the blue appearance of the sky 3 9 6 The result is a Tyndall blue structural color that varies with external lighting conditions The inheritance pattern followed by blue eyes was previously assumed to be a mendelian recessive trait however eye color inheritance is now recognized as a polygenic trait meaning that it is controlled by the interactions of several genes 54 In 2008 team of researchers from the University of Copenhagen have located a single mutation that causes the phenomenon of blue eyes The research was published in the Journal of Human Genetics The same DNA sequence of the OCA2 gene among blue eyed people suggests they may have a single common ancestor The researchers hypothesized that the OCA2 mutation responsible for blue eyes arose in an individual who lived in the northwestern part of the Black Sea region in Europe sometime between 6 000 and 10 000 years ago during the Neolithic period 55 56 However more recent ancient DNA research has identified human remains much older than the Neolithic period with the OCA2 mutation for blue eyes It is now believed that the OCA2 allele dates back to the recent migration of modern humans out of Africa roughly 50 000 years ago and entered Europe from western Asia 17 Eiberg and colleagues suggested 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 55 57 58 Blue eyes are predominant in northern and eastern Europe particularly around the Baltic Sea Blue eyes are also found in southern Europe Central Asia South Asia North Africa and West Asia 59 60 A Birman kitten with distinctive sapphire blue eyes Jungle crow The first blue eyed koala known to be born in captivity 61 Actor Daniel Craig has the most common eye color in the U K as of 2014 update blue 48 green 30 brown 22 62 Approximately 8 to 10 of the global population have blue eyes 26 A 2002 study found that the prevalence of blue eye color among the white population 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 14 As of 2006 update one out of every six Americans or 16 6 of the total US population has blue eyes 63 including 22 3 of whites The incidence of blue eyes continues to decline among American children 64 Of Slovenes 56 have blue green eyes 65 Sclater s lemur also known as the blue eyed black lemur Gray Gray eyes Like blue eyes gray eyes have a dark epithelium at the back of the iris and a relatively clear stroma at the front One possible explanation for the difference in the appearance of gray and blue eyes is that gray eyes have larger deposits of collagen in the stroma so that the light that is reflected from the epithelium undergoes Mie scattering which is not strongly frequency dependent rather than Rayleigh scattering in which shorter wavelengths of light are scattered more This would be analogous to the change in the color of the sky from the blue given by the Rayleigh scattering of sunlight by small gas molecules when the sky is clear to the gray caused by Mie scattering of large water droplets when the sky is cloudy 66 Alternatively it has been suggested that gray and blue eyes might differ in the concentration of melanin at the front of the stroma 66 Gray eyes can also be found among the Algerian Shawia people 67 of the Aures Mountains in Northwest Africa in the Middle East West Asia Central Asia and South Asia The Greek goddess Athene appears with gray eyes glaykῶpis 68 Under magnification gray eyes exhibit small amounts of yellow and brown color in the iris Gray is the second rarest natural eye color after green with 3 of the world s population having it 69 Special cases Red and violet Red appearing albino eyes The eyes of people with severe forms of albinism may appear red under certain lighting conditions owing to the extremely low quantities of melanin 70 allowing the blood vessels to show through In addition flash photography can sometimes cause a red eye effect in which the very bright light from a flash reflects off the retina which is abundantly vascular causing the pupil to appear red in the photograph 71 Although the deep blue eyes of some people such as Elizabeth Taylor can appear purple or violet at certain times true violet colored eyes occur only due to albinism 72 73 74 Eyes that appear red or violet under certain conditions due to albinism are less than 1 percent of the world s population 75 Two different colors Two different eye colors are known as heterochromia iridum As a result of heterochromia iridum it is also possible to have two different eye colors This occurs in humans and certain breeds of domesticated animals and affects less than 1 percent of the world s population 75 Medical implicationsThe most important role of melanin in the iris is to protect the eyes from the sun s harmful rays 76 People with lighter eye colors such as blue or green have lessened protection from the sun and so need greater protection from the sun s rays than those with darker eye colors citation needed 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 41 lighter eye color is also associated with an increased risk of ARMD progression 77 A gray iris may indicate the presence of a uveitis and an increased risk of uveal melanoma has been found in those with blue green or gray eyes 78 79 However a study in 2000 suggests that people with dark brown eyes are at increased risk of developing cataracts and therefore should protect their eyes from direct exposure to sunlight 80 Wilson s disease A Kayser Fleischer ring in a patient with Wilson s disease Wilson s disease involves a mutation of the gene coding for the enzyme ATPase 7B which prevents copper within the liver from entering the Golgi apparatus in cells Instead the copper accumulates in the liver and in other tissues including the iris of the eye This results in the formation of Kayser Fleischer rings which are dark rings that encircle the periphery of the iris 81 Coloration of the sclera Eye color outside of the iris may also be symptomatic of disease Yellowing of the sclera the whites of the eyes is associated with jaundice 82 and may be symptomatic of liver diseases such as cirrhosis or hepatitis 83 A blue coloration of the sclera may also be symptomatic of disease 82 Aniridia Main article Aniridia Aniridia is a congenital condition characterized by an extremely underdeveloped iris which appears absent on superficial examination 84 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 iris 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 a total lack of melanin Transillumination defects can almost always be observed during an eye examination due to lack of iridial pigmentation 85 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 more pronounced in albino individuals and this can emphasize the red eye effect in photographs Heterochromia Main article Heterochromia iridum This section needs additional citations for verification Please help improve this article by adding citations to reliable sources in this section Unsourced material may be challenged and removed November 2012 Learn how and when to remove this template message An example of complete heterochromia The subject has one brown eye and one hazel eye An example of sectoral heterochromia The subject has a blue iris with a brown section Heterochromia heterochromia iridum or heterochromia iridis is an eye condition in which one iris is a different color from the other complete heterochromia or where a 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 86 This uncommon condition usually results due to uneven melanin content A number of causes are responsible including genetic such as chimerism Horner s syndrome and Waardenburg syndrome A chimera can have two different colored eyes just like any two siblings can because each cell has different eye color genes A mosaic can have two different colored eyes if the DNA difference happens to be in an eye color gene There are many other possible reasons for having two different colored eyes For example the film actor Lee Van Cleef was born with one blue eye and one green eye a trait that reportedly was common in his family suggesting that it was a genetic trait This anomaly which film producers thought would be disturbing to film audiences was corrected by having Van Cleef wear brown contact lenses 87 David Bowie on the other hand had the appearance of different eye colors due to an injury that caused one pupil to be permanently dilated Another hypothesis about heterochromia is that it can result from a viral infection in utero affecting the development of one eye possibly through some sort of genetic mutation Occasionally heterochromia can be a sign of a serious medical condition A common cause in females with heterochromia is X inactivation which can result in a number of heterochromatic traits such as calico cats Trauma and certain medications such as some prostaglandin analogues can also cause increased pigmentation in one eye 88 On occasion a difference in eye color is caused by blood staining the iris after injury Impact on visionAlthough people with lighter eye color are generally more sensitive to light because they have less pigment in the iris to protect them from sunlight there is little to no evidence that eye color has a direct impact on vision qualities such as visual acuity 89 However there is a study that found that dark eyed people perform better at reactive type tasks which suggests they may have better reaction times 90 People with light colored eyes however performed better at so called self paced tasks which include activities like hitting a golf ball or throwing baseballs 90 In another study people with darker eyes performed better at hitting racquetballs 91 There are also other studies that challenge these findings 92 According to scientists more study is needed to verify these results 89 Classification of colorIris color can provide a large amount of information about a person and a classification of colors may be useful in documenting pathological changes or determining how a person may respond to ocular pharmaceuticals 93 Classification systems have ranged from a basic light or dark description to detailed gradings employing photographic standards for comparison 93 Others have attempted to set objective standards of color comparison 94 The Martin Schultz scale developed from the Martin scale is one standard color scale used in physical anthropology to establish the eye color of an individual It was created by the anthropologists Rudolf Martin and Bruno K Schultz in the first half of the 20th century The scale consists of 20 colors 95 ranging from light blue to dark brown black corresponding to natural eye colors caused by the amount of melanin in the iris 96 97 Normal eye colors range from the darkest shades of brown to the lightest tints of blue 13 To meet the need for standardized classification at once simple yet detailed enough for research purposes Seddon et al developed a graded system based on the predominant iris color and the amount of brown or yellow pigment present 98 There are three pigment colors that determine depending on their proportion the outward appearance of the iris along with structural color Green irises for example have some yellow and the blue structural color Brown irises contain more or less melanin Some eyes have a dark ring around the iris called a limbal ring Eye color in non human animals is regulated differently For example instead of blue as in humans autosomal recessive eye color in the skink species Corucia zebrata is black and the autosomal dominant color is yellow green 99 As the perception of color depends on viewing conditions e g the amount and kind of illumination as well as the hue of the surrounding environment so does the perception of eye color 100 See alsoHair color Iridology Human skin color XanthophoreReferences Wielgus AR Sarna T 2005 Melanin in human irides of different color and age of donors Pigment Cell Res 18 6 454 64 doi 10 1111 j 1600 0749 2005 00268 x PMID 16280011 Prota G Hu DN Vincensi MR McCormick SA Napolitano A 1998 Characterization of melanins in human irides and cultured uveal melanocytes from eyes of different colors Exp Eye Res 67 3 293 9 doi 10 1006 exer 1998 0518 PMID 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3406 bmsap 1968 1417 Seddon JM CR Sahagian RJ Glynn RD Sperduto ES Gragoudas 1 August 1990 Evaluation of an iris color classification system Investigative Ophthalmology amp Visual Science 31 8 1592 8 PMID 2201662 Retrieved 19 October 2011 Jones S L Schnirel B L 2006 Subspecies comparison of the Genus Corucia Polyphemos 4 1 1 25 Archived from the original on 2 February 2009 Color Perception Archived 20 October 2006 at the Wayback Machine Edromanguitars com Retrieved on 23 December 2011 External links Wikimedia Commons has media related to Eyes by color genetics thetech org Eye Color and Human Diseases Retrieved from https en wikipedia org w index php title Eye color amp oldid 1150194890, wikipedia, wiki, book, books, library,

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