Brown hair with purple tint 2018

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

Close up of a /green human .

Eye color is a determined by two distinct factors: the of the 's and the frequency-dependence of the of light by the medium in the .:9

In humans, the pigmentation of the iris varies from light brown to black, depending on the concentration of in the (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. The appearance of blue and green, as well as hazel eyes, results from the of light in the stroma, a phenomenon similar to that which accounts for the blueness of the sky called . Neither blue nor green pigments are ever present in the human iris or ocular fluid. Eye color is thus an instance of and varies depending 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 , , and . Humans and other animals have many variations in eye color. The genetics of eye color are complicated, and color is determined by multiple genes. So far, as many as 15 genes have been associated with eye color inheritance. Some of the eye-color genes include and . The earlier belief that blue eye color is a simple 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. However, OCA2 , close to proximal , explains most human eye-color variation.

Contents

Genetic determination

See also:

Eye color is an inherited trait influenced by more than one . These genes are sought using associations to small changes in the genes themselves and in neighboring genes. These changes are known as or SNPs. The actual number of genes that contribute to eye color is currently unknown, but there are a few likely candidates. A study in (2009) found that it was possible to predict eye color with more than 90% accuracy for brown and blue using just six SNPs. 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 and , and both are localized in .

The gene OCA2 (OMIM: ), when in a variant form, causes the pink eye color and common in human . (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 , counts, and . The polymorphisms may be in an OCA2 , where they may influence the expression of the gene product, which in turn affects pigmentation. A specific mutation within the HERC2 gene, a gene that regulates OCA2 expression, is partly responsible for blue eyes. Other genes implicated in eye color variation are and . A 2010 study on eye color variation into hue and saturation values using high-resolution digital full-eye photographs found three new loci for a total of ten genes, and now about 50% of eye colour variation can be explained.

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

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

Ancient DNA and eye color in Europe

People of European descent show the greatest variety in eye color of any population worldwide. Recent advances in technology have revealed some of the history of eye color in Europe. All European hunter-gatherer remains so far investigated have shown genetic markers for light-colored eyes, in the case of western and central European hunter-gatherers combined with dark skin color. The later additions to the European , the Early farmers from and the / pastoralists (possibly the population) from the area north of the appear to have had much higher incidences of dark eye color alleles, and alleles giving rise to lighter skin, than the original European population.

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. Classification systems have ranged from a basic light or dark description to detailed gradings employing photographic standards for comparison. Others have attempted to set objective standards of color comparison.

Eye colors range from the darkest shades of brown to the lightest tints of blue. 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. There are three pigment colors that determine, depending on their proportion, the outward appearance of the iris, along with . Green irises, for example, have blue and some yellow. Brown irises contain mostly brown. Some eyes have a dark ring around the iris, called a .

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

As the 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.

Changes in eye color

Percentage of light eyes in and near Europe according to anthropologist Robert Frost.

Most new-born 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 . 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. 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 colors during early childhood, puberty, pregnancy, and sometimes after serious trauma (like ) do represent cause for a plausible argument stating that some eyes can or do change, based on chemical reactions and 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.

Eye color chart (Martin scale)

created a chart by the original . The numbering is reversed on the scale below in the (later) , which is (still) used in .

Light and light-mixed eyes (16–12 in Martin scale)

Pure light (16–15 in Martin scale)

  • 16: pure light blue
  • 15: gray

Light-mixed (14–12 in Martin scale)

  • 14: Very light-mixed (blue with gray or green or green with gray)
  • 13-12: Light-mixed (light or very light-mixed with small admixture of brown)

Mixed eyes (11–7 in Martin scale) Mixture of light eyes (blue, gray or green) with brown when light and brown appearance is at the same level.

Dark and dark-mixed eyes (6–1 in Martin scale)

  • Dark-mixed: 6–5 in Martin scale. Brown with small admixture of light
  • Dark: 4–1 in Martin scale. Brown (light brown and dark brown) and very dark brown (almost black)

Amber

Amber eyes in sunlight – displaying an orange color rather than brown

Amber eyes are of a solid color and have a strong yellowish/golden and russet/coppery tint. This may be due to the deposition of the yellow pigment called in the iris (which is also found in green eyes). Amber eyes should not be confused with hazel eyes; although hazel eyes may contain specks of amber or gold, they usually tend to comprise 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 considered to be like gold, some people have russet or copper colored amber eyes that many people mistake 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 . The bright yellow eyes of the are thought to be due to the presence of the pteridine pigment within certain (called xanthophores) located in the iris stroma. In humans, yellowish specks or patches are thought to be due to the pigment , also known as lipochrome. Many animals such as canines, domestic cats, owls, eagles, pigeons and fish have amber eyes as a common color, whereas in humans this color occurs less frequently.

Blue

"Blue eyes" redirects here. For other uses, see .

A light blue iris

There is no blue pigmentation either in the iris or in the ocular fluid. Dissection reveals that the iris pigment is brownish black due to the presence of . Unlike brown eyes, blue eyes have low concentrations of 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 in the medium of the stroma. This is the same frequency-dependence of scattering that accounts for the blue appearance of the sky.:9 The result is a " blue" that varies with external lighting conditions.

In humans, the inheritance pattern followed by blue eyes is considered similar to that of a trait (in general, eye color inheritance is considered a , meaning that it is controlled by the interactions of several genes, not just one). In 2008, new research tracked down a single genetic mutation that leads to blue eyes. "Originally, we all had brown eyes," said Eiberg. Eiberg and colleagues suggested in a study published in Human Genetics that a mutation in the 86th of the gene, which is hypothesized to interact with the OCA2 gene , reduced expression of OCA2 with subsequent reduction in melanin production. The authors suggest that the mutation may have arisen in the northwestern part of the region, but add that it is "difficult to calculate the age of the mutation."

Blue eyes are common in northern and eastern Europe, particularly around the . Blue eyes are also found in southern Europe, , , and . In West Asia, a proportion of Israelis are of origin, among whom the trait is relatively elevated (a study taken in 1911 found that 53.7% of Ukrainian Jews had blue eyes).

  • The first blue-eyed known to be born in captivity.

The same DNA sequence in the region of the OCA2 gene among blue-eyed people suggests they may have a single common ancestor.

DNA studies on ancient human remains confirm that light skin, hair and eyes were present at least tens of thousands of years ago on , who lived in Eurasia for 500,000 years. As of 2016, the earliest light-pigmented and blue-eyed remains of were found in 7,700 years old Mesolithic from , .

A 2002 study found that the prevalence of blue eye color among the in the United States to be 33.8% for those born from 1936 through 1951 compared with 57.4 percent for those born from 1899 through 1905. As of 2006, one out of every six people, or 16.6% of the total population, and 22.3% of whites, has blue eyes. Blue eyes are continuing to become less common among American children.

Blue eyes are rare in mammals; one example is the recently discovered marsupial, the (Spilocuscus wilsoni). The trait is hitherto known only from a single other than humans – (Eulemur flavifrons) of Madagascar. While some and have blue eyes, this is usually due to another mutation that is . But in cats alone, there are four identified gene mutations that produce blue eyes, some of which are associated with . The mutation found in the is associated with strabismus (crossed eyes). The mutation found in blue-eyed solid white cats (where the coat color is caused by the gene for "epistatic white") is linked with deafness. However, there are phenotypically identical, but genotypically different, blue-eyed white cats (where the coat color is caused by the gene for white spotting) where the coat color is not strongly associated with deafness. In the blue-eyed breed, there may be other neurological defects. Blue-eyed non-white cats of unknown genotype also occur at random in the cat population.

Brown

"Brown eyes" redirects here. For other uses, see .

In humans, 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.

Dark brown eyes are dominant in humans and in many parts of the world, it is nearly the only iris color present. Dark pigment of brown eyes is common in , , , , , , , , , , etc. as well as parts of and . The majority of people in the world overall have brown eyes to dark brown eyes.

Light or medium-pigmented brown eyes can also be commonly found in , among the , and parts of Central ( and ).

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 in the stroma, so that the light that is reflected from the epithelium undergoes (which is not strongly frequency-dependent) rather than (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. Alternatively, it has been suggested that gray and blue eyes might differ in the concentration of melanin at the front of the stroma.

Gray eyes are most common in Northern and Eastern Europe. Gray eyes can also be found among the of the in Northwest Africa, in the , , and . Under magnification, gray eyes exhibit small amounts of yellow and brown color in the iris.

Green

Green eye Green eyes

As with blue eyes, the color of green eyes does not result simply from the pigmentation of the iris. 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 of reflected light. Green eyes contain the yellowish pigment .

Green eyes probably result from the interaction of multiple variants within the and other genes. They were present in south during the .

Green-Hazel eyes

They are most common in , and . In and 14% of people have brown eyes and 86% have either blue or green eyes, In , 89% of women and 87% of men have either blue or green eye color. A study of Icelandic and Dutch adults found green eyes to be much more prevalent in women than in men. Among , green eyes are most common among those of recent and ancestry, about 16%.[] 37.2% of Italians from and 56% of Slovenes have blue/green eyes.

Green eyes are common in cats as well as the and its shorthaired equivalents are notable for their black-rimmed sea-green eyes.

Hazel

Hazel eyes Hazel eye

Hazel eyes are due to a combination of and a moderate amount of melanin in the iris' anterior border layer. 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 how many people mistake hazel eyes to be amber and vice versa. This 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 shell.

Hazel eyes occur throughout populations, in particular in regions where blue, green and brown eyed peoples are intermixed.

Red and violet

"Red" albino eyes

The eyes of people with severe forms of may appear red under certain lighting conditions owing to the extremely low quantities of , allowing the blood vessels to show through. In addition, can sometimes cause a "", 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. Although the deep blue eyes of some people such as can appear violet at certain times, "true" violet-colored eyes occur only due to albinism.

Spectrum of eye color

  • Eye55.jpg
  • Blue-green eye closeup.JPG
  • Eye Central Heterochromia crop and lighter.jpg
  • Humaniris.jpg
  • Hazel Eyes, Caucasian Male, Age 23.jpg
  • Human Iris JD052007.jpg
  • Greeneye.jpg
  • Hazel green eye close up.jpg
  • Hazel eye1.png
  • Green Eye.jpg
  • Green light mixed type B.JPG
  • Eye big.JPG
  • Amber Brown Green.jpg
  • Amber eye1.jpg
  • Menschliches auge.jpg
  • MyStrangeIris.JPG
  • Indianeye.png
  • Lens5.jpg

Medical implications

Those with lighter iris color have been found to have a higher prevalence of (ARMD) than those with darker iris color; lighter eye color is also associated with an increased risk of ARMD progression. A gray iris may indicate the presence of a , and an increased risk of has been found in those with blue, green or gray eyes. However, a study in 2000 suggests that people with dark brown eyes are at increased risk of developing and therefore should protect their eyes from direct exposure to sunlight.

Wilson's disease

involves a mutation of the gene coding for the enzyme 7B, which prevents within the liver from entering the 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 , which are dark rings that encircle the periphery of the iris.

Coloration of the sclera

Eye color outside of the iris may also be symptomatic of disease. Yellowing of the (the "whites of the eyes") is associated with , and may be symptomatic of liver diseases such as or . A blue coloration of the sclera may also be symptomatic of disease. In general, any sudden changes in the color of the sclera should be addressed by a medical professional.[]

Aniridia

Main article:

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

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 , 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. defects can almost always be observed during an due to lack of iridial pigmentation. 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 is much more pronounced in albino individuals, and this can emphasize the in photographs.

Heterochromia

Main article:

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 ( 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 within an iris or part of an iris, which may be or acquired by or . This uncommon condition usually results due to uneven content. A number of causes are responsible, including genetic, such as , and .

A can have two different colored eyes just like any two siblings can—because each cell has different eye color genes. A 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 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., 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 , which can result in a number of heterochromatic traits, such as . Trauma and certain medications, such as some , can also cause increased pigmentation in one eye. On occasion, a difference in eye color is caused by blood staining the iris after injury.

Mate selection and traits that have been linked to iris color

Selection for rare iris colors

A study compared the frequency of eye color in commercial advertising models in Brazil and the UK, these countries were chosen because they have inverted frequencies of eye-coloration, with Brazil having an excess of brown and the UK an excess of light-colored eyes. Models are chosen for their attractiveness, and it was found that, in Brazil, models with light eyes are in a significant excess compared to the levels found in the general population, while, in the UK, models with brown or intermediate eyes were in significant excess over their frequency in the general population. This suggests that eye color rarity plays a role in sexual attraction, people with rare eye colors being perceived as being more attractive. Some research indicates that eye color variation is greater in women than in men, which may reflect of mates with rare eye colors.

Selection though imprinting of parental eye color

In contrast to the phenomenon of selection for rarity, scholarship has implied the existence another form of eye color involvement in mate selection. A study found a significant incidence of the partners of heterosexual people possessing similar eye and hair color to that of their opposite-sex parent. This is suggestive of a form of parental imprinting on eventual mate selection.

See also

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