Eye color

Eye color is a polygenic trait and is determined primarily by the amount and type of pigments 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]

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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 genes 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

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 on viewing conditions (e.g. the amount and type of illumination, as well as the hue of the surrounding environment), so is the perception of eye color.^[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 the eyes in turn depends on how much of these colors are present. For example, green eyes have 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] The above is true for the species Homo sapiens. The iris color can vary in the animal world. Instead of blue in humans, autosomal recessive color in the species Corucia zebrata is black. whereas, the autosomal dominant color is yellow - green {52} ( Subspecies comparison of the Genus: Corucia - Leeway Corucia Research Center - LCRC, Polyphemos, {2006} Volume 4, Issue 1. pp. 1-25.).

Brown

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]

Brown is predominant^[21] and, in many populations, it is (with few exceptions) one of the only iris colors present.^[22] It is least common in the countries around the Baltic Sea.

Those of mixed European and non-European ancestry generally have dark eyes, but with more variability in eye color within their families than those from more homogenous origins.^[23] Inhabitants indigenous to Africa, Asia and the Americas generally have brown eyes. Brown eyes are equally found in Europe and Oceania, though within some European populations they are not predominant to the same extent.

Hazel

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 (and vice versa) when it is open to the elements of the sun/shined in the sunlight. Hazel is mostly found in some regions of the Americas and Europe and certain populations of Asia including Iran and Turkey. Rarely, hazel eyes can be found in people with African or Asian descent.

There is some difficulty in defining the eye color "hazel" as it is sometimes considered to be synonymous with light-brown 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 gray, depending on current lighting in the environment. They are nicknamed "cat eyes."

Amber

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][citation needed]} They, like hazel eyes, are also nicknamed "cat eyes".

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. They are most often found among people of North and Eastern European descent, and to a slightly lesser extent, in Southern Europe, and parts of Central and South Asia.[1][2][3][4]. Green eyes are the rarest eye color; only 1-2% of human population possesses this trait. In Iceland 88% of the population have green or blue eyes. Other ethnically nordic countries like Sweden, Denmark, Germany, Austria, Norway, and the Netherlands [5] have a high percentage of green eyes.

Blue

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 considered 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 the Americas and Oceania. While most common among people of Finno-Ugric descent, there are also rare populations in West Asia, Central and South Asia with fairly uncommon natural occurrences of blue eyes.

Blue eyes are mostly common throughout northern Europe, especially in Sweden, Iceland, Norway, Denmark, Germany, Finland, the Netherlands, Estonia, and Austria. One survey estimated that nearly 90% of Icelanders have blue or green eyes,^[41] while another reported 50-80%, in contrast to 20%-49% 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

Gray eyes are a variant of blue eyes and are sometimes very hard to tell apart, but are most common in Finland and the Baltic States. 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.

A gray iris may indicate the presence of a uveitis. However, other visual signs make a uveitis obvious.

The Greek goddess Athena was renowned for having "owl-gray" (in Greek, γλαυκῶπις – glaukōpis) or "sea-gray" eyes.^[43]

Violet

The appearance of violet eyes is thought to occur from the mixing of red and blue reflections.^[44] Some albinos have eyes that appear violet., though many people with violet eyes are not albino.^[45]

Red

'Red' eyes are also found in an extremely small (<0.001%) percentage of the world's population. This is believed to be a result of either large quantities of the normally scarce red areas in the eye, or a small leakage of blood into the iris; however the number of people on record as having red eyes apart from albinos, is believed not to exceed 20 (Recording of this condition started in 1964). This should be noted as being very different from the 'red-eye' condition found in photography.

Anomalous conditions

Aniridia

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

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

Main article: Heterochromia

Heterochromia (also known as a heterochromia iridis or heterochromia iridium) is an ocular condition in which one iris is a different color from the other iris (complete heterochromia), or where the part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia). It is a result of the relative excess or lack of pigment within an iris or part of an iris, which may be inherited or acquired by disease or injury.^[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. David Bowie (Jones) is a famous person often wrongly attributed with heterochromia. His apparent condition is due to a teenage injury. (One eye appears darker because the pupil is permanently dilated.) American actress Kate Bosworth has sectoral heterochromia, resulting in a hazel section at the bottom of her right blue eye, while the left is completely blue. The lead vocalist of American band Rise Against, Tim McIlrath, has heterochromia; his left eye is blue while his right is brown. American actress Mila Kunis also has heterochromia, resulting in one green eye and one brown eye.

Eye color change

Often, paler newborns have blue eyes, which change to green, hazel, light brown or dark brown, as they grow older. This is probably the origin of the idiom "being blue-eyed" (= naïve; gullible).

It is thought that exposure to light after birth triggers the production of melanin in the iris of the eye. By three years of age, the eyes produce and store enough melanin to indicate their natural shade. While changes in eye color of infants are more common, even in adults, eye color changes are seen, most often as a result of exposure to the sun. Sunlight triggers melanin production in the eye, as it does to the skin.

Eyedrops containing a prostaglandin analogue (such as latanoprost) may result in a permanently darkened iris; these eyedrops are commonly used to treat open-angle glaucoma.^[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]

Eye color can also be symptomatic of disease. In particular, yellow eyes are associated with jaundice and symptomatic of liver disease, including cirrhosis, hepatitis and malaria.

References

• 52. Jones, S.L., Schnirel, B.L., " Subspecies Comparison of the Genus: Corucia" Polyphemos, (2006). pp 1-25.

See also

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