Eye color

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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] In humans, three loci associated with eye color are currently known: EYCL1, EYCL2, and EYCL3.^[8][9] These genes account for three phenotypic eye colors (brown, green, and blue) in humans.^[3] Eye color is demanding, as all four genes must determine the same color for the eye color to be pure; otherwise a mixed color such as blue-green will result.^[6][10] 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 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

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 irises are sometimes said to have whiskey eyes. Brown eyes are also most commonly linked to those with dark brown or black hair colors.

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 of Africa, Asia, and the Americas have brown eyes. Brown eyes are equally found in Europe, Oceania, and North America, 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

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 brown and dark brown.^{[25][26][27][28][29][30][31]} This can sometimes produce a multicolored iris, i.e., an eye that is brown near the pupil and charcoal or amber 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 South and East European nations.

There is some difficulty in defining the eye color "hazel" as it is sometimes considered to be synonymous with brown and other times with green.^{[32][19][25][27][30][33]} They have been described as light brown or yellowish brown,^[34] or as a lighter shade of brown.^[35] Hazel eyes have also been described as being equivalent to a dark black or charcoal-colored/light black eyes.^[36] In North America, "hazel" is often used to describe eyes that change color, ranging from light brown to green and even blue, depending on what color clothing the person is wearing or what color is predominant in their immediate environment. (Eyes that change only between blue and green are not called "hazel"; the term only describes changeable eye color that includes a brownish shade within its range.)

Amber and yellow

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."^{[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. They are most often found in people of Celtic, Germanic and Slavic descent, and to a lesser extent in southern Europe (Portugal, Greece, Italy, Spain) as well as Afghanistan and Iran.^{[citation needed]} Only 2% of the world's population have green eyes, making it the most uncommon color.^{[citation needed]} Deities in both Chinese and Greek mythology had green eyes, possibly because of their rarity.

Blue

Blue eyes are relatively common throughout Europe and other areas with populations of European descent, such as Canada, the United States, Australia and some countries of South America, in lower proportions, like Argentina, Uruguay or Brazil. Throughout the rest of the world they are relatively rare. Approximately 8% of the world's population has blue eyes. Blue eyes are often (but not exclusively) associated with those who have light skin and fairer hair colors, such as blond and red.

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]

Blue eyes are found in people of European descent. Finland and Lithuania have the highest proportions of blue-eyed people; they are also very common in other countries around the Baltic Sea.^{[citation needed]} One survey estimated that nearly 90% of Icelanders have blue or green eyes.^[41] A 2002 study found the prevalence of blue eye color among non-Hispanic 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] apparently due to high birth rate among Italian-Americans and other predominantly brown-eyed whites in the middle of the century, and perhaps also an increased number of southern European immigrants.^{[citation needed]}

Blue is the color of the indole monomer that when polymerised forms melanin.^{[citation needed]} If both alleles for brown eyes (a polymerase gene) are absent or damaged, the blue color remains.^{[citation needed]} The inheritance pattern followed by blue eyes is thought to be similar to that of a recessive trait.^[7] As melanin production generally increases during the first few years of life, the blue eyes of some babies may darken as they get older.

Gray

Gray eyes are a variant of blue eyes. Usually, gray eyes are considered more dominant than blue. This is probably not true, as gray eyes contain the least amount of melanin of any eye color. An example of gray eyes being lighter would be in albinism. OCA1A is the most complete form of albinism, and in that group most have gray eyes. While in group OCA1B (the next less severe form) most have light blue eyes. Under magnification, gray eyes exhibit the least amount of yellow and brown color in the iris. These colors, when mixed with gray, create the blue and green hues. 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-grey" eyes.^[42]

Violet eyes

The appearance of violet eyes is thought to occur due to the mixing of red and blue reflections.^[43] Some albinos have eyes that appear to be violet.^[44] Elizabeth Taylor's physical trademark is her violet eyes.^[45]

Anomalous conditions

Aniridia

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

Ocular albinism

In those with albinism, the color of the irises is typically blue, but can vary from blue to brown (also it is often pink due to no pigment whatsoever); transillumination defects can almost always be observed during an eye examination due to lack of iridial pigmentation.^[47]

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

Heterochromia

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.

Actress Kate Bosworth has one blue eye and one hazel eye. Actor/Rapper Young Rome (also known as Jerome Jones), has one hazel eye and one blue eye, though he wears contacts to cover them up. Lead singer Tim Mcllrath of Rise Against has one brown eye and one blue eye.

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

Typically, caucasian newborns have blue eyes, which change to green, light blue, light brown and 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 have their natural shade. While changes in eye color of infants are more common, even in adults, eye color changes in approximately 12% of the people as they get older.

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.^[49] An increased risk of uveal melanoma has been found in those with blue or grey iris color.^[50]

References

See also

• Iridology
• Hair color
• Human skin color
• Xanthophore

Wikipedia information about Eye color This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Eye color". It may not have been reviewed by professional editors (see full disclaimer). Help contribute to Americola and edit this article.