Eye color.html
 
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Eye color is a polygenic trait and is determined by the amount and type of pigments in the eye's iris.12 Humans and animals have many phenotypic variations in eye color.3 In human eyes, 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.25 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 OCA2 gene polymorphism, close to proximal 5′ regulatory region, explains most human eye-color variation. 6

Contents

Genetic determination of eye color

Eye colors can range from the most common color, brown, to the least common, gray. Rare genetic specialties can even lead to unusual eye colors: black, red, and violet. Eye color is an inherited trait influenced by more than one gene.78 These genes are being sought using associations to small changes in the genes themselves and in neighboring genes. These changes are known as single nucleotide polymorphisms or SNPs. The actual number of genes that contribute to eye color is unknown at present, but there are a few likely candidates.

The gene OCA2 (OMIM: 203200), when in a variant form the gene 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.9 A specific mutation within the HERC2 gene, a gene that regulates OCA2 expression, is partly responsible for blue eyes.10 Other genes implicated in eye color variation are: SLC24A4,11 TYR.11

Blue eyes with a brown spot, green eyes and gray eyes are caused by an entirely different part of the genome. As Eiberg said: "The SNP rs12913832 [of theHerc2 gene] is found to be associated with the brown and blue eye color, but this single DNA variation cannot explain all the brown eye color variation from dark brown over hazel to blue eyes with brown spots".

The same study found an associaton between hair color and eye color. The BEY1 pheonotype (Hazel eyes) demonstrated association with blond hair while the BEY2 phenotype (Brown eyes) is linked with dark hair.12

Eye color usually stabilizes when an infant is around 6 months old.13

Synopsis

Schematic representation of different eye colors resulting from different conditions in the iris

Classification of colors

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

Iris color can provide a large amount of information about an individual and a classification of various colors may be useful in documenting pathological changes or determining how a person may respond to various ocular pharmaceuticals.14 Various classification systems have ranged from a basic "light" or "dark" description to detailed gradings employing photographic standards for comparison.14 Others have attempted to set objective standards of color comparison.15

As the perception of color is dependent 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.7 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 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 blue. How much of each color one has determines the appearance of the eye color. The color of the eyes in turn depends on how much of these colors are present. For example, green eyes have yellow and some blue, making them appear green. Brown eyes appear brown because most of the eye contains the brown color. The above is true for 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.17

Amber

Human amber eyes displaying the yellow pigments.

Amber 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).1819. 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.

The eyes of some pigeons contain yellow fluorescing pigments known as pteridines.20 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.21 In humans, yellowish specks or patches are thought to be due to the pigment lipofuscin, also known as lipochrome.22

Blue

A blue eye

Blue eyes contain low amounts of melanin within the iris stroma; longer wavelengths of light tend to be absorbed by the underlying iris pigment epithelium, and shorter wavelengths are reflected and undergo Rayleigh scattering.5 The type of melanin present is eumelanin.23 The inheritance pattern followed by blue eyes is considered similar to that of a recessive trait, however it is a polygenic trait (meaning that it is controlled by the interactions of several genes, not just one).8 Eiberg and colleagues showed 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.12

The authors concluded that the mutation may have arisen in a single individual around the Black Sea region 6,000-10,000 years ago24, perhaps suggesting that all people with true blue eyes are more closely related. However, blue eyes with brown spots around the pupil is not related to this mutation.12

Blue eyes are most common in Northern and Central Europe and to a lesser degree in Southern Europe 25, they are also found in the Middle East, Afghanistan, Iran and the north of India. A 2002 study found the prevalence of blue eye color among Caucasians 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 1905.8

Brown

Brown human iris
Light brown human iris

Brown eyes are predominant in humans26 and, in many populations, it is (with few exceptions) the only iris color present.27 It is less common in countries around the Baltic Sea, such as Finland, Norway, Denmark, Iceland, Estonia, Latvia, Lithuania, Sweden, Poland, Germany and The Netherlands.citation needed

In humans, brown eyes contain large amounts of melanin within the iris stroma, which serves to absorb light, particularly at the shorter wavelengths. Brown eyes are the most common eye color, with over half of the world population having them. They are also the most dominant eye color gene.523 Very dark brown irises may appear at a glance to be black.2829

Gray

A steel blue-gray eye

Gray eyes have less melanin than blue eyes, even though they are considered a darker shade of blue (like blue-green). They are most common in European Russia, Finland and the Baltic States.citation needed 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.

Visually, gray eyes often tend to appear to change between the shades of blue, green and gray; This is because gray eyes are extremely light, as mentioned before. The color change for gray eyes is usually influenced by the lighting and the strange surroundings (such as clothes, makeup, etc.).

The Greek goddess Athena was renowned for having "owl-gray" or "sea-gray" eyes (in Greek, γλαυκῶπις – glaukōpis).30

Green

Green eyes

Green eyes are the product of moderate amounts of melanin. According to some researchers, green eyes are the result of mutations that change the melanin structure.31 Green eyes are among the rarest eye color and they are most common in Northern Europe and Eastern Europe 32 to a lesser extent it is also found in Southern Europe and occasionally in the Middle East, Afghanistan, Pakistan, and north India. A study of Icelandic and Dutch adults found that green eyes are much more prevalent in women than in men.33 Almost 90% of the Icelandic population has either green or blue eye color.34

Hazel

This eye shows a mixture of brown, green and amber colors.
Some eye colors are too mixed to identify properly, and are identified as hazel for simplicity's sake.

Hazel eyes are due to a combination of a Rayleigh scattering and a more than moderate amount of melanin in the iris' anterior border layer.225 Hazel eyes often appear to shift in color from a light brown to a medium golden-green. A number of studies using three-point scales have assigned "hazel" to be the medium-color between the lightest shade of blue and darkest shade of brown.35363738394041 This can sometimes produce a multicoloured 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 observed in sunlight. Hazel is commonly found in Europe, some regions of the Middle East, North America, parts of Central Asia, parts of North India, Northern Pakistan, Iran and in Afghanistan. Rarely, hazel eyes can be found in people of sub-saharan African, Native American and East Asian descent. A large percent of Hazel eyes have a brown ring around the iris and have either green or blue around the outside of the ring.

Definitions of the eye color "hazel" vary: it is sometimes considered to be synonymous with light-brown and gold.422835374043444546 In North America, "hazel" is often used to describe eyes that appear to change color, ranging from light brown to green and even grey, or grey to blue, depending on lighting in the environment.

Red

The eyes of a person with albinism may appear red under certain lighting conditions due to the very low quantities of melanin. "True" red eyes also exist in albinistic and even some non-albinistic populations, but are very rare. Only about 20 cases of natural red eyes are recognized throughout the world. Red eyes are caused by the complete lack of pigment in the iris leaving the blood vessels as the only visible part of the iris, or a blood leakage into the iris. 47

Violet

The appearance of "violet" eyes is thought to occur from the mixing of red and blue reflections.48 Some albinos have eyes that appear violet. Violet eyes are genetically similar to blue eyes, i.e. they are a reflection, pigment, or variant of blue.4749

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;39 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, green or gray iris color.5152

Eye color may also be symptomatic of disease. Aside from the iris, yellowing of the whites of the eyes is associated with jaundice and symptomatic of liver disease, including cirrhosis, hepatitis and malaria.

Anomalous conditions

Aniridia

Main article: Aniridia
Aniridia: Eyes wherein the irises are not present; the eyes appear to be two large pupils.

Aniridia is a congenital condition characterized by an extremely underdeveloped iris which appears absent on superficial examination.53

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.54 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.

Heterochromia

An example of heterochromia. The subject has one brown and one hazel eye.
An example of sectoral heterochromia. The subject has a blue iris with a brown section.
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.55 This uncommon condition usually results due to uneven melanin content. A number of causes are responsible, including genetics such as chimerism and Waardenburg syndrome. 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 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.

Eye color change

Often, newborns have blue eyes, which change to green, hazel, light brown or dark brown. This is possibly the origin of the idiom "being blue-eyed" (i. e. naïve; gullible) or having "baby blues", or striking blue eye color.

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.56

Eye color and red-eye effect

The photographic red-eye effect is more prominent in people with pale (blue, green, or grey) eyes, and a similar effect (possibly the same) is observed in the eyeshine of blue-eyed cats and dogs.

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See also

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