Eye colour

Eye color or eye colour is a polygenic trait and is determined primarily by the amount and type of pigments present in the eye's iris. Humans and other animals have many phenotypic variations in eye color. In humans, these variations in color are attributed to varying ratios of the two types of melanin produced by melanocytes in the iris: eumelanin and pheomelanin. The brightly colored eyes of many bird species are largely determined by other pigments, such as pteridines, purines, and carotenoids.

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. In eyes of all colors, the iris pigment epithelium contains the black pigment, eumelanin. Color variations among different irises are typically attributed to the melanin content within the iris stroma. The density of cells within the stroma affects how much light is absorbed by the underlying pigment epithelium.

Determination of eye color
Eye color is an inherited trait influenced by more than one gene. In humans, three genes coding for eye color are currently known: EYCL1, EYCL2, and EYCL3. These genes account for three phenotypic eye colors (brown, green, and blue) in humans. Although it was once thought that brown eye color was always dominant and blue eye color was always recessive, the fact that two blue-eyed parents can give birth to a brown-eyed child has shown that the determination of eye color does not follow the simple rules of Mendelian inheritance. In fact, most babies of European or Southern Asian descent tend to have blue eyes at first, then either remains that colour or turns green to brown depending on pigmentation levels. Eye colour usually stabilizes when an infant is around 6 months old.

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. Various 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.

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.

Eye color exists on a continuum from the darkest shades of brown to the lightest shades of blue. 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.

Brown
In humans, brown eyes contain large amounts of melanin (primarily eumelanin) within the iris stroma which serves to absorb light, particularly at the shorter wavelengths. Very dark brown irises may appear to be black.

Brown is one of the most common eye colors and, in many populations, it is the only iris color present.

Those with non-European ancestry generally have darker eyes and less variability in eye color than those of European descent. Virtually all of the original inhabitants of Africa, Asia, and the Americas have brown eyes. Brown eyes are also found in Europe, Oceania, and North America, though within European populations they are not predominant to the same extent. Brown had been considered to be the most dominant eye color in any gene, but new studies have revealed that this is not always true.

Hazel


Hazel eyes are due to a combination of a Rayleigh scattering and a moderate amount of melanin in the iris' anterior border layer. A number of studies using three-point scales have assigned "hazel" to be the medium-color between brown and blue. This can sometimes produce a multicolored iris, i.e., an eye that is brown near the pupil and green on the outside.

There is some difficulty in defining the eye color "hazel" as it sometimes considered to be synonymous with brown and other times with green. They have been described as light brown or yellowish brown, or as a lighter shade of brown. Hazel eyes have also been described as being equivalent to blue/gray.

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). Amber eyes are much more common in other animals than they are in humans. They are also nicknamed "cat eyes."

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

Green
Green eyes are also the product of moderate amounts of melanin.

Green eye color has been linked to brown hair color.

Green eyes are most often found in people of Celtic, Germanic, and Slavic descent, and to a lesser extent southern Europe. Green eyes are also found, though in much lower proportions, among other Caucasian populations in the Middle East and South Asia. They are so common among Pashtuns that in Pakistan, Pashtuns are often called "Hare Ankheian Vaale": those with green eyes (Urdu translation). They may also be found in many areas of Afghanistan (the native land of the Pashtuns) and parts of India - most common in the north, with the highest percentages respectively in Punjab, Uttar Pradesh, Gujarat, and Rajasthan.

One of the most famous photographs ever published by National Geographic was a close-up of Sharbat Gula, a Pashtun girl with startling green eyes, taken in western Afghanistan by Steve McCurry in 1984. Details of her irises captured by the photograph were used to confirm her identity after she was relocated in 2002.

Gray
Gray eyes are a variant of blue eyes. Indian actress Celina Jaitley has gray eyes. A gray iris may indicate the presence of a uveitis. This is however obvious too the viewer that an eye problem exists.

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. The type of melanin present is primarily eumelanin.

Blue eyes are found mainly in people of northern European and eastern European descent, and to a lesser extent, in people of southern Europe, the Middle East and Central Asia, most commonly in people who live at higher elevations. South Asians may also have blue eyes, but this is uncommon, except amongst Pathans in Afghanistan, Pakistan, and India. It also occurs in other Indians and Pakistanis, generally in highest percentages among Punjabiis - though even then it is not exactly common. Finland and Lithuania have the highest proportions of blue-eyed people, with at least 80% in both countries respectively. Ireland and Great Britain also have high proportions of blue eyes, with estimates of around 70% for Ireland and about 60% for Britain. Blue is the color of the indole monomer that when polymerised forms melanin. If both alleles for brown eyes (a polymerase gene) are absent or damaged, the blue color remains. The inheritance pattern followed by blue eyes is thought to be similar to that of a recessive trait.

A 2002 study found the prevalence of blue eye color among non-Hispanic whites in the United States to be 57.4% for those born between 1899 and 1905 compared to 33.8% for those born between 1936 and 1951.

One survey estimated that nearly 90% of Icelanders have blue or green eyes.

As melanin production generally increases during the first few years of life, the blue eyes of some babies may darken as they get older. Eye color typically stabilizes by 6 years of age.

Violet eyes
The appearance of violet eyes is thought to occur due to the mixing of red and blue reflections. Some albinos have eyes that appear to be violet. Elizabeth Taylor's physical trademark is her "violet" eyes.

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

Ocular albinism
In those with albinism, the color of the irises is typically blue (but can vary from blue to brown); transillumination defects can almost always be observed during an eye examination due to lack of iridial pigmentation.

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


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. 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.

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; lighter eye color is also associated with an increased risk of ARMD progression. An increased risk of uveal melanoma has been found in those with blue or grey iris color. Darker iris colors have been found to have slightly higher intraocular pressures than lighter iris colors. An increased incidence of age-related cataracts has been found in those with dark brown irises.