Everything about Eye Color totally explained
Eye color is a
polygenic trait and is determined primarily by the amount and type of
pigments in the
eye's
iris. Humans and animals have many
phenotypic variations in eye color. In humans, these variations in color are attributed to varying ratios of
eumelanin produced by
melanocytes in the iris.
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. Two major genes and multiple minor genes account for the tremendous variation of human eye color. In humans, three genes associated with eye color are currently known:
EYCL1,
EYCL2, and
EYCL3. These genes account for three
phenotypic eye colors (brown, green, and blue) in humans.
In 2006, the molecular basis of the
EYCL3 locus was resolved. 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 .
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. A 2008 study demonstrated that a specific mutation within the HERC2 gene that regulates OCA2 expression is responsible for blue eyes (see below).
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 is found to be associated with the brown and blue eye color, but this single DNA variation can't explain all the brown eye color variation from dark brown over hazel to blue eyes with brown spots".
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.
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.
Eye color exists on a continuum from the darkest shades of brown to the lightest shades of blue.
Amber
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). They are nicknamed "Wolf eyes" due to the high rate of the amber eye color in wolves.
The 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, yellowish specks or patches are thought to be due to the pigment
lipofuscin, also known as
lipochrome.
The authors concluded that the mutation may have arisen in a single individual around the
Black Sea region 6,000-10,000 years ago, perhaps suggesting that all people with true blue eyes are more closely related. However, blue eyes with brown spots around the pupil are not related to this mutation.,
Germany,
Netherlands,
Iceland,
Austria,
Sweden,
Norway,
Denmark,
Finland,
Estonia, and the
United Kingdom They are also present in Southern Europe, the
Middle East (especially in
Israel,
Lebanon and
Iran) and are also found in
Afghanistan. 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. and, in many populations, it's (with few exceptions) the only iris color present. It is less common in countries around the
Baltic Sea, such as Finland and Estonia.
In humans, brown eyes contain large amounts of melanin (eumelanin) within the iris stroma, which serves to absorb light, particularly at the shorter wavelengths. Brown is the most common eye color. Very dark brown irises may appear to be black.
Violet
The appearance of "violet" eyes is thought to occur from the mixing of red and blue reflections. Some
albinos have eyes that appear
violet. Violet eyes are either a form of blue eyes or a mutation.
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; An increased risk of
uveal melanoma has been found in those with blue, green or gray iris color.
Eye color can also be symptomatic of disease. In particular, yellowing of the whites of the eyes is associated with
jaundice and symptomatic of liver disease, including
cirrhosis,
hepatitis and
malaria.
Anomalous conditions
Aniridia
Aniridia is a congenital condition characterized by an extremely underdeveloped iris which appears absent on superficial examination.
Ocular albinism and eye color
Normally, there's 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's 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. 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
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 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.
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. American Actress
Elizabeth Berkley has sectoral heterochromia; her right eye is half green and half brown, and her entire left eye is green. So does actor
Anthony Head - he's a patch of hazel in his left eye where both eyes are blue-green overall. 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 blue eye and one brown-green eye. American Actress
Demi Moore also has heterochromia, by having one green eye, and the other hazel.
Eye color change
Often, paler 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).
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.
Further Information
Get more info on 'Eye Color'.
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