Eye color



"Brown eyes" redirects here. For other uses, see Brown eyes (disambiguation).
Brown iris
Light brown iris

Brown eyes are dominant in humans[37] and in many parts of the world, it is nearly the only iris color present.[38] It is less common in countries around the Baltic Sea and in Scandinavia. Dark pigment of brown eyes are most common in East Asia, Southeast Asia, Oceania, Africa, South America and the Middle East.

Light-pigmented brown eyes are common in Europe, Northern India and some parts of the Middle East. Light-pigmented brown eyes are sometimes referred as "honey eyes".

In humans, brown eyes contain large amounts of melanin within the iris stroma, which serves to absorb light at both shorter and longer wavelengths. Brown eyes are the most common eye color, with over half of the world's population having them.

Gray

A steel blue-gray iris
Gray iris under magnification, exhibiting small amounts of yellow

Gray eyes have less melanin than blue eyes,[citation needed] even though they are considered a darker shade of blue (like blue-green). Gray eyes are most common in European Russia, Sweden, Finland and the Baltic States[citation needed]. It can also be found to a lesser extent in parts of India[32]. Under magnification, gray eyes exhibit small amounts of yellow and brown color in the iris. Ultimately, there are at least two things that could determine gray eye color. The first is the amount of melanin made, and the second is the density of the proteins in the stroma.[39]

A gray iris may indicate the presence of a uveitis. However, other visual signs make a uveitis obvious. Gray iris color, as well as blue, are at increased risk of uveal melanoma.[40]

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.

Green

A light green iris with small amounts of brown
Olive green eyes


Green eyes are the product of low to moderate amounts of melanin and probably represent the interaction of multiple variants within the OCA2 and other genes. Green eyes are most common in Northern and Central Europe.[41][42] They can also be found in parts of South Asia[32], West Asia, and North Africa. In Iceland, 89% of women and 87% of men have either blue or green eye color.[43] A study of Icelandic and Dutch adults found green eyes to be much more prevalent in women than in men. Among European Americans, green eyes are most common among those of Celtic and Germanic ancestry, about 16%.[44]

Hazel

This iris shows a mixture of brown and green colors.
This hazel iris shows a mix of gray and golden brown

Hazel eyes are due to a combination of Rayleigh scattering and a moderate amount of melanin in the iris's anterior border layer.[7][25] Hazel eyes often appear to shift in color from a light brown to a dark 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.[citation needed] Hazel mostly consists of brown and green. The dominant color in the eye can either be green or light brown/gold.[45][46][47][48][49][50][51] This can sometimes produce a multicolored iris, i.e., an eye that is light brown/amber near the pupil and charcoal or dark green on the outer part of the iris (and vice versa) when observed in sunlight. Hazel is commonly found in Europe, the Middle East, the Americas, Central Asia, and parts of South Asia.

Definitions of the eye color hazel vary: it is sometimes considered to be synonymous with light brown or gold, as in the color of a hazelnut shell.[45][47][50][52]

Red

"Red" albino eyes

The eyes of people with severe forms of albinism may appear red under certain lighting conditions owing to the extremely low quantities of melanin,[53] allowing the blood vessels to show through. In addition, flash photography can sometimes cause a "red-eye effect", in which the very bright light from the flash reflects off the back of the eyeball, causing the iris to appear red in the photograph.[54]

Spectrum of eye color

Human Iris JD052007.jpg
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Iris of the Human Eye.jpg
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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;[49] lighter eye color is also associated with an increased risk of ARMD progression.[55] An increased risk of uveal melanoma has been found in those with blue, green or gray iris color.[40][43] However a study in 2000 suggests that people with dark brown eyes are at increased risk of developing cataracts and therefore should protect their eyes from direct exposure to sunlight.[56]

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. Yellowing of the whites of the eyes in people with darker pigmented skin is often due to melanin being present in the whites of the eyes. However, any sudden changes in the color of the whites of the eyes should be addressed by a medical professional.

Anomalous conditions

Aniridia

Main article: Aniridia

Aniridia is a congenital condition characterized by an extremely underdeveloped iris, which appears absent on superficial examination.[57]

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 a total lack of melanin. Transillumination defects can almost always be observed during an eye examination due to lack of iridial pigmentation.[58] 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 complete heterochromia. The subject has one brown eye and one hazel eye. The subject also appears to have a faint star pattern in the hazel iris.
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.[59] This uncommon condition usually results due to uneven melanin content. A number of causes are responsible, including genetic, such as chimerism, Horners Syndrome and Waardenburg syndrome.


An example of sectoral heterochromia. A green eye with a brown section.

A chimera can have two different colored eyes just like any two siblings can—because each cell has different eye color genes. A mosaic can have two different colored eyes if the DNA difference happens to be in an eye color gene.

There are many other possible reasons for having two different colored eyes. For example, David Bowie's different eye colors comes from an injury that caused one pupil to be permanently dilated. Another idea about how this can happen is if an early viral infection while in the womb turns an eye color gene on or off in just one eye. Occasionally it can be a sign of a serious disease.

Green brown eyes.png

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.[60] On occasion, the condition of having two different-colored eyes is caused by blood staining the iris after sustaining injury.

See also

References

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