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Revision as of 09:00, 27 March 2005
- This article is about the geographical distribution and social connotations of eye color in humans. For an anatomical explanation of the pigmentation of the iris, see iris (anatomy).
Human beings have many variations in eye color. Eye color is a polygenic trait and is determined by the amount of melanin present in the eye. A small amount of melanin will result in blue eyes, a larger amount will result in green eyes, and brown eyes represent melanin-rich irises.
Colors
Blue eyes
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Blue eyes are a relatively rare eye color. They are found mainly in people of northern European descent, and to a lesser extent, in people of southern Europe. Many Caucasian babies are born with blue eyes, though their eyes will darken, or change color. Most infants' eye color is set within a couple of days to a couple of weeks, though some people's eye color will continue to change for a number of years.
Blue eye color is a recessive trait, and the gene must be present (though not necessarily apparent) in both parents for a child to be born with blue eyes.
Blue eye color also contains less melanin, which protects from sun damage. Consequently, blue-eyed people are more susceptible to retinal damage or related conditions due to extensive sun exposure.
The Nazi Party promoted a supposed blue-eyed, blonde-haired ideal.
In the mid-20th century, blue eyes were considered very desirable in those aspiring to be Hollywood actors and actresses. This became less true toward the latter half of that century.
Brown eyes
The vast majority of the world's people have dark eyes, ranging from brown to nearly black. Most of the original inhabitants of Africa and Asia have brown eyes. Brown eyes are also found in Europe, though within that continent they do not predominate to the same extent as the other two continents mentioned. In this population, brown eyes are genetically linked to brown hair.
Light brown eyes are often characterized as hazel eyes.
Green eyes
Green eyes are most often found in people of Celtic, Slavic and Germanic descent. Green eyes may be confused with hazel eyes.
Sometimes a person's eye color can change as that person ages. Blue eyes can become green but green eyes can't become blue.
Grey eyes
Very light blue eyes can be confused with grey eyes.
Red eyes
The red-eye effect commonly appears in photographs, especially in those with light eyes. Also, irises entirely lacking in the pigment melanin appear red. This trait is characteristic of albinism.
Heterochromia iridium
Heterochromia iridium is the condition of having two different colored eyes due to uneven melanin content. This is a rare trait in people. A number of causes are responsible, including genetics and waardenburg syndrome. Trauma and certain medications can also cause increased or decreased pigmentation in one eye. Rock star David Bowie has heterochromia iridium, apparently due to an eye injury incurred in his youth. Actor Joe Pesci's eyes are also different colors.
- See Heterochromia iridium for more information.
Genetics
Two genes have been identified which control eye color. EYCL3 is found on chromosome 15 and codes for brown and blue eyes (BEY). EYCL1, found on chromosome 19, codes for green and blue eyes (GEY). However, the determination of a precise eye color is complex, requiring even other genes, and how these genes interact is unknown.
External links
- Genetics of eye colour
- Scientific American - Ask the Experts, "How does someone get two different-colored eyes?"
- Eye colour: portals into pigmentation genes and ancestry