Eye color

Categories: Eye | Human appearance

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).

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. Human beings have many variations in eye color.

Colors

Blue eyes

Among human phenotypes, 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 eastern Europe, southern Europe, the Middle East, and Central Asia. South Asians may also have blue eyes, but this is uncommon, except amongst Pathans and in Kashmir. In Finland there is the highest percentage of blue-eyed people, 90 per cent. 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. Although extremely rare, people of black/African descent may have blue eyes, some examples being Gary Dourdan, Michael Ealy, Blu Cantrell and Vanessa Williams

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.

Eyes without melanin appear blue due to selective scattering of light in the transparent iris; no blue pigment is involved (the same phenomenon that causes the blue color of the sky).

In the mid-20th century after the dawn of color films, 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. Light brown are also present in many people, but to a lesser extent. Most of the original inhabitants of Africa, Asia and the Americas have brown eyes. Brown eyes are also found in Europe, though within European populations they are not predominate to the same extent. In this population, brown eyes are genetically linked to brown hair. [1]

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.

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 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 2003.

Grey eyes

Very light blue eyes may give the impression of being grey.

Hazel eyes

Hazel is usually used to describe eyes that contain elements of both green eyes and brown eyes. Some hazel-eyed people have irises which transition from green at the edges to a light or darker brown near the center.

Red eyes

The red-eye effect commonly appears in photographs, especially in those with light eyes. In animals with albinism, the irises may appear red due to a lack of the pigment melanin. This does not happen in humans—human albinos normally have blue eyes. Under certain lighting conditions, in humans with albinism, the iris and/or pupil will appear bright red to violet in color. This is due to the translucency of their eyes and the light reflecting off their retina.

Heterochromia

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). The condition usually results 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. Gracie Allen had two eyes of startlingly different colors, which some believe influenced her decision to stop acting when color television was widely adopted. Actor Joe Pesci and actresses Mila Kunis and Kate Bosworth also have different colored eyes. The French scientist Louis Pasteur, who proved the germ theory of disease, is also known to have had a green and a blue eye. On occasion the condition of having two different colored eyes is caused by blood staining the iris after sustaining injury.

Genetics

Three gene pairs controlling human eye color are known. Two of the gene pairs occur on chromosome pair 15 and one occurs on chromosome pair 19. The bey 2 gene (EYCL3), on chromosome 15, has a brown and a blue allele. A second gene (EYCL1), located on chromosome 19 (the gey gene) has a blue and a green allele. A third gene, bey 1, located on chromosome 15, is a central brown eye color gene.

Geneticists have designed a model using the bey 2 and gey gene pairs that explains the inheritance of blue, green and brown eyes. In this model the bey 2 gene has a brown and a blue allele. The brown allele is always dominant over the blue allele so even if a person is heterozygous (one brown and one blue allele) for the bey 2 gene on chromosome 15 the brown allele will be expressed. The gey gene also has two alleles, one green and one blue. The green allele is dominant to the blue allele on either chromosome but is recessive to the brown allele on chromosome 15. This means that there is a dominance order among the two gene pairs. If a person has a brown allele on chromosome 15 and all other alleles are blue or green the person will have brown eyes. If there is a green allele on chromosome 19 and the rest of the alleles are blue, eye color will be green. Blue eyes will occur only if all four alleles are for blue eyes. This model explains the inheritance of blue, brown and green eyes but cannot account for gray, hazel or multiple shades of brown, blue, green and gray eyes. It cannot explain how two blue-eyed parents can produce a brown-eyed child or how eye color can change over time. This suggests that there are other genes, yet to be discovered, that determine eye color or that modify the expression of the known eye color genes.

External links

• Genetics of eye color
• Scientific American - Ask the Experts, "How does someone get two different-colored eyes?"
• Eye colour: portals into pigmentation genes and ancestryde:Augenfarbe