Eye color
Eye color is a polygenic trait and is determined by the amount and type of pigments present in the eye's iris[1][2]. Although there seem to be three genotypic eye colors (brown, green, and blue), humans and other animals have many phenotypic variations in eye color[3]. These variations in color are attributed to varying ratios of eumelanin and pheomelanin melanocytes in the iris[2].
Colors
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Brown eyes
The vast majority of the world's people have dark eyes, ranging from brown to nearly black. Light brown eyes 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, 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[citation needed].
Black eyes
People with very dark brown irises may appear to have black eyes. This is fairly common in people of African, Asian and Native American descent. It should be noted that while dark brown eyes may appear black they are still technically brown. Truly black eyes occurs only in the absence of the iris in the case of the congenital defect aniridia, in which case the black retina will appear as the contracted iris.
Hazel eyes
Hazel is usually used to describe eyes that contain elements of both green eyes and brown eyes, sometimes transitioning from green at the edges to brown around the pupil. This should not be confused with irises that display a brown, yellow, or copper-colored ring around the pupil, as in the grey iris shown below. Hazel eyes are also known as light brown eyes. They are dominant in middle European countries such as the middle of Russia, Belarus, Ukraine, south and middle of Poland, south and middle of Germany, north of France, Switzerland, north of Italy, Slovenia and others.[citation needed]
Amber eyes
Amber eyes are often mistakenly referred to as "hazel." They have, however, a much stronger yellowish/golden and russet/coppery tint than hazel eyes and are not as common in humans. The golden tint appears at its brightest in direct lighting or sunlight, but turns into a golden green-brown in the shade or a coppery brown in candlelight. Amber eyes are also nicknamed "cat eyes."
Blue eyes
Among human phenotypes, blue eyes are a relatively rare eye color. They are found mainly in people of northern European and eastern European descent, and to a lesser extent, in people of southern Europe[citation needed] 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 and Parsis and people in Pakistan, and also in India. For example, blue eyes are most common in Punjab and Kashmir, with fewer numbers in Gujarat and Rajasthan. Finland has the highest percentage of blue-eyed people. Ireland and Great Britain also have high percentages of blue eyes. Although very rare, there are instances of blue eyes occurring in people of African descent.[citation needed] Many caucasian babies are born with blue eyes, though their eyes will darken, or change color. Most infants' eye color will set within a couple of days to a couple of weeks, though some people's will continue to change for a number of years.
Blue is the color of the indole monomer that when polymerised forms melanin[citation needed]. If both alleles for brown eyes (a polymerase gene) are absent or damaged, the blue color remains. Hence blue eye color is a recessive trait.
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.
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[4]. This seems to have been caused by the large wave of Italian, Jewish, and other Eastern and Southern European immigration between 1890 and 1914. The largest groups (Irish, German, and English) all have about 40% blue eyes, but the proportion of whites born in groups other than these had shifted from predominately Scottish, Scandinavian and Dutch around 1900 (who have a majority of blue-eyed people) to largely Italian (8% blue eyes), Polish (32% blue), and French Canadian (30% blue), among others.
Gray eyes
Gray eyes are a (often lighter) variant of blue eyes. A wide variety of shades of grey exist, from the almost white (light grey) to dark.
The underlying gray color may be tinted with various other colors, as in the greenish-gray eyes in the picture. "Steel blue" eyes are also found, ranging from a slightly desaturated blue to light blue-gray.
A yellow-, amber- or copper-colored ring is commonly seen around the pupil. This is a normal part of the iris, and should not be confused with Kayser-Fleischer rings. As with other colors, gray eyes are often seen to change color depending on the surrounding colors.
Green eyes
Green eyes are rarer than brown, black, hazel, blue and gray eyes. Green eyes are most often found in people of Celtic, Germanic, and Slavic descent. Hungarians have the highest percentage of green eyes of any population, close to 20%.[citation needed] Green eyes are also found, though in far 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": the green-eyed people.[citation needed]. They may also be found in many areas of northern India, especially in Punjab and Kashmir.
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.
Blue-Green eyes
Blue-green eyes are relatively uncommon and usually consist of an iris with a predominantly blue color, usually darker blue, with green streaks or stippling caused by a yellow- or copper-colored overlay. Yellow-, amber-, or copper-colored rings are often present around the pupil.
The exact color is often perceived to vary according to its surroundings. Wearing lighter blue colors or getting tanned tends to emphasize the blue components. Wearing bright red, green, and shades of yellow or brown can emphasize green components, or even give the impression of gray.
Violet Eyes
Violet eyes are extremely rare and are a variation of blue eyes. They are believed to be caused by eyes having so little pigmentation that the red and blue blood vessels permeate through, causing a violet color of the iris. A noteworthy subject is Elizabeth Taylor, whose physical trademark is her violet eyes.
Red eye effect
The red-eye effect commonly appears in photographs taken with a flash, especially in those with light eyes. It is caused by light reflecting off of the retina, which is highly reflective, in much the same way light reflects off of the eyes of animals such as cats and alligators, making their eyes appear to glow.
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[5]. This uncommon condition usually results due to uneven melanin content. A number of causes are responsible, including genetics 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.
Genetics
At one time scientists thought that a single gene pair, in a dominant/recessive inheritance pattern, controlled human eye color. The allele for brown eyes was considered dominant over the allele for blue eyes. The genetic basis for eye color is actually far more complex. At the present, 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, on chromosome 15, has a brown and a blue allele. A second gene, 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.[citation needed] 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 gray 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.
Eye color typically stabilizes by 6 years of age[6].
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[7]; lighter eye color is also associated with an increased risk of ARMD progression[8]. An increased risk of uveal melanoma has been found in those with blue or grey iris color[9]. Darker iris colors have been found to have slightly higher intraocular pressures than lighter iris colors [10]. An increased incidence of age-related cataracts has been found in those with dark brown irises[11][12].
References
- ^ Wielgus AR, Sarna T. "Melanin in human irides of different color and age of donors." Pigment Cell Res. 2005 Dec;18(6):454-64. PMID 16280011.
- ^ a b Prota G, Hu DN, Vincensi MR, McCormick SA, Napolitano A. "Characterization of melanins in human irides and cultured uveal melanocytes from eyes of different colors." Exp Eye Res. 1998 Sep;67(3):293-9. PMID 9778410.
- ^ Morris, PJ. "Phenotypes and Genotypes for human eye colors." Athro Limited website. Retrieved May 10, 2006.
- ^ Grant MD, Lauderdale DS. "Cohort effects in a genetically determined trait: eye colour among US whites." Ann Hum Biol. 2002 Nov-Dec;29(6):657-66. PMID 12573082.
- ^ Imesch PD, Wallow IH, Albert DM. "The color of the human eye: a review of morphologic correlates and of some conditions that affect iridial pigmentation." Surv Ophthalmol. 1997 Feb;41 Suppl 2:S117-23. PMID 9154287.
- ^ Bito LZ, Matheny A, Cruickshanks KJ, Nondahl DM, Carino OB. "Eye color changes past early childhood. The Louisville Twin Study." Arch Ophthalmol. 1997 May;115(5):659-63. PMID 9152135.
- ^ Frank RN, Puklin JE, Stock C, Canter LA. "Race, iris color, and age-related macular degeneration." Trans Am Ophthalmol Soc. 2000;98:109-15; discussion 115-7. PMID 11190014.
- ^ Nicolas CM, Robman LD, Tikellis G, Dimitrov PN, Dowrick A, Guymer RH, McCarty CA. "Iris colour, ethnic origin and progression of age-related macular degeneration." Clin Experiment Ophthalmol. 2003 Dec;31(6):465-9. PMID 14641151.
- ^ Stang A, Ahrens W, Anastassiou G, Jockel KH. "Phenotypical characteristics, lifestyle, social class and uveal melanoma." Ophthalmic Epidemiol. 2003 Dec;10(5):293-302. PMID 14566630.
- ^ Mitchell R, Rochtchina E, Lee A, Wang JJ, Mitchell P; Blue Mountains Eye Study. "Iris color and intraocular pressure: the Blue Mountains Eye Study." Am J Ophthalmol. 2003 Mar;135(3):384-6. PMID 12614760.
- ^ Younan C, Mitchell P, Cumming RG, Rochtchina E, Wang JJ. "Iris color and incident cataract and cataract surgery: the Blue Mountains Eye Study." Am J Ophthalmol. 2002 Aug;134(2):273-4. PMID 12140040.
- ^ Hammond BR Jr, Nanez JE, Fair C, Snodderly DM. "Iris color and age-related changes in lens optical density." Ophthalmic Physiol Opt. 2000 Sep;20(5):381-6. PMID 11045246.
External links
- What Color Eyes Would Your Child Have?
- Eye color inheritance chart, discussing six different eye colors
- Genetics of eye color
- Scientific American - Ask the Experts, "How does someone get two different-colored eyes?"
- Eye colour: portals into pigmentation genes and ancestry
- Various links on human eye color
- Explanation of how someone of African descent has blue eyes
- A random Eye color generator. - Choose the genes and it produces the random eye colors.