Eye color
Overview
Eye color is a polygenic phenotypic character and is determined by two distinct factors: the pigmentation
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

 of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

's iris
Iris (anatomy)
The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...

 and the frequency-dependence of the scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...

 of light by the turbid
Turbidity
Turbidity is the cloudiness or haziness of a fluid caused by individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality....

 medium in the stroma of the iris
Stroma of iris
The stroma of the iris consists of fibers and cells.The former are made up of delicate bundles of fibrous tissue; a few fibers at the circumference of the iris have a circular direction; but the majority radiate toward the pupil, forming by their interlacement delicate meshes in which the vessels...

.

In humans, the pigmentation of the iris varies from light brown to black, depending on the concentration of melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...

 in the iris pigment epithelium
Iris pigment epithelium
The iris pigment epithelium is a two-cell thick layer of cuboidal cells lying behind the iris. Both layers are jet black due to the numerous large melanosomes which pack the cytoplasm of each cell. Towards the central axis, the IPE terminates at the pupillary margin...

 (located the back of the iris), the melanin content within the iris stroma (located at the front of the iris), and the cellular density of the stroma.
Encyclopedia
Eye color is a polygenic phenotypic character and is determined by two distinct factors: the pigmentation
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

 of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

's iris
Iris (anatomy)
The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...

 and the frequency-dependence of the scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...

 of light by the turbid
Turbidity
Turbidity is the cloudiness or haziness of a fluid caused by individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality....

 medium in the stroma of the iris
Stroma of iris
The stroma of the iris consists of fibers and cells.The former are made up of delicate bundles of fibrous tissue; a few fibers at the circumference of the iris have a circular direction; but the majority radiate toward the pupil, forming by their interlacement delicate meshes in which the vessels...

.

In humans, the pigmentation of the iris varies from light brown to black, depending on the concentration of melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...

 in the iris pigment epithelium
Iris pigment epithelium
The iris pigment epithelium is a two-cell thick layer of cuboidal cells lying behind the iris. Both layers are jet black due to the numerous large melanosomes which pack the cytoplasm of each cell. Towards the central axis, the IPE terminates at the pupillary margin...

 (located the back of the iris), the melanin content within the iris stroma (located at the front of the iris), and the cellular density of the stroma. The appearance of blue, green, and hazel eyes results from the Rayleigh scattering
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...

 of light in the stroma, a phenomenon similar to that which accounts for the blueness of the sky. Neither blue nor green pigments are ever present in the human iris or ocular fluid. Eye color is thus an instance of structural color and varies depending on the lighting conditions, especially for lighter-colored eyes.

The brightly colored eyes of many bird species result from the presence of other pigments, such as pteridine
Pteridine
Pteridine is a chemical compound composed of fused pyrimidine and pyrazine rings. A pteridine is also a group of heterocyclic compounds containing a wide variety of substitutions on this structure. Pterins and flavins are classes of substituted pteridines that have important biological...

s, purine
Purine
A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycle in nature....

s, and carotenoid
Carotenoid
Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...

s. Humans and other animals have many phenotypic variations in eye color. The genetics of eye color are complicated, and color is determined by multiple genes. Some of the eye-color genes include EYCL1 (a green/blue eye-color gene located on chromosome 19), EYCL2 (a brown eye-color gene) and EYCL3 (a brown/blue eye-color gene located on chromosome 15). The once-held view that blue eye color is a simple recessive trait has been shown to be incorrect. The genetics of eye color are so complex that almost any parent-child combination of eye colors can occur. OCA2
OCA2
P protein is a protein that in humans is encoded by the OCA2 gene.OCA2 encodes the human homologue of the mouse p gene. The P protein is believed to be an integral membrane protein involved in small molecule transport, specifically tyrosine - a precursor of melanin...

 gene polymorphism, close to proximal 5′ regulatory region, explains most human eye-color variation.

Genetic determination of eye color

Eye colors can range from the most common color, brown, to the least common, green. Eye color is an inherited trait influenced by more than one gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

. These genes are being sought using associations to small changes in the genes themselves and in neighboring genes. These changes are known as single-nucleotide polymorphisms or SNPs. The actual number of genes that contribute to eye color is currently unknown, but there are a few likely candidates. A study in Rotterdam
Rotterdam
Rotterdam is the second-largest city in the Netherlands and one of the largest ports in the world. Starting as a dam on the Rotte river, Rotterdam has grown into a major international commercial centre...

 (2009) found that it was possible to predict the color of eyes with more than 90% accuracy for brown and blue, using just six SNPs (from six genes).

The gene OCA2 , when in a variant form, causes the pink eye color and hypopigmentation
Hypopigmentation
Hypopigmentation is the loss of skin color. It is caused by melanocyte or melanin depletion, or a decrease in the amino acid tyrosine, which is used by melanocytes to make melanin.-Treatments:...

 common in human albinism
Albinism
Albinism is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of an enzyme involved in the production of melanin...

. (The name of the gene is derived from the disorder it causes, oculocutaneous albinism type II.) Different SNPs within OCA2 are strongly associated with blue and green eyes as well as variations in freckling, mole
Mole (skin marking)
A melanocytic nevus is a type of lesion that contains nevus cells .Some sources equate the term mole with "melanocytic nevus". Other sources reserve the term "mole" for other purposes....

 counts, hair
Hair color
Hair color is the pigmentation of hair follicles due to two types of melanin: eumelanin and pheomelanin. Generally, if more melanin is present, the color of the hair is darker; if less melanin is present, the hair is lighter...

 and skin tone
Human skin color
Human skin color is primarily due to the presence of melanin in the skin. Skin color ranges from almost black to white with a pinkish tinge due to blood vessels underneath. Variation in natural skin color is mainly due to genetics, although the evolutionary causes are not completely certain...

. The polymorphisms may be in an OCA2 regulatory sequence
Regulatory sequence
A regulatory sequence is a segment of DNA where regulatory proteins such as transcription factors bind preferentially. These regulatory proteins bind to short stretches of DNA called regulatory regions, which are appropriately positioned in the genome, usually a short distance 'upstream' of the...

, where they may influence the expression of the gene product, which in turn affects pigmentation. A specific mutation within the HERC2 gene, a gene that regulates OCA2 expression, is partly responsible for blue eyes. Other genes implicated in eye color variation are: SLC24A4 and TYR.

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 [of the Herc2 gene] is found to be associated with the brown and blue eye color, but this single DNA variation cannot explain all the brown eye color variation from dark brown over hazel to blue eyes with brown spots."

Classification of color

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. Others have attempted to set objective standards of color comparison.

As the perception of color
Color vision
Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths of the light they reflect, emit, or transmit...

 depends on viewing conditions (e.g., the amount and kind of illumination, as well as the hue of the surrounding environment), so does the perception of eye color.

Eye colors range from the darkest shades of brown to the lightest tints of blue. To meet the need for standardized classification, at once simple yet detailed enough for research purposes, Seddon et al. developed a graded system based on the predominant iris color and the amount of brown or yellow pigment present. There are three pigment colors that determine, depending on their proportion, the outward appearance of the iris: brown, yellow, and blue. Green irides, for example, have blue and some yellow. Brown irides contain mostly brown. Eye color in animals other than Homo sapiens is differently regulated. For example, instead of blue as in humans, autosomal recessive
Recessive
In genetics, the term "recessive gene" refers to an allele that causes a phenotype that is only seen in a homozygous genotype and never in a heterozygous genotype. Every person has two copies of every gene on autosomal chromosomes, one from mother and one from father...

 eye color in the skink species Corucia zebrata is black, and the autosomal dominant color is yellow-green.

Changes in eye color

In all populations, children are most commonly born with unpigmented eyes. However, most babies who have European ancestry have light-colored eyes before the age of one. As the child develops, melanocytes (cells found within the iris of human eyes, as well as skin and hair follicles) slowly begin to produce melanin. Because melanocyte cells continually produce pigment, in theory eye color can be changed. Most eye changes happen when the infant is around one year old, although it can happen up to three years of age. Observing the iris of an infant from the side using only transmitted light with no reflection from the back of the iris, it is possible to detect the presence or absence of low levels of melanin. An iris that appears blue under this method of observation is more likely to remain blue as the infant ages. An iris that appears golden contains some melanin even at this early age and is likely to turn green or brown as the infant ages.

Changes (lightening or darkening) of eye colors during puberty, early childhood, pregnancy, and sometimes after serious trauma (like heterochromia
Heterochromia
In anatomy, heterochromia refers to a difference in coloration, usually of the iris but also of hair or skin. Heterochromia is a result of the relative excess or lack of melanin...

) do represent cause for plausible argument to state that some eyes can or do change, based on chemical reactions and hormonal changes within the body.

Studies on Caucasian twins, both fraternal and identical, have shown that eye color over time can be subject to change, and major demelanization of the iris may also be genetically determined. Most eye-color changes have been observed or reported in the Caucasian population with hazel eyes.

Eye color chart (Martin–Schultz scale)

Carleton Coon
Carleton S. Coon
Carleton Stevens Coon, was an American physical anthropologist, Professor of Anthropology at the University of Pennsylvania, lecturer and professor at Harvard, and president of the American Association of Physical Anthropologists.-Biography:Carleton Coon was born in Wakefield, Massachusetts to a...

 created this chart by the Martin–Schultz scale often used in physical anthropology
Physical anthropology
Biological anthropology is that branch of anthropology that studies the physical development of the human species. It plays an important part in paleoanthropology and in forensic anthropology...

.


  1. Light eyes

  2. Eyes light and light mixed are 16–12 in Martin scale.
    Light: Gray, blue, green. 16–15 in Martin scale.
    Light-mixed: 14–12 in Martin scale.

    1. Very light-mixed (blue with gray or green or green with gray)

    2. Light-mixed (light or very light-mixed with small admixture of brown pigment)


  3. Mixed eyes

  4. Mixed: 12–6 in Martin scale. Mixture of light eyes (blue, gray or green) with brown pigment when light and brown pigment are the same level.
  5. Dark eyes

  6. Dark-mixed: 6–4 in Martin scale. Brown with small admixture of light pigment.
    Dark: 4–1 in Martin scale. Brown (light brown and dark brown) and very dark brown (almost black).


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
Lipochrome
Lipochrome is a form of pigment associated with amber and gold tones in human eyes and the eyes of other mammals.The term carotenoid is sometimes considered a synonym, but that term usually refers to specific molecules, while lipochrome refers to accumulations of varied molecules....

 in the iris (which is also found in green eyes). Amber eyes should not be confused with hazel eyes; although hazel eyes may contain specks of amber or gold, they usually tend to comprise many other colors, including green, brown and orange. Also, hazel eyes may appear to shift in color and consist of flecks and ripples, while amber eyes are of a solid gold hue. Even though amber is considered to be like gold, some people have russet or copper colored amber eyes which many people mistake for hazel, though hazel tends to be duller and contains green with red/gold flecks, like mentioned above. Amber eyes may also contain amounts of very light gold-ish gray, found in animals like wolves.

The eyes of some pigeons contain yellow fluorescing pigments known as pteridine
Pteridine
Pteridine is a chemical compound composed of fused pyrimidine and pyrazine rings. A pteridine is also a group of heterocyclic compounds containing a wide variety of substitutions on this structure. Pterins and flavins are classes of substituted pteridines that have important biological...

s. The bright yellow eyes of the Great Horned Owl
Great Horned Owl
The Great Horned Owl, , also known as the Tiger Owl, is a large owl native to the Americas. It is an adaptable bird with a vast range and is the most widely distributed true owl in the Americas.-Description:...

 are thought to be due to the presence of the pteridine pigment xanthopterin
Xanthopterin
Xanthopterin is a yellow, crystalline solid, that occurs mainly in the wings of butterflies and in the urine of mammals. Small microorganisms convert it into folic acid. It is the end product of a non-conjugated pteridine compound, and inhibits the growth of lymphocytes produced by concanovalin...

 within certain chromatophore
Chromatophore
Chromatophores are pigment-containing and light-reflecting cells found in amphibians, fish, reptiles, crustaceans, and cephalopods. They are largely responsible for generating skin and eye colour in cold-blooded animals and are generated in the neural crest during embryonic development...

s (called xanthophores) located in the iris stroma. In humans, yellowish specks or patches are thought to be due to the pigment lipofuscin
Lipofuscin
Lipofuscin is the name given to finely granular yellow-brown pigment granules composed of lipid-containing residues of lysosomal digestion. It is considered one of the aging or "wear-and-tear" pigments, found in the liver, kidney, heart muscle, adrenals, nerve cells, and ganglion cells...

, also known as lipochrome. Many animals such as canines, domestic cats, owls, eagles, pigeons and fish have amber eyes as a common color, whereas in humans this color occurs less frequently.

Blue

There is no blue pigmentation either in the iris or in the ocular fluid. Dissection reveals that the iris pigment epithelium is brownish black due to the presence of melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...

. Unlike brown eyes, blue eyes have low concentrations of melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...

 in the stroma of the iris, which lies in front of the dark epithelium. Longer wavelengths of light tend to be absorbed by the dark underlying epithelium, while shorter wavelengths are reflected and undergo Rayleigh scattering
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...

 in the turbid
Turbidity
Turbidity is the cloudiness or haziness of a fluid caused by individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality....

 medium of the stroma. This is the same frequency-dependence of scattering that accounts for the blue appearance of the sky. The result is a "Tyndall
John Tyndall
John Tyndall FRS was a prominent Irish 19th century physicist. His initial scientific fame arose in the 1850s from his study of diamagnetism. Later he studied thermal radiation, and produced a number of discoveries about processes in the atmosphere...

 blue" structural color that varies with external lighting conditions.

Distribution

Blue eyes are most common in the Baltic Sea
Baltic Sea
The Baltic Sea is a brackish mediterranean sea located in Northern Europe, from 53°N to 66°N latitude and from 20°E to 26°E longitude. It is bounded by the Scandinavian Peninsula, the mainland of Europe, and the Danish islands. It drains into the Kattegat by way of the Øresund, the Great Belt and...

 region, Northern
Northern Europe
Northern Europe is the northern part or region of Europe. Northern Europe typically refers to the seven countries in the northern part of the European subcontinent which includes Denmark, Estonia, Latvia, Lithuania, Norway, Finland and Sweden...

, Eastern
Eastern Europe
Eastern Europe is the eastern part of Europe. The term has widely disparate geopolitical, geographical, cultural and socioeconomic readings, which makes it highly context-dependent and even volatile, and there are "almost as many definitions of Eastern Europe as there are scholars of the region"...

 and Central Europe
Central Europe
Central Europe or alternatively Middle Europe is a region of the European continent lying between the variously defined areas of Eastern and Western Europe...

, and in Southern Europe
Southern Europe
The term Southern Europe, at its most general definition, is used to mean "all countries in the south of Europe". However, the concept, at different times, has had different meanings, providing additional political, linguistic and cultural context to the definition in addition to the typical...

. A number of blue-eyed minorities are also found in parts of the Middle East
Middle East
The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East...

, most notably in Iran
Iran
Iran , officially the Islamic Republic of Iran , is a country in Southern and Western Asia. The name "Iran" has been in use natively since the Sassanian era and came into use internationally in 1935, before which the country was known to the Western world as Persia...

, Iraq
Iraq
Iraq ; officially the Republic of Iraq is a country in Western Asia spanning most of the northwestern end of the Zagros mountain range, the eastern part of the Syrian Desert and the northern part of the Arabian Desert....

, Afghanistan
Afghanistan
Afghanistan , officially the Islamic Republic of Afghanistan, is a landlocked country located in the centre of Asia, forming South Asia, Central Asia and the Middle East. With a population of about 29 million, it has an area of , making it the 42nd most populous and 41st largest nation in the world...

 ([the Indo-Iranian
Indo-Iranians
Indo-Iranian peoples are a linguistic group consisting of the Indo-Aryan, Iranian, Dardic and Nuristani peoples; that is, speakers of Indo-Iranian languages, a major branch of the Indo-European language family....

 Nuristani people); as well as in the Levant
Levant
The Levant or ) is the geographic region and culture zone of the "eastern Mediterranean littoral between Anatolia and Egypt" . The Levant includes most of modern Lebanon, Syria, Jordan, Israel, the Palestinian territories, and sometimes parts of Turkey and Iraq, and corresponds roughly to the...

).

A 2009 study suggested that blue eyes were present in Siberia
Siberia
Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...

 during the Bronze Age
Bronze Age
The Bronze Age is a period characterized by the use of copper and its alloy bronze as the chief hard materials in the manufacture of some implements and weapons. Chronologically, it stands between the Stone Age and Iron Age...

; 15 of 25 Andronovo culture
Andronovo culture
The Andronovo culture, is a collection of similar local Bronze Age cultures that flourished ca. 21200–1400 BCE in western Siberia and the west Asiatic steppe. It is probably better termed an archaeological complex or archaeological horizon...

 specimens (60%) from the Krasnoyarsk
Krasnoyarsk
Krasnoyarsk is a city and the administrative center of Krasnoyarsk Krai, Russia, located on the Yenisei River. It is the third largest city in Siberia, with the population of 973,891. Krasnoyarsk is an important junction of the Trans-Siberian Railway and one of Russia's largest producers of...

 area had blue (or green) eyes.

Y-Chromosome DNA testing performed on ancient Scythian skeletons dating to the Bronze
Bronze Age
The Bronze Age is a period characterized by the use of copper and its alloy bronze as the chief hard materials in the manufacture of some implements and weapons. Chronologically, it stands between the Stone Age and Iron Age...

 and Iron Age
Iron Age
The Iron Age is the archaeological period generally occurring after the Bronze Age, marked by the prevalent use of iron. The early period of the age is characterized by the widespread use of iron or steel. The adoption of such material coincided with other changes in society, including differing...

s in the Siberian Krasnoyarsk
Krasnoyarsk
Krasnoyarsk is a city and the administrative center of Krasnoyarsk Krai, Russia, located on the Yenisei River. It is the third largest city in Siberia, with the population of 973,891. Krasnoyarsk is an important junction of the Trans-Siberian Railway and one of Russia's largest producers of...

 region found that all but one of 11 subjects carried Y-DNA R1a1, with blue or green eye color and light hair common, suggesting mostly European origin of that particular population.

In Estonia
Estonia
Estonia , officially the Republic of Estonia , is a state in the Baltic region of Northern Europe. It is bordered to the north by the Gulf of Finland, to the west by the Baltic Sea, to the south by Latvia , and to the east by Lake Peipsi and the Russian Federation . Across the Baltic Sea lies...

, 99% of people have blue eyes, stated Hans Eiberg from the Department of Cellular and Molecular Medicine at the University of Copenhagen
University of Copenhagen
The University of Copenhagen is the oldest and largest university and research institution in Denmark. Founded in 1479, it has more than 37,000 students, the majority of whom are female , and more than 7,000 employees. The university has several campuses located in and around Copenhagen, with the...

. In Denmark
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...

 30 years ago, only 8% of the population had brown eyes, though through immigration, today that number is about 11%. In Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

, about 75% have blue eyes.

A 2002 study found that the prevalence of blue eye color among Caucasians in the United States to be 33.8 percent for those born from 1936 through 1951 compared with 57.4 percent for those born from 1899 through 1905. Blue eyes are continuing to become less common among American children, with only one out of every six or 16.6% of the total population, and 22.3% of the white population having blue eyes.

Worldwide, only 150 million people (some 2.2%) have blue eyes.

Genetics

In humans, the inheritance pattern followed by blue eyes is considered similar to that of a recessive
Recessive
In genetics, the term "recessive gene" refers to an allele that causes a phenotype that is only seen in a homozygous genotype and never in a heterozygous genotype. Every person has two copies of every gene on autosomal chromosomes, one from mother and one from father...

 trait (in general, eye color inheritance is considered a polygenic trait, meaning that it is controlled by the interactions of several genes, not just one). In 2008, new research suggested that people with blue eyes have a single common ancestor. Scientists tracked down a genetic mutation that leads to blue eyes. "Originally, we all had brown eyes," said Eiberg. Eiberg and colleagues showed in a study published in Human Genetics that a mutation in the 86th intron
Intron
An intron is any nucleotide sequence within a gene that is removed by RNA splicing to generate the final mature RNA product of a gene. The term intron refers to both the DNA sequence within a gene, and the corresponding sequence in RNA transcripts. Sequences that are joined together in the final...

 of the HERC2
HERC2
Probable E3 ubiquitin-protein ligase HERC2 is an enzyme that in humans is encoded by the HERC2 gene.A mutation in the HERC2 gene adjacent to OCA2, affecting OCA2's expression in the human iris, is found common to nearly all people with blue eyes...

gene, which is hypothesized to interact with the OCA2 gene promoter, reduced expression of OCA2 with subsequent reduction in melanin production. The authors concluded that the mutation may have arisen in a single individual probably living in the northwestern part of the Black Sea
Black Sea
The Black Sea is bounded by Europe, Anatolia and the Caucasus and is ultimately connected to the Atlantic Ocean via the Mediterranean and the Aegean seas and various straits. The Bosphorus strait connects it to the Sea of Marmara, and the strait of the Dardanelles connects that sea to the Aegean...

 region (around modern Romania
Romania
Romania is a country located at the crossroads of Central and Southeastern Europe, on the Lower Danube, within and outside the Carpathian arch, bordering on the Black Sea...

) 6,000–10,000 years ago during the Neolithic revolution
Neolithic Revolution
The Neolithic Revolution was the first agricultural revolution. It was the transition from hunting and gathering to agriculture and settlement. Archaeological data indicates that various forms of plants and animal domestication evolved independently in 6 separate locations worldwide circa...

. Eiberg stated, "A genetic mutation affecting the OCA2 gene in our chromosomes resulted in the creation of a 'switch,' which literally 'turned off' the ability to produce brown eyes."
The genetic switch is located in the gene adjacent to OCA2 and rather than completely turning off the gene, the switch limits its action, which reduces the production of melanin in the iris. In effect, the turned-down switch diluted brown eyes to blue. If the OCA2 gene had been completely shut down, our hair, eyes and skin would be melanin-less, a condition known as albinism
Albinism
Albinism is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of an enzyme involved in the production of melanin...

.

Blue eyes are rare in mammals; one example is the quite recently-discovered Blue-eyed Spotted Cuscus
Blue-eyed Spotted Cuscus
The Blue-eyed Spotted Cuscus or Biak Spotted Cuscus is a species of marsupial in the Phalangeridae family. It is endemic to the islands of Biak and Supiori in the Cenderawasih Bay in West Papua of Indonesia...

 (Spilocuscus wilsoni). The trait is hitherto known only from a single primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

 other than humans – Sclater's Lemur (Eulemur flavifrons) of Madagascar. While some cat
Cat
The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...

s and dog
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...

s have blue eyes, this is usually due to another mutation which is associated with deafness
Deaf white cat
Some white cats suffer from congenital deafness caused by degeneration of the inner ear. This condition is associated with blue irises. In white cats with mixed-coloured eyes , it has been found that deafness is more likely to affect the ear on the blue-eyed side...

. But in cats alone, there are three known mutations which produce blue eyes – the one found in Siamese cats does not have adverse health effects, while the others – that of blue-eyed white cats and the Ojos Azules
Ojos Azules
-Origin:Cats with dark blue eyes were discovered in New Mexico among feral cat populations. The first cat, discovered in 1984, was a tortoiseshell named Cornflower. She was bred to males without the trait, which proved to be dominant, as all her kittens showed it...

 breed – are associated with congenital neurological disorder
Neurological disorder
A neurological disorder is a disorder of the body's nervous system. Structural, biochemical or electrical abnormalities in the brain, spinal cord, or in the nerves leading to or from them, can result in symptoms such as paralysis, muscle weakness, poor coordination, loss of sensation, seizures,...

s.

Brown

Dark brown eyes are dominant in humans and in many parts of the world, it is nearly the only iris color present. Dark pigment of brown eyes is most common in East Asia
East Asia
East Asia or Eastern Asia is a subregion of Asia that can be defined in either geographical or cultural terms...

, Southeast Asia
Southeast Asia
Southeast Asia, South-East Asia, South East Asia or Southeastern Asia is a subregion of Asia, consisting of the countries that are geographically south of China, east of India, west of New Guinea and north of Australia. The region lies on the intersection of geological plates, with heavy seismic...

, South Asia
South Asia
South Asia, also known as Southern Asia, is the southern region of the Asian continent, which comprises the sub-Himalayan countries and, for some authorities , also includes the adjoining countries to the west and the east...

, West Asia, Oceania
Oceania
Oceania is a region centered on the islands of the tropical Pacific Ocean. Conceptions of what constitutes Oceania range from the coral atolls and volcanic islands of the South Pacific to the entire insular region between Asia and the Americas, including Australasia and the Malay Archipelago...

, Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...

, Americas
Americas
The Americas, or America , are lands in the Western hemisphere, also known as the New World. In English, the plural form the Americas is often used to refer to the landmasses of North America and South America with their associated islands and regions, while the singular form America is primarily...

, etc. as well as parts of Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

 such as Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...

 and southern Italy. Dark brown is also the eye color of the semitic people seen among Jew and Arab
Arab
Arab people, also known as Arabs , are a panethnicity primarily living in the Arab world, which is located in Western Asia and North Africa. They are identified as such on one or more of genealogical, linguistic, or cultural grounds, with tribal affiliations, and intra-tribal relationships playing...

 populations in the Middle East
Middle East
The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East...

.

Light or medium-pigmented brown eyes are common in Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, Afghanistan
Afghanistan
Afghanistan , officially the Islamic Republic of Afghanistan, is a landlocked country located in the centre of Asia, forming South Asia, Central Asia and the Middle East. With a population of about 29 million, it has an area of , making it the 42nd most populous and 41st largest nation in the world...

 and Northern India
North India
North India, known natively as Uttar Bhārat or Shumālī Hindustān , is a loosely defined region in the northern part of India. The exact meaning of the term varies by usage...

, as well as some parts of the Middle East
Middle East
The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East...

. Light-pigmented brown eyes are sometimes referred to as "honey eyes".

In humans, brown eyes result from a relatively high concentration of melanin in the stroma of the iris, which causes light of both shorter and longer wavelengths to be absorbed. Brown is by far the most common eye color, with over half of the world's population having them.

Gray

Like blue eyes, gray eyes have a dark epithelium at the back of the iris and a relatively clear stroma at the front. One possible explanation for the difference in the appearance of gray and blue eyes is that that gray eyes have larger deposits of collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...

 in the stroma, so that the light that is reflected from the epithelium undergoes Mie scattering (which is not strongly frequency-dependent) rather than Rayleigh scattering
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...

 (in which shorter wavelengths of light are scattered more). This would be analogous to the change in the color of the sky, from the blue given by the Rayleigh scattering of sunlight by small gas molecules when the sky is clear, to the gray caused by Mie scattering off large water droplets when the sky is cloudy. Alternatively, it has been suggested that gray and blue eyes might differ in the concentration of melanin at the front of the stroma.

Gray eyes are most common in Northern and Eastern Europe. Gray eyes can also be found in parts of North West Africa (Aurès Mountains
Aurès Mountains
The Aurès , or Aurea, refers to an Amazigh language-speaking region in East Algeria, as well as an extension of the Atlas mountain range that lies to the east of the Saharan Atlas in eastern Algeria and northwestern Tunisia...

) among the Algeria
Algeria
Algeria , officially the People's Democratic Republic of Algeria , also formally referred to as the Democratic and Popular Republic of Algeria, is a country in the Maghreb region of Northwest Africa with Algiers as its capital.In terms of land area, it is the largest country in Africa and the Arab...

n Shawia people and in the Middle East
Middle East
The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East...

, most notably in Iran
Iran
Iran , officially the Islamic Republic of Iran , is a country in Southern and Western Asia. The name "Iran" has been in use natively since the Sassanian era and came into use internationally in 1935, before which the country was known to the Western world as Persia...

, Afghanistan
Afghanistan
Afghanistan , officially the Islamic Republic of Afghanistan, is a landlocked country located in the centre of Asia, forming South Asia, Central Asia and the Middle East. With a population of about 29 million, it has an area of , making it the 42nd most populous and 41st largest nation in the world...

 (the Indo-Iranian
Indo-Iranians
Indo-Iranian peoples are a linguistic group consisting of the Indo-Aryan, Iranian, Dardic and Nuristani peoples; that is, speakers of Indo-Iranian languages, a major branch of the Indo-European language family....

 Nuristani people), and Pakistan
Pakistan
Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan...

 (the Kalasha, another Indo-Iranian people); Under magnification, gray eyes exhibit small amounts of yellow and brown color in the iris.

A gray iris may indicate the presence of a uveitis
Uveitis
Uveitis specifically refers to inflammation of the middle layer of the eye, termed the "uvea" but in common usage may refer to any inflammatory process involving the interior of the eye....

. However, other visual signs make a uveitis obvious. Gray iris color, as well as blue, are at increased risk of uveal melanoma
Uveal melanoma
Uveal melanoma is a cancer of the eye involving the iris, ciliary body, or choroid . Tumors arise from the pigment cells that reside within the uvea giving color to the eye...

.

Green

As in the case of blue eyes, the color of green eyes does not result simply from the pigmentation of the iris. Rather, their appearance is caused by the combination of an amber or light brown pigmentation of the stroma, given by a moderate concentration of melanin, with the blue tone imparted by the Rayleigh scattering of the reflected light.

Green eyes probably result from the interaction of multiple variants within the OCA2
OCA2
P protein is a protein that in humans is encoded by the OCA2 gene.OCA2 encodes the human homologue of the mouse p gene. The P protein is believed to be an integral membrane protein involved in small molecule transport, specifically tyrosine - a precursor of melanin...

and other genes. They were present in south Siberia
Siberia
Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...

 during the Bronze Age
Bronze Age
The Bronze Age is a period characterized by the use of copper and its alloy bronze as the chief hard materials in the manufacture of some implements and weapons. Chronologically, it stands between the Stone Age and Iron Age...

. They are most common in Northern
Northern Europe
Northern Europe is the northern part or region of Europe. Northern Europe typically refers to the seven countries in the northern part of the European subcontinent which includes Denmark, Estonia, Latvia, Lithuania, Norway, Finland and Sweden...

 and Central Europe
Central Europe
Central Europe or alternatively Middle Europe is a region of the European continent lying between the variously defined areas of Eastern and Western Europe...

. They can also be found in Southern Europe
Southern Europe
The term Southern Europe, at its most general definition, is used to mean "all countries in the south of Europe". However, the concept, at different times, has had different meanings, providing additional political, linguistic and cultural context to the definition in addition to the typical...

 and North Africa
North Africa
North Africa or Northern Africa is the northernmost region of the African continent, linked by the Sahara to Sub-Saharan Africa. Geopolitically, the United Nations definition of Northern Africa includes eight countries or territories; Algeria, Egypt, Libya, Morocco, South Sudan, Sudan, Tunisia, and...

. In Iceland
Iceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...

, 89% of women and 87% of men have either blue or green eye color. 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 recent Celtic and Slavic ancestry, about 16%.

Hazel

Hazel eyes are due to a combination of Rayleigh scattering
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...

 and a moderate amount of melanin in the iris's anterior border layer. Hazel eyes often appear to shift in color from a brown to a green. Although hazel mostly consists of brown and green, the dominant color in the eye can either be brown/gold or green. This is how many people mistake hazel eyes to be amber and vice versa. 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 (or vice versa) when observed in sunlight.

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
Hazelnut
A hazelnut is the nut of the hazel and is also known as a cob nut or filbert nut according to species. A cob is roughly spherical to oval, about 15–25 mm long and 10–15 mm in diameter, with an outer fibrous husk surrounding a smooth shell. A filbert is more elongated, being about twice...

 shell.

Red and violet

The eyes of people with severe forms of albinism
Albinism
Albinism is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of an enzyme involved in the production of melanin...

 may appear red under certain lighting conditions owing to the extremely low quantities of melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...

, allowing the blood vessels to show through. In addition, flash photography can sometimes cause a "red-eye effect
Red-eye effect
The red-eye effect in photography is the common appearance of red pupils in color photographs of eyes. It occurs when using a photographic flash very close to the camera lens , in ambient low light. The effect appears in the eyes of humans and animals that have no tapetum lucidum, hence no...

", in which the very bright light from a flash reflects off the back of the eyeball, which is abundantly vascular, causing the pupil to appear red in the photograph.

Although the deep blue eyes of some people such as Elizabeth Taylor
Elizabeth Taylor
Dame Elizabeth Rosemond "Liz" Taylor, DBE was a British-American actress. From her early years as a child star with MGM, she became one of the great screen actresses of Hollywood's Golden Age...

 can appear violet at certain times, true violet-colored eyes occur only due to albinism.

Medical implications

Those with lighter iris color have been found to have a higher prevalence of age-related macular degeneration
Macular degeneration
Age-related macular degeneration is a medical condition which usually affects older adults and results in a loss of vision in the center of the visual field because of damage to the retina. It occurs in “dry” and “wet” forms. It is a major cause of blindness and visual impairment in older adults...

 (ARMD) than those with darker iris color; lighter eye color is also associated with an increased risk of ARMD progression. An increased risk of uveal melanoma
Uveal melanoma
Uveal melanoma is a cancer of the eye involving the iris, ciliary body, or choroid . Tumors arise from the pigment cells that reside within the uvea giving color to the eye...

 has been found in those with blue, green or gray iris color. 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.

Eye color may also be symptomatic of disease. Aside from the iris, yellowing of the whites
Sclera
The sclera , also known as the white or white of the eye, is the opaque , fibrous, protective, outer layer of the eye containing collagen and elastic fiber. In the development of the embryo, the sclera is derived from the neural crest...

 of the eyes is associated with jaundice
Jaundice
Jaundice is a yellowish pigmentation of the skin, the conjunctival membranes over the sclerae , and other mucous membranes caused by hyperbilirubinemia . This hyperbilirubinemia subsequently causes increased levels of bilirubin in the extracellular fluid...

 and symptomatic of liver disease, including cirrhosis
Cirrhosis
Cirrhosis is a consequence of chronic liver disease characterized by replacement of liver tissue by fibrosis, scar tissue and regenerative nodules , leading to loss of liver function...

, hepatitis
Hepatitis
Hepatitis is a medical condition defined by the inflammation of the liver and characterized by the presence of inflammatory cells in the tissue of the organ. The name is from the Greek hepar , the root being hepat- , meaning liver, and suffix -itis, meaning "inflammation"...

 and malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

. 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.

Wilson's disease

Wilson's disease
Wilson's disease
Wilson's disease or hepatolenticular degeneration is an autosomal recessive genetic disorder in which copper accumulates in tissues; this manifests as neurological or psychiatric symptoms and liver disease...

 involves a mutation of the gene coding for the enzyme ATPase
ATPase
ATPases are a class of enzymes that catalyze the decomposition of adenosine triphosphate into adenosine diphosphate and a free phosphate ion. This dephosphorylation reaction releases energy, which the enzyme harnesses to drive other chemical reactions that would not otherwise occur...

7B, which prevents copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 within the liver from entering the Golgi apparatus
Golgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....

 in cells. Instead, the copper accumulates in the liver and in other tissues, including the iris of the eye. This results in the formation of Kayser-Fleischer ring
Kayser-Fleischer ring
Kayser-Fleischer rings are dark rings that appear to encircle the iris of the eye. They are due to copper deposition as a result of particular liver diseases. They are named after Dr Bernhard Kayser and Dr Bruno Fleischer, the German doctors who first described them in 1902 and 1903...

s, which are dark rings that encircle the periphery of the iris.

Aniridia

Aniridia
Aniridia
Aniridia is the absence of the iris. Aniridia usually involves both eyes. It can be congenital or caused by a penetrant injury. Isolated aniridia is a congenital disorder which is not limited to a defect in iris development, but is a panocular condition with macular and optic nerve hypoplasia,...

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

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
Albinism
Albinism is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of an enzyme involved in the production of melanin...

, the color of the iris 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
Transillumination
Transillumination is the technique of sample illumination by transmission of light through the sample. Transillumination is used in a variety of methods of imaging.-Microscopy:...

 defects can almost always be observed during an eye examination
Eye examination
An eye examination is a battery of tests performed by an ophthalmologist, optometrist, or orthoptist assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes....

 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 more pronounced in albino individuals, and this can emphasize the red eye effect in photographs.

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
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

 within an iris or part of an iris, which may be inherited or acquired by disease
Disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...

 or injury
Injury
-By cause:*Traumatic injury, a body wound or shock produced by sudden physical injury, as from violence or accident*Other injuries from external physical causes, such as radiation injury, burn injury or frostbite*Injury from infection...

. This uncommon condition usually results due to uneven melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...

 content. A number of causes are responsible, including genetic, such as chimerism
Chimera (genetics)
A chimera or chimaera is a single organism that is composed of two or more different populations of genetically distinct cells that originated from different zygotes involved in sexual reproduction. If the different cells have emerged from the same zygote, the organism is called a mosaic...

, Horners Syndrome
Horner's syndrome
Horner's syndrome is the combination of drooping of the eyelid and constriction of the pupil , sometimes accompanied by decreased sweating of the face on the same side; redness of the conjunctiva of the eye is often also present...

 and Waardenburg syndrome
Waardenburg syndrome
Waardenburg syndrome Waardenburg syndrome Waardenburg syndrome (also Waardenburg­ Shah Syndrome, Waardenburg-Klein syndrome, Mende's syndrome II, Van der Hoeve-Halbertsma-Waardenburg syndrome, Ptosis-Epicanthus syndrome, Van der Hoeve-Halbertsma-Gualdi syndrome, Waardenburg type Pierpont,[5] Van...

.
A chimera
Chimera (genetics)
A chimera or chimaera is a single organism that is composed of two or more different populations of genetically distinct cells that originated from different zygotes involved in sexual reproduction. If the different cells have emerged from the same zygote, the organism is called a mosaic...

 can have two different colored eyes just like any two siblings can — because each cell has different eye color genes. A mosaic
Mosaic (genetics)
In genetic medicine, a mosaic or mosaicism denotes the presence of two populations of cells with different genotypes in one individual who has developed from a single fertilized egg...

 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
David Bowie
David Bowie is an English musician, actor, record producer and arranger. A major figure for over four decades in the world of popular music, Bowie is widely regarded as an innovator, particularly for his work in the 1970s...

 has the appearance of different eye colors due to 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.

A common cause in females with heterochromia is X-inactivation
X-inactivation
X-inactivation is a process by which one of the two copies of the X chromosome present in female mammals is inactivated. The inactive X chromosome is silenced by packaging into transcriptionally inactive heterochromatin...

, which can result in a number of heterochromatic traits, such as calico cats
Calico cats
Calico cats are domestic cats with a spotted or parti-colored coat, usually predominantly white with orange and black patches. Outside of North America the pattern is more usually called tortoiseshell-and-white. In the province of Quebec, they are unofficially considered a breed by the...

. Trauma and certain medications, such as some prostaglandin analogue
Prostaglandin analogue
Synthetic prostaglandin analogues are molecules which are manufactured to bind to a prostaglandin receptor.Wider use of prostaglandin analogues is limited by unwanted side effects and their abortive potential.-Uses:...

s, can also cause increased pigmentation in one eye. On occasion, the condition of having two different-colored eyes is caused by blood staining the iris after sustaining injury.

See also

  • Iridology
    Iridology
    Iridology Iridology Iridology (also known as iridodiagnosis or iridiagnosis is an alternative medicine technique whose proponents claim that patterns, colors, and other characteristics of the iris can be examined to determine information about a patient's systemic health...

  • Hair color
    Hair color
    Hair color is the pigmentation of hair follicles due to two types of melanin: eumelanin and pheomelanin. Generally, if more melanin is present, the color of the hair is darker; if less melanin is present, the hair is lighter...

  • Human skin color
    Human skin color
    Human skin color is primarily due to the presence of melanin in the skin. Skin color ranges from almost black to white with a pinkish tinge due to blood vessels underneath. Variation in natural skin color is mainly due to genetics, although the evolutionary causes are not completely certain...

  • Xanthophore
  • List of Mendelian traits in humans

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK