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Eye color is a
polygenicA polygene is a member of a group of non-epistatic gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, Mendelian units of heredity..." (Greek language, Greek) meani ...
phenotypic character determined by two distinct factors: the
pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compound , CH4; is among the simplest organic compou ...
ation of the
eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light L ...

eye
's iris and the frequency-dependence of the
scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light Light or visible light is electromagnetic radiation within the portion of the electromagnetic ...

scattering
of light by the
turbid Turbidity is the cloudiness or haziness of a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are a Phase (matter), phase of matter a ...

turbid
medium in the stroma of the iris. In humans, the pigmentation of the iris varies from light brown to black, depending on the concentration of
melanin Melanin (; from el, μέλας ''melas'', "black, dark") is a broad term for a group of natural pigments found in most organisms. Melanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the ami ...

melanin
in the iris pigment epithelium (located on 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 and green, as well as hazel eyes, results from the
Tyndall scattering
Tyndall scattering
of light in the stroma, a phenomenon similar to that which accounts for the blueness of the sky called
Rayleigh scattering Rayleigh scattering ( ), named after the nineteenth-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength o ...

Rayleigh scattering
. Neither blue nor green pigments are ever present in the human iris or ocular fluid. Eye color is thus an instance of
structural color A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as o ...

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 is an aromatic forms of benzene (top) combine to produce an average structure (bottom) In chemistry, aromaticity is a property of cyclic compound, cyclic (ring (chemistry), ring-shaped), plane (geometry), planar (flat) structures with ...
s,
purine Purine is a heterocyclic 125px, Pyridine, a heterocyclic compound A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different chemical element, elements as members of its ring(s). Heterocyclic chemi ...

purine
s, and
carotenoid Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), organ Chemistry * Organ ...
s. Humans and other animals have many
phenotypic In genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inter ...
variations in eye color.Morris, PJ
"Phenotypes and Genotypes for human eye colors."
Athro Limited website. Retrieved 10 May 2006.
The genetics and inheritance of eye color in humans is complicated. So far, as many as 15 genes have been associated with eye color inheritance. Some of the eye-color genes include ''
OCA2 P protein, also known as melanocyte-specific transporter protein or pink-eyed dilution protein homolog, is a protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within org ...
'' and ''
HERC2HERC2 is a giant E3 ubiquitin protein ligase, implicated in DNA repair regulation, pigmentation and neurological disorders. It is encoded by a gene of the same name belonging to the HERC family, which typically encodes large protein products with C- ...
''. The earlier belief that blue eye color is a simple
recessive trait In genetics Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed for millennia, Gregor Mendel, Moravia, Moravian scientis ...
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. However, OCA2
gene polymorphism Genes which control hair colour are polymorphic. A gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, Mendelian units of heredity..." (Greek language, Greek) mean ...
, close to proximal 5' regulatory region, explains most human eye-color variation.


Genetic determination

Eye color is an inherited trait influenced by more than one
gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...

gene
. These genes are sought using associations to small changes in the genes themselves and in neighboring genes. These changes are known as
single-nucleotide polymorphism In genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interacti ...
s 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 ( , , ) is the second largest city A city is a large human settlement.Goodall, B. (1987) ''The Penguin Dictionary of Human Geography''. London: Penguin.Kuper, A. and Kuper, J., eds (1996) ''The Social Science Encyclopedia''. 2n ...

Rotterdam
(2009) found that it was possible to predict eye color with more than 90% accuracy for brown and blue using just six SNPs. There is evidence that as many as 16 different genes could be responsible for eye color in humans; however, the main two genes associated with eye color variation are ''
OCA2 P protein, also known as melanocyte-specific transporter protein or pink-eyed dilution protein homolog, is a protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within org ...
'' and ''
HERC2HERC2 is a giant E3 ubiquitin protein ligase, implicated in DNA repair regulation, pigmentation and neurological disorders. It is encoded by a gene of the same name belonging to the HERC family, which typically encodes large protein products with C- ...
'', and both are localized in
Chromosome 15 Chromosome 15 is one of the 23 pairs of chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by chape ...
. The gene ''OCA2'' (), when in a variant form, causes the pink eye color and
hypopigmentation Hypopigmentation is characterized specifically as an area of skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticl ...
common in human
albinism Albinism is the congenital absence of any pigmentation or colouration in a person, animal or plant, resulting in white hair, feathers, scales and skin and pink eyes in mammals, birds, reptiles, amphibians and fish and invertebrates as well. V ...
. (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 (or Molé) may refer to: Animals * Mole (animal) or "true mole", mammals in the family Talpidae, found in Eurasia and North America * Golden moles, southern African mammals in the family Chrysochloridae, similar to but unrelated to Talpidae ...
counts,
hair Hair is a protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Ph ...

hair
and
skin tone Afghan children with fair skin Human skin color ranges from the darkest brown to the lightest hues. Differences in skin color among individuals is caused by variation in pigment A pigment is a colored material that is completely or nearly i ...
. The polymorphisms may be in an ''OCA2''
regulatory sequence A regulatory sequence is a segment of a nucleic acid Nucleic acids are biopolymers, macromolecules, essential to all Organism, known forms of life. They are composed of nucleotides, which are the monomers made of three components: a pentose, 5-car ...
, 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 Sodium/potassium/calcium exchanger 4 also known as solute carrier family 24 member 4 is a protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins ...
and TYR. A 2010 study on eye color variation into hue and saturation values using high-resolution digital full-eye photographs found three new loci for a total of ten genes, and now about 50% of eye colour variation can be explained. Blue eyes with a brown spot, green eyes, and gray eyes are caused by an entirely different part of the genome.


Ancient DNA and eye color in Europe

People of European descent show the greatest variety in eye color of any population worldwide. Recent advances in
ancient DNA Ancient DNA (aDNA) is DNA The structure of part of a DNA double helix Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear ...

ancient DNA
technology have revealed some of the history of eye color in Europe. All European
Mesolithic The Mesolithic (Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appro ...

Mesolithic
hunter-gatherer remains so far investigated have shown genetic markers for light-colored eyes, in the case of western and central European hunter-gatherers combined with dark skin color. The later additions to the European
gene pool The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. Description A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can surviv ...
, the Early
Neolithic The Neolithic period is the final division of the Stone Age The Stone Age was a broad prehistoric Prehistory, also known as pre-literary history, is the period of human history Human history, also known as world history, is t ...
farmers from
Anatolia Anatolia,, tr, Anadolu Yarımadası), and the Anatolian plateau. also known as Asia Minor, is a large peninsula in Western Asia and the westernmost protrusion of the Asian continent. It makes up the majority of modern-day Turkey. The region ...
and the
Yamnaya The Yamnaya culture (/ˈjamnaja/), from Russian Я́мная культу́ра, or Yamnaya Horizon, (mistakenly) Yamna culture, (translated) Pit Grave culture, or Ochre Grave culture, was a late Copper Age to early Bronze Age The Bronze ...

Yamnaya
Copper Age The Chalcolithic (),The New Oxford Dictionary of English (1998) , p. 301: "Chalcolithic /,kælkəl'lɪθɪk/ adjective ''Archaeology'' of, relating to, or denoting a period in the 4th and 3rd millennium BC, chiefly in the Near East and SE Europe, ...
/
Bronze Age The Bronze Age is a prehistoric Periodization, period that was characterized by the use of bronze, in some areas proto-writing, and other early features of urban civilization. The Bronze Age is the second principal period of the Three-age sys ...
pastoralists (possibly the
Proto-Indo-European Proto-Indo-European (PIE) is the theorized common ancestor of the Indo-European language family The Indo-European languages are a language family A language is a structured system of communication used by humans, including speech ( ...
population) from the area north of the
Black Sea , with the skyline of Batumi Batumi (; ka, ბათუმი ) is the second largest city of Georgia Georgia usually refers to: * Georgia (country) Georgia ( ka, საქართველო; ''Sakartvelo''; ) is a country locat ...

Black Sea
appear to have had much higher incidences of dark eye color alleles, and alleles giving rise to lighter skin, than the original European population.


Classification of color

Iris color can provide a large amount of information about a person, and a classification of colors may be useful in documenting pathological changes or determining how a person may respond to ocular pharmaceuticals. 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.Fan S, Dyer CR, Hubbard L
Quantification and Correction of Iris Color."
Technical report 1495, University of Wisconsin–Madison, Dec 2003.
Normal 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, along with
structural color A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as o ...

structural color
. Green irises, for example, have some yellow and the blue structural color. Brown irises contain more or less melanin. Some eyes have a dark ring around the iris, called a
limbal ring A limbal ring is a dark ring around the iris of the eye, where the sclera The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective, outer layer of the human eye ...
. Eye color in non-human animals is regulated differently. For example, instead of blue as in humans,
autosomal An autosome is any chromosome that is not a sex chromosome (an allosome). The members of an autosome pair in a diploid Ploidy () is the number of complete sets of chromosome A chromosome is a long DNA molecule with part or all of the ...
recessive In genetics Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed for millennia, Gregor Mendel, Moravia, Moravian scientist ...
eye color in the skink species '''' is black, and the autosomal
dominant Domination or dominant may refer to: Society * World domination, which is mainly a conspiracy theory * Colonialism in which one group (usually a nation) invades another region for material gain or to eliminate competition * Chauvinism in which a p ...
color is yellow-green. As the 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.


Changes in eye color

Most newborn babies who have European ancestry have light-colored eyes. 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 Melanin (; from el, μέλας ''melas'', "black, dark") is a broad term for a group of natural pigments found in most organisms. Melanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the ami ...

melanin
. Because melanocyte cells continually produce pigment, in theory eye color can be changed. Adult eye color is usually established between 3 and 6 months of age, though this can be later. 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 from blue to green or brown as the infant ages. Changes (lightening or darkening) of eye colors during early childhood, puberty, pregnancy, and sometimes after serious trauma (like
heterochromia Heterochromia is a variation in coloration. The term is most often used to describe color differences of the iris (anatomy), iris, but can also be applied to color variation of hair or skin. Heterochromia is determined by the production, deliver ...

heterochromia
) do represent cause for a plausible argument stating 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 and amber eyes. Under the same environmental conditions, there may be disagreement over the color of an object between two different people, and the factor that causes this disagreement is pigments called Melanin, which are the main factors in determining eye color. The higher the amount of these pigments in the Iris and the denser their texture, the darker the color of a person's eyes; The same melanin concentration also depends on many factors such as hereditary, environmental, and other factors. The most important role of eye melanins is to protect the eyes from the sun's harmful rays. People with bright eyes (honey, blue, or green) have fewer sunscreens than other people. Therefore, they should protect their eyes from the sun more than other people. The human eye consists of two types of light and color receptors in the Retina. Cylindrical cells are the photoreceptors of the eye that have a black and white vision and, depending on the amount of light received from the environment, determine the amount of darkness and brightness of objects. The number of cylindrical cells is more than the number of color receptors and reaches about 120 million; Cone cells, which are smaller in number than light receptors, have color vision and are divided into three distinct categories, each of which recognizes one of the colors blue, red, and green, allowing the individual to distinguish colors. When a person is exposed to sunlight, the rays of sunlight hit a part of the back of the eye called the Yellow spot. The cylindrical cells receive these rays, make a neural message from them, and send them to the occipital region of the brain, where they are examined and answered if necessary. At this time, a large amount of ultraviolet light is received by the eye and destroys parts of the light receptors. Because people with bright eyes have less melanin in their eyes than people with dark eyes, the lack of this protective factor means that the light receptors in their eyes are more damaged and destroyed than in other people. This difference in the number of light receptors in the eyes of different people causes a difference in the amount of light they receive from the environment; For this reason, people with bright eyes see colors a little darker than others; But this amount is so small that it is not very visible in everyday life and only appears as a slight difference of opinion between people with different eye colors.


Eye color chart (Martin scale)

Carleton Coon created a chart by the original Martin scale. The numbering is reversed on the scale below in the (later) Martin–Schultz scale, which is (still) used in
physical anthropology Biological anthropology, also known as physical anthropology, is a scientific discipline concerned with the biological and behavioral aspects of human beings, their extinct hominin The Hominini form a Tribe (biology), taxonomic tribe of the subf ...
. Light and light-mixed eyes (16–12 in Martin scale) Pure light (16–15 in Martin scale) * 16: pure light blue * 15: gray Light-mixed (14–12 in Martin scale) * 14: Very light-mixed (blue with gray or green or green with gray) * 13-12: Light-mixed (light or very light-mixed with small admixture of brown) Mixed eyes (11–7 in Martin scale) * Mixture of light eyes (blue, gray or green) with brown when light and brown appearance is at the same level Dark and dark-mixed eyes (6–1 in Martin scale) * Dark-mixed: 6–5 in Martin scale. Brown with small admixture of light * 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 may be due to the deposition of the yellow pigment called lipochrome 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 that many people mistake for hazel, though hazel tends to be duller and contains green with red/gold flecks, as mentioned above. Amber eyes may also contain amounts of very light gold-ish gray. The eyes of some pigeons contain yellow fluorescing pigments known as
pteridine Pteridine is an aromatic forms of benzene (top) combine to produce an average structure (bottom) In chemistry, aromaticity is a property of cyclic compound, cyclic (ring (chemistry), ring-shaped), plane (geometry), planar (flat) structures with ...
s. The bright yellow eyes of the
great horned owl The great horned owl (''Bubo virginianus''), also known as the tiger owl (originally derived from early naturalists' description as the "winged tiger" or "tiger of the air"), or the hoot owl,Austing, G.R. & Holt, Jr., J.B. (1966). ''The World of ...
are thought to be due to the presence of the pteridine pigment within certain
chromatophore Chromatophores are cells that produce color, of which many types are pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Genera ...
s (called xanthophores) located in the iris stroma. In humans, yellowish specks or patches are thought to be due to the pigment
lipofuscin Lipofuscin is the name given to fine yellow-brown composed of -containing residues of digestion. It is considered to be one of the aging or "wear-and-tear" pigments, found in the , , muscle, retina, s, cells, and cells. Formation and turn ...
, 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. With a world population share of 5%, amber eyes are extremely rare anywhere in the world. It is possible to find people with that eye color in the , as well as in
Hungary Hungary ( hu, Magyarország ) is a in . Spanning of the , it is bordered by to the north, to the northeast, to the east and southeast, to the south, and to the southwest and to the west. Hungary has a population of 10 million, mostl ...

Hungary
, in
Southern France Southern France, also known as the South of France or colloquially in French as , is a defined geographical area consisting of the regions of France France France (), officially the French Republic (french: link=no, République fr ...
,
Italy Italy ( it, Italia ), officially the Italian Republic ( it, Repubblica Italiana, links=no ), is a country consisting of a peninsula delimited by the Alps The Alps ; german: Alpen ; it, Alpi ; rm, Alps; sl, Alpe ) are the highest ...

Italy
, and to a lesser degree in the
Iberian Peninsula The Iberian Peninsula , ** * Aragonese Aragonese or Aragones may refer to: * Something related to Aragon, an autonomous community and former kingdom in Spain * the Aragonese people, those originating from or living in the historical region ...

Iberian Peninsula
,
Southern Cone The Southern Cone ( es, Cono Sur, pt, Cone Sul) is a geographic and cultural region composed of the southernmost areas of South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisp ...
and
Middle East The Middle East ( ar, الشرق الأوسط, ISO 233 The international standard An international standard is a technical standard A technical standard is an established norm (social), norm or requirement for a repeatable technical task whi ...

Middle East
. In
Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At 8.5 million square kilometers (3.2 million square miles) and with over 211 mill ...

Brazil
, it can be found particularly in the Southern Region, as well as in
São Paulo São Paulo (, ; Portuguese for 'Saint Paul') is a city in the Southeast Region, Brazil, Southeast Region of Brazil. Listed by the Globalization and World Cities Research Network, GaWC as an global city, alpha global city, the Municipalities of ...
,
Minas Gerais Minas Gerais () is a state in Southeastern Brazil. It ranks as the second most populous, the third by Gross Domestic Product, gross domestic product (GDP), and the fourth largest by area in the country. The state's capital and largest city, Belo ...

Minas Gerais
and other places that received strong European immigration.


Blue

There is no blue pigmentation either in the iris or in the ocular fluid. Dissection reveals that the iris pigment
epithelium Epithelium is one of the four basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as ...
is brownish black due to the presence of
melanin Melanin (; from el, μέλας ''melas'', "black, dark") is a broad term for a group of natural pigments found in most organisms. Melanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the ami ...

melanin
. Unlike brown eyes, blue eyes have low concentrations of melanin 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 ( ), named after the nineteenth-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength o ...

Rayleigh scattering
in the
turbid Turbidity is the cloudiness or haziness of a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are a Phase (matter), phase of matter a ...

turbid
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 Tyndall (the original spelling, also Tyndale, "Tindol", Tyndal, Tindoll, Tindall, Tindal, Tindale, Tindle, Tindell, Tindill, and Tindel) is the name of an English family taken from the land they held as tenants in chief of the Kings of England ...

Tyndall
blue"
structural color A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as o ...

structural color
that varies with external lighting conditions. In humans, the inheritance pattern followed by blue eyes is considered similar to that of a
recessive In genetics Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed for millennia, Gregor Mendel, Moravia, Moravian scientist ...
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 tracked down a single genetic mutation that leads to blue eyes. "Originally, we all had brown eyes," said Eiberg. Eiberg and colleagues suggested in a study published in ''Human Genetics'' that a mutation in the 86th
intron An intron (for ''intragenic region'') is any Nucleic acid sequence, nucleotide sequence within a gene that is removed by RNA splicing during Post-transcriptional modification, maturation of the final RNA product. In other words, introns are non-c ...

intron
of the ''
HERC2HERC2 is a giant E3 ubiquitin protein ligase, implicated in DNA repair regulation, pigmentation and neurological disorders. It is encoded by a gene of the same name belonging to the HERC family, which typically encodes large protein products with C- ...
'' gene, which is hypothesized to interact with the ''OCA2'' gene promoter, reduced expression of ''OCA2'' with subsequent reduction in melanin production. The authors suggest that the mutation may have arisen in the northwestern part of the
Black Sea , with the skyline of Batumi Batumi (; ka, ბათუმი ) is the second largest city of Georgia Georgia usually refers to: * Georgia (country) Georgia ( ka, საქართველო; ''Sakartvelo''; ) is a country locat ...

Black Sea
region, and add that it is "difficult to calculate the age of the mutation." Blue eyes are common in northern and eastern Europe, particularly around the
Baltic Sea The Baltic Sea is an arm of the Atlantic Ocean, enclosed by Denmark Denmark ( da, Danmark, ) is a Nordic country The Nordic countries, or the Nordics, are a geographical and cultural region In geography, regions are areas that a ...

Baltic Sea
. Blue eyes are also found in southern Europe,
Central Asia Central Asia is a region in Asia which stretches from the Caspian Sea in the west to China and Mongolia in the east, and from Afghanistan and Iran in the south to Russia in the north, including the former Soviet Union, Soviet republics of the Sov ...

Central Asia
,
South Asia South Asia is the southern region of Asia Asia () is 's largest and most populous , located primarily in the and . It shares the continental of with the continent of and the continental landmass of with both Europe and . Asia cov ...

South Asia
,
North Africa North Africa or Northern Africa is a region encompassing the northern portion of the African continent. There is no singularly accepted scope for the region, and it is sometimes defined as stretching from the Atlantic shores of Mauritania in th ...

North Africa
and
West Asia Western Asia, also West Asia, is the westernmost subregion of Asia Asia () is Earth's largest and most populous continent, located primarily in the Eastern Hemisphere, Eastern and Northern Hemisphere, Northern Hemisphere of the Earth, Hem ...
. File:Bimbam.jpg, A with distinctive
sapphire Sapphire is a precious gemstone A gemstone (also called a fine gem, jewel, precious stone, or semi-precious stone) is a piece of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical c ...

sapphire
blue eyes File:Karasu2a.JPG,
Jungle crow Jungle crow is a common name that refers to three species of crow A crow is a bird of the genus ''Corvus'', or more broadly a synonym for all of ''Corvus''. The word "crow" is used as part of the common name of species including: * ''Corvus albus' ...
File:FrankieakaLogan.jpg, The first blue-eyed
koala The koala or, inaccurately, koala bear (''Phascolarctos cinereus''), is an arboreal herbivorous marsupial native to Australia. It is the only Extant taxon, extant representative of the family Phascolarctidae and its closest living relative ...

koala
known to be born in captivity
The same DNA sequence in the region of the OCA2 gene among blue-eyed people suggests they may have a single common ancestor. , the earliest remains of
Homo sapiens Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedality, bipedalism and large, complex brains. This has enabled the development of advanced tools, culture, and language. Humans are highl ...

Homo sapiens
with genes for both light-pigmentation and blue-eyes were found in 7,700 years old Mesolithic
hunter-gatherers A hunter-gatherer is a human Humans (''Homo sapiens'') are the most populous and widespread species of primates, characterized by bipedality, opposable thumbs, hairlessness, and intelligence allowing the use of culture, language and tools. T ...
from
Motala Motala () is a Urban areas in Sweden, locality and the seat of Motala Municipality, Östergötland County, Sweden with 29,823 inhabitants (41,956 in the entire municipality) in 2010. It is the third largest city of Östergötland, following Linkö ...
,
Sweden Sweden ( sv, Sverige ), officially the Kingdom of Sweden ( sv, links=no, Konungariket Sverige ), is a Nordic country The Nordic countries, or the Nordics, are a geographical and cultural region In geography, regions are areas that ...

Sweden
. Approximately 8% to 10% of the global population have blue eyes. A 2002 study found that the prevalence of blue eye color among the white population in the United States to be 33.8% for those born from 1936 through 1951, compared with 57.4% for those born from 1899 through 1905. , one out of every six Americans, or 16.6% of the total US population, has blue eyes, including 22.3% of whites. Blue eyes are continuing to become less common among American children. 56% of
Slovenes The Slovenes, also known as Slovenians ( sl, Slovenci ), are a nation A nation is a community A community is a social unit (a group of living things) with commonality such as Norm (social), norms, religion, values, Convention (norm), cust ...
have blue/green eyes. Blue eyes are rare in mammals; one example is the recently discovered marsupial, the blue-eyed spotted cuscus (''Spilocuscus wilsoni''). The trait is hitherto known only from a single
primate A primate ( ) (from Latin , from 'prime, first rank') is a eutherian mammal constituting the Taxonomy (biology), taxonomic order (biology), order Primates (). Primates arose 85–55 million years ago first from small Terrestrial animal, ...

primate
other than humans – Sclater's lemur (''Eulemur flavifrons'') of Madagascar. While some
cat The cat (''Felis catus'') is a domestic Domestic may refer to: In the home * Anything relating to the human home A home, or domicile, is a space used as a permanent or semi-permanent residence for an individual, group or famil ...

cat
s and
dog The dog or domestic dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the ...

dog
s have blue eyes, this is usually due to another mutation that is associated with deafness. But in cats alone, there are four identified gene mutations that produce blue eyes, some of which are associated with
congenital A birth defect, also known as a congenital disorder, is a condition present at birth Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiate ...
neurological disorder A neurological disorder is any disorder of the nervous system In biology, the classical doctrine of the nervous system determines that it is a Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sen ...
s. The mutation found in the
Siamese cat The Siamese cat is one of the first distinctly recognized Cat breed, breeds of Asian cat. Derived from the Wichianmat landrace, one of several varieties of cat native to Thailand (formerly known as Siam), the original Siamese became one of the m ...

Siamese cat
s is associated with
strabismus Strabismus is a condition in which the eyes do not properly align with each other when looking at an object. The eye that is focused on an object can alternate. The condition may be present occasionally or constantly. If present during a large ...

strabismus
(crossed eyes). The mutation found in blue-eyed solid white cats (where the coat color is caused by the gene for "epistatic white") is linked with deafness. However, there are phenotypically identical, but genotypically different, blue-eyed white cats (where the coat color is caused by the gene for white spotting) where the coat color is not strongly associated with deafness. In the blue-eyed breed, there may be other neurological defects. Blue-eyed non-white cats of unknown genotype also occur at random in the cat population.


Brown

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. Dark brown eyes are dominant in humans and in many parts of the world, it is nearly the only iris color present. Brown eyes are common in
Europe Europe is a continent A continent is any of several large landmass A landmass, or land mass, is a large region In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of scienc ...

Europe
,
East Asia East Asia is the eastern region of Asia, which is defined in both Geography, geographical and culture, ethno-cultural terms. The modern State (polity), states of East Asia include China, Japan, Mongolia, North Korea, South Korea, and Taiwan. ...

East Asia
,
Southeast Asia Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia or SEA, is the geographical United Nations geoscheme for Asia#South-eastern Asia, southeastern subregion of Asia, consisting of the regions ...

Southeast Asia
,
Central Asia Central Asia is a region in Asia which stretches from the Caspian Sea in the west to China and Mongolia in the east, and from Afghanistan and Iran in the south to Russia in the north, including the former Soviet Union, Soviet republics of the Sov ...

Central Asia
,
South Asia South Asia is the southern region of Asia Asia () is 's largest and most populous , located primarily in the and . It shares the continental of with the continent of and the continental landmass of with both Europe and . Asia cov ...

South Asia
,
West Asia Western Asia, also West Asia, is the westernmost subregion of Asia Asia () is Earth's largest and most populous continent, located primarily in the Eastern Hemisphere, Eastern and Northern Hemisphere, Northern Hemisphere of the Earth, Hem ...
,
Oceania Oceania (, , ) is a geographic region In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Eart ...

Oceania
,
Africa Africa is the world's second-largest and second-most populous continent A continent is any of several large landmass A landmass, or land mass, is a large region In geography Geography (from Greek: , ''geographia'', ...

Africa
and the
Americas The Americas (also collectively called America) is a landmass comprising the totality of North America, North and South America. The Americas make up most of the land in Earth's Western Hemisphere and comprise the New World. Along with th ...

Americas
. Brown is by far the most common eye color, with approximately 79% of people in the world having it. Light or medium-pigmented brown eyes can also be commonly found in
South Europe Southern Europe is the southern subregion of Europe Europe is a continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are ...
, among the
Americas The Americas (also collectively called America) is a landmass comprising the totality of North America, North and South America. The Americas make up most of the land in Earth's Western Hemisphere and comprise the New World. Along with th ...

Americas
, and parts of Central
Asia Asia () is Earth's largest and most populous continent, located primarily in the Eastern Hemisphere, Eastern and Northern Hemisphere, Northern Hemisphere of the Earth, Hemispheres. It shares the continental landmass of Eurasia with the cont ...

Asia
,
West Asia Western Asia, also West Asia, is the westernmost subregion of Asia Asia () is Earth's largest and most populous continent, located primarily in the Eastern Hemisphere, Eastern and Northern Hemisphere, Northern Hemisphere of the Earth, Hem ...
and
South Asia South Asia is the southern region of Asia Asia () is 's largest and most populous , located primarily in the and . It shares the continental of with the continent of and the continental landmass of with both Europe and . Asia cov ...

South Asia
.


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 gray eyes have larger deposits of
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

collagen
in the stroma, so that the light that is reflected from the epithelium undergoes
Mie scattering The Mie solution to Maxwell's equations Maxwell's equations are a set of coupled partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, and electric cir ...

Mie scattering
(which is not strongly frequency-dependent) rather than Rayleigh scattering (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 of 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 can also be found among the
Algeria ) , image_map = Algeria (centered orthographic projection).svg , map_caption = , image_map2 = , capital = Algiers , coordinates = , largest_city = capital , religion = , official_languages = , languages_type = Oth ...

Algeria
n Shawia people of the Aurès Mountains in Northwest Africa, in the
Middle East The Middle East ( ar, الشرق الأوسط, ISO 233 The international standard An international standard is a technical standard A technical standard is an established norm (social), norm or requirement for a repeatable technical task whi ...

Middle East
/
West Asia Western Asia, also West Asia, is the westernmost subregion of Asia Asia () is Earth's largest and most populous continent, located primarily in the Eastern Hemisphere, Eastern and Northern Hemisphere, Northern Hemisphere of the Earth, Hem ...
,
Central Asia Central Asia is a region in Asia which stretches from the Caspian Sea in the west to China and Mongolia in the east, and from Afghanistan and Iran in the south to Russia in the north, including the former Soviet Union, Soviet republics of the Sov ...

Central Asia
, and
South Asia South Asia is the southern region of Asia Asia () is 's largest and most populous , located primarily in the and . It shares the continental of with the continent of and the continental landmass of with both Europe and . Asia cov ...

South Asia
. The Greek goddess Athene appears with grey eyes (γλαυκῶπις). Under magnification, gray eyes exhibit small amounts of yellow and brown color in the iris. Gray is the second-rarest natural eye color after green, with 3% of the world's population having it.


Green

As with blue eyes, the color of green eyes does not result simply from the pigmentation of the iris. The green color is caused by the combination of: 1) an amber or light brown pigmentation in the stroma of the iris (which has a low or moderate concentration of melanin) with: 2) a blue shade created by the Rayleigh scattering of reflected light. Green eyes contain the yellowish pigment lipochrome. Green eyes probably result from the interaction of multiple variants within the ''
OCA2 P protein, also known as melanocyte-specific transporter protein or pink-eyed dilution protein homolog, is a protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within org ...
'' and other genes. They were present in south Siberia during the
Bronze Age The Bronze Age is a prehistoric Periodization, period that was characterized by the use of bronze, in some areas proto-writing, and other early features of urban civilization. The Bronze Age is the second principal period of the Three-age sys ...
. They are most common in Northern Europe, Northern, Western Europe, Western and Central Europe. In Scotland and Ireland, 86% of people have either blue or green eyes. In Iceland, 89% of women and 87% of men have either blue or green eye color. Among European Americans, green eyes are most common among those of recent Celts (modern), Celtic and Germanic peoples, Germanic ancestry, about 16%. 40.8% of Italians from Verona, 22.5% of Spaniards from Alicante and 15.4% of Greeks from Athens have green, grey, and blue eyes. Globally, however, green is considered the rarest natural eye color; only 2% of the world's population have it.


Hazel

Hazel eyes are due to a combination of
Rayleigh scattering Rayleigh scattering ( ), named after the nineteenth-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength o ...

Rayleigh scattering
and a moderate amount of melanin in the iris' 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 shell. Around 18% of the US population and 5% of the world population have hazel eyes.


Special cases


Red and violet

The eyes of people with severe forms of
albinism Albinism is the congenital absence of any pigmentation or colouration in a person, animal or plant, resulting in white hair, feathers, scales and skin and pink eyes in mammals, birds, reptiles, amphibians and fish and invertebrates as well. V ...
may appear red under certain lighting conditions owing to the extremely low quantities of
melanin Melanin (; from el, μέλας ''melas'', "black, dark") is a broad term for a group of natural pigments found in most organisms. Melanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the ami ...

melanin
,NOAH – What is Albinism?
. Albinism.org. Retrieved on 23 December 2011.
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 a flash reflects off the retina, 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 can appear violet at certain times, "true" violet-colored eyes occur only due to albinism. Eyes that appear red or violet under certain conditions due to albinism are less than 1 percent of the world's population.


Two different colors

As a result of heterochromia iridum, it is also possible to have two different eye colors. This occurs in humans and certain breeds of domesticated animals and affects less than 1 percent of the world's population.


Spectrum of eye color

File:eye55.jpg File:Blue-green eye closeup.JPG File:Eye Central Heterochromia crop and lighter.jpg File:Humaniris.jpg File:Hazel Eyes, Caucasian Male, Age 23.jpg File:Human Iris JD052007.jpg File:Greeneye.jpg File:Hazel green eye close up.jpg File:Hazel eye1.png File:Green Eye.jpg File:Green light mixed type B.JPG File:Eye big.JPG File:Amber Brown Green.jpg File:amber-eyes.jpg File:MyStrangeIris.JPG File:Indianeye.png File:Lens5.jpg


Medical implications

Those with lighter iris color have been found to have a higher prevalence of macular degeneration, age-related macular degeneration (ARMD) than those with darker iris color; lighter eye color is also associated with an increased risk of ARMD progression. A gray iris may indicate the presence of a uveitis, and an increased risk of uveal melanoma has been found in those with blue, green or gray eyes. 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.


Wilson's disease

Wilson's disease involves a mutation of the gene coding for the enzyme Wilson disease protein, ATPase 7B, which prevents copper within the liver from entering the Golgi apparatus 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 rings, which are dark rings that encircle the periphery of the iris.


Coloration of the sclera

Eye color outside of the iris may also be symptomatic of disease. Yellowing of the sclera (the "whites of the eyes") is associated with jaundice, and may be symptomatic of liver diseases such as cirrhosis or hepatitis. A blue coloration of the sclera may also be symptomatic of disease.


Aniridia

Aniridia 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 is the congenital absence of any pigmentation or colouration in a person, animal or plant, resulting in white hair, feathers, scales and skin and pink eyes in mammals, birds, reptiles, amphibians and fish and invertebrates as well. V ...
, 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 defects can almost always be observed during an eye examination 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 light reflex, pupillary reflex is much more pronounced in albino individuals, and this can emphasize the red eye effect in photographs.


Heterochromia

Heterochromia (''heterochromia iridum'' or ''heterochromia iridis'') is an eye condition in which one iris is a different color from the other (complete heterochromia), or where a 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 A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compound , CH4; is among the simplest organic compou ...
within an iris or part of an iris, which may be Biological inheritance, inherited or acquired by disease or injury. This uncommon condition usually results due to uneven
melanin Melanin (; from el, μέλας ''melas'', "black, dark") is a broad term for a group of natural pigments found in most organisms. Melanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the ami ...

melanin
content. A number of causes are responsible, including genetic, such as Chimera (genetics), chimerism, Horner's syndrome and Waardenburg syndrome. A Chimera (genetics), chimera can have two different colored eyes just like any two siblings can—because each cell has different eye color genes. A Mosaic (genetics), 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, the film actor Lee Van Cleef was born with one blue eye and one green eye, a trait that reportedly was common in his family, suggesting that it was a genetic trait. This anomaly, which film producers thought would be disturbing to film audiences, was "corrected" by having Van Cleef wear brown contact lenses. David Bowie, on the other hand, had the appearance of different eye colors due to an injury that caused one pupil to be permanently dilated. Another hypothesis about heterochromia is that it can result from a viral infection ''in utero'' affecting the development of one eye, possibly through some sort of genetic mutation. Occasionally, heterochromia can be a sign of a serious medical condition. 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 pigmentation in one eye. On occasion, a difference in eye color is caused by blood staining the iris after injury.


Mate selection and traits that have been linked to iris color


Selection for rare iris colors

A study compared the frequency of eye color in commercial advertising models in Brazil and the UK; these countries were chosen because they have inverted frequencies of eye-coloration, with Brazil having an excess of brown and the UK an excess of light-colored eyes. Models are chosen for their attractiveness, and it was found that, in Brazil, models with light eyes are in a significant excess compared to the levels found in the general population, while, in the UK, models with brown or intermediate eyes were in significant excess over their frequency in the general population. This suggests that eye color rarity plays a role in sexual attraction, people with rare eye colors being perceived as being more attractive. Which may reflect sexual selection of mates with rare eye colors.


Impact on vision

Although people with lighter eye color are generally more sensitive to light because they have less pigment in the iris to protect them from sunlight, there is little to no evidence that eye color has a direct impact on vision qualities such as visual acuity. However, there is a study that found that dark-eyed people perform better at "reactive-type tasks", which suggests they may have better reaction times. People with light-colored eyes, however, performed better at so-called "self-paced tasks", which include activities like hitting a golf ball or throwing baseballs. In another study, people with darker eyes performed better at hitting racquetballs. There are also other studies that challenge those findings. According to scientists, more study is needed to verify these results.


See also

* Hair color * Iridology * Human skin color * Xanthophore * List of Mendelian traits in humans


References


External links


genetics.thetech.org


{{DEFAULTSORT:Eye Color Eye color, Eye Facial features Color