Eye color
Eye color is a polygenic phenotypic trait determined by two factors: the pigmentation of the eye's iris and the frequency-dependence of the scattering of light by the 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 in the iris pigment epithelium, the melanin content within the iris stroma, and the cellular density of the stroma. The appearance of blue, green, and hazel eyes results from the Tyndall scattering of light in the stroma, a phenomenon similar to Rayleigh scattering which accounts for the blue sky. Neither blue nor green pigments are present in the human iris or vitreous humour. This is an example of structural color, which depends 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 pteridines, purines, and carotenoids. Humans and other animals have many phenotypic variations in eye color. Mammals are born with blue eyes and all variations in darkening develop after birth.
The genetics and inheritance of eye color in humans is complicated., as many as 16 genes have been associated with human eye color inheritance. Some of the eye-color genes include OCA2 and HERC2. The earlier belief that blue eye color is a recessive trait has been shown to be incorrect, and the genetics of eye color are so complex that almost any parent-child combination of eye colors can occur.
Genetic determination
Eye color is an inherited trait determined by multiple genes. These genes are sought by studying small changes in the genes themselves and in neighboring genes, called single-nucleotide polymorphisms or SNPs. The total number of genes that contribute to eye color is unknown, but there are a few likely candidates. A study in Rotterdam found that it was possible to predict eye color with more than 90% accuracy for brown and blue using just six SNPs.In humans, eye color is a highly sexually dimorphic trait. Several studies have shown that men are more likely to have blue eyes than women, while women are more likely to have darker eye colors than men. Sex is therefore a major factor in the expression of eye color genotypes. One study suggested that women's higher levels of the sex hormone estrogen may explain why women tend to have darker eyes than men.
People of European descent show the greatest variety in eye color of any population worldwide. Recent advances in ancient DNA technology have revealed some of the history of eye color in Europe. Through the analysis of ancient DNA, a 2020 study published in Experimental Dermatology suggested that the common gene for blue eye color likely originated in the Near East and arrived in Europe around 42,000 years ago, after the exodus out of Africa.
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 and HERC2, and both are localized in chromosome 15.
The gene OCA2, when in a variant form, causes the pink eye color and hypopigmentation common in human albinism. Different SNPs within OCA2 are strongly associated with blue and green eyes as well as variations in freckling, mole counts, hair and skin tone. The polymorphisms may be in an OCA2 regulatory sequence, 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. A 2010 study of eye color variation in hue and saturation values using high-resolution digital full-eye photographs found three new loci for a total of ten genes, allowing the explanation of about 50% of eye color variation.
| Gene name | Effect on eye color |
| OCA2 | Associated with melanin producing cells. Central importance to eye color. |
| HERC2 | Affects function of OCA2, with a specific mutation strongly linked to blue eyes. |
| SLC24A4 | Associated with differences between blue and green eyes. |
| TYR | Associated with differences between blue and green eyes. |
Blue eyes with a brown spot, green eyes, and gray eyes are caused by an entirely different part of the genome.
Changes in eye color
A 1997 study of White Americans found that eye color may be subject to change in infancy, and from adolescence to adulthood. Seventeen percent of children experienced a change of eye color by adulthood. Of those children, 50% of developed lighter eyes as they got older. The other 50% developed darker eyes.Generally, children with hazel and light brown eyes tended to experience a lightening of their eye color by adulthood. Children with green eyes often experienced a darkening of their eye color. It was also found that 11% of the children's mothers experienced an eye color change during the same period, with most developing lighter eyes, relative to their original color at the time of their child's birth.
Eye color range
As is the case with hair color and skin color, the melanin that is usually produced in the bodies of almost all mammals is either eumelanin or pheomelanin, resulting in a high proportion of black, brown, red, and yellow coloring in the biological features that make use of it. As such, most mammals generally have a predominantly orange-based color palette, which generally varies from light orange colors like peach to dark orange colors like brown; sometimes even more varied colors like maroon are exhibited.Brown
Almost all mammals have brown or darkly-pigmented irises. In humans, brown is by far the most common eye color, with approximately 79% of people in the world having it. 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.In many parts of the world, it is nearly the only iris color present. Brown eyes are common in Europe, East Asia, Southeast Asia, Central Asia, South Asia, West Asia, Oceania, West Africa and the Americas. Dark-pigmented brown eyes are the most common variant of brown eyes due to it being found in various parts of the world such as the Americas, Africa, West Asia, South Asia, East Asia, Southeast Asia, Central Asia, Oceania, and Europe. Light or medium-pigmented brown eyes can also be commonly found in Europe, among the Americas, and parts of Central Asia, West Asia, South Asia, and East Africa. Light brown eyes bordering amber and hazel coloration are more common in Europe, but can also be observed in East Asia, Southeast Asia, North Africa and East Africa.
Amber
Amber eyes are a solid color with a strong yellowish/golden or russet/coppery tint, which may be due to a yellow pigment called lipochrome . Amber eyes should not be confused with hazel eyes. Although hazel eyes may contain specks of amber or gold, they usually tend to have 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 similar to gold, some people have russet- or copper-colored amber eyes that are mistaken 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 pteridines. The bright yellow eyes of the great horned owl are thought to be due to the presence of the pteridine pigment xanthopterin within certain chromatophores located in the iris stroma. In humans, yellowish specks or patches are thought to be due to the pigment lipofuscin, also known as lipochrome. Many animals such as canines, domestic cats, owls, eagles, pigeons, and fish have amber eyes, whereas in humans this color occurs less frequently. Amber is the third-rarest natural eye color after green and gray, occurring in 5% of the world's population. People with amber eyes are found in Europe, and in fewer numbers in the Middle East, North Africa, and South America.
Hazel
The hazel color of eyes is caused by a combination of 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 why hazel eyes can be mistaken as amber, and why amber is often counted as hazel in studies, and vice versa. The combination 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 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. 55.2% of Spanish subjects in a series of 221 photographs were judged to have hazel eyes. Hazel eyes are found in Europe, most commonly in the Netherlands and the United Kingdom, and have also been observed to be very common among the Low Saxon-speaking populations of northern Germany.
Green
Green eyes probably result from the interaction of multiple allelic variants of OCA2 and other genes. They may have been present in southern Siberia during the Bronze Age.Green is the rarest human eye color, seen in about 2% of all people worldwide. Green eyes are most common in Northern, Western, and Central Europe. Around 8–10% of men and 18–21% of women in Iceland and 6% of men and 17% of women in the Netherlands have green eyes. Ireland is generally regarded as having the highest percentage of green eyed people in the world followed by Scotland, although the exact percentage is relatively unknown due to a lack of major research. Among European Americans, green eyes are most common among those of recent Celtic and Germanic ancestry, about 16%.
The green color is caused by the combination of: 1) an amber or light brown pigmentation in the stroma of the iris a blue shade created by the Rayleigh scattering of reflected light. Green eyes contain the yellowish pigment lipochrome.