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 (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, 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.Sturm R.A. & Larsson M., Genetics of human iris colour and patterns, Pigment Cell Melanoma Res, 22:544-562, 2009. 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.Morris, PJ. "Phenotypes and Genotypes for human eye colors." Athro Limited website. Retrieved 10 May 2006. 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.
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 some likely candidates. A study in Rotterdam (2009) found that it was possible to predict eye color with more than 90% accuracy for brown and blue using just six SNPs."DNA test for eye colour could help fight crime", New Scientist 14 March 2009.
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. "The genetic history of today's European populations is based on continuous migrations over the past 40 000 years. Homo sapiens arrived in Europe from Near East some 42 000 years ago.[48, 49] Like in their African origin, these humans had dark skin but due to variations of their OCA2 gene (causing iris depigmentation) many of them had blue eyes[48,50] (Figure 2, left)." Figure 2: "Phenotype information was retrieved from supplementary files of the according publications or additionally assessed following the instructions of HirisPlex-S, which is a forensic DNA phenotyping tool based on an array of different marker SNPs providing additional support to the SNPs of the genes SLC24A5, SLC45A2 and OCA2."
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. (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 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% 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.Brenda J Bradley, Anja Pedersen, Nicholas I Mundy: Blue eyes in lemurs and humans: same phenotype, different genetic mechanism Am J Phys Anthropol. 2009 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, bordering on black, 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, often bordering on hazel, can also be commonly found in Europe and in the Americas, particularly amongst European descendants, and in smaller numbers in parts of Central Asia, West Asia, South Asia, and East Africa.
Hazel
Definitions of the eye color "hazel" vary: it is traditionally considered to be synonymous with light brown or gold, as in the color of a hazelnut shell, thus making it synonymous with amber. They typically have a strong yellowish/golden or russet/coppery tint, primarily noticeable under sunlight, which is due to a yellow pigment called lipochrome (also found in green eyes).Howard Hughes Medical Institute: Ask A Scientist . Hhmi.org. Retrieved on 23 December 2011.Larry Bickford Eye Color Amber Eyecarecontacts.com. Retrieved on 23 December 2011."OCA2: The Gene for Color" . allaboutgenes.weebly.com. Retrieved on 8 September 2016. They may also contain amounts of very light goldish gray.
In looser modern definitions, it can also be considered a mixed eye color of brown and green, although in anthropology this was typically referred to as a mixed eye color, rather than hazel. The mixed brown-green 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 (or vice versa) when observed in sunlight, often appearing to shift from brown to green.
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 (called xanthophores) 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 golden hazel, or amber, eyes.
Around 18% of the US population and 5% of the world population have hazel eyes. Hazel eyes are found primarily in northern and central Europe, most commonly in the United Kingdom, and the traditionally Low Saxon speaking populations of the Netherlands, and Germany Additionally, in mythology, Apollonius Rhodius described the sun god Helios, and his descendants, as having golden eyes.
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. 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.
The green color is caused by the combination of: 1) An iris containing the yellowish pigment lipochrome and 2) a blue shade created by the Rayleigh scattering of reflected light.
Blue
There is no intrinsically blue pigmentation either in the iris or in the vitreous body; in fact, a form of melanin that would produce a blue coloration does not currently exist in the bodies of most mammals. Rather, blue eyes result from structural color in combination with certain concentrations of non-blue pigments. The iris pigment epithelium is brownish black due to the presence of 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 in the turbid medium of the stroma. This is the same scattering that accounts for the blue appearance of the sky. The result is a "Tyndall blue" structural color that varies with external lighting conditions.
In 2008, a team of researchers from the University of Copenhagen located a single mutation that causes the phenomenon of blue eyes. The research was published in the Journal of Human Genetics. The same DNA sequence of the OCA2 gene among blue-eyed people suggests they may have a single common ancestor. The researchers hypothesized that the OCA2 mutation responsible for blue eyes arose in an individual who lived in the northwestern part of the Black Sea region in Europe sometime between 6,000 and 10,000 years ago, during the Neolithic period. "Everyone with blue eyes alive today can trace their ancestry back to one person who probably lived about 10,000 years ago in the Black Sea region, a study has found." However, more recent ancient DNA research has identified human remains much older than the Neolithic period which possess the OCA2 mutation for blue eyes. It is now believed that the OCA2 allele responsible for blue eyes dates back to the migration of modern humans out of Africa roughly 50,000 years ago, and entered Europe from western Asia.