Eye color

Eye color is an inherited trait determined by multiple genes.^[13][14] 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 (2009) found that it was possible to predict eye color with more than 90% accuracy for brown and blue using just six SNPs.^[15][16]

In humans, eye color is a highly sexually dimorphic trait.^[17] 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 (green and brown eyes) than men.^[17] Gender is thus a major factor in the expression of eye color genotypes.^[17]

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.^[18]

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

The gene OCA2 (OMIM: 203200), when in a variant form, causes the pink eye color and

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. 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.^[23] 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 color during early childhood, puberty, pregnancy, and sometimes after serious trauma (like

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.^[24] Under the same environmental conditions, there may be disagreement over the color of an object between two different people;^{[further explanation needed]} the factor that causes this discrepancy is the presence of melanin in the iris, which is the main factor in determining eye color. The higher the amount of melanin in the iris and the denser the texture of the melanin, the darker the color of a person's eyes; the same melanin concentration also depends on many factors such as hereditary and environmental ones.

Eye color range

Brown

Almost all mammals have brown or darkly-pigmented irises.^[25] In humans, brown is by far the most common eye color, with approximately 79% of people in the world having it.^[26] 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.^[27]

Dark brown eyes are dominant in humans.

Amber eyes are a solid color with a strong yellowish/golden and russet/coppery tint, which may be due to the yellow pigment called

The eyes of some pigeons contain yellow fluorescing pigments known as pteridines.^[32] 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.^[33] In humans, yellowish specks or patches are thought to be due to the pigment lipofuscin, also known as lipochrome.^[34] 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, in 5% of the world's population.^[35] People with that eye color are found in the Balkan region, Southern France, the Iberian Peninsula, Italy, and Hungary, and in fewer numbers in the Middle East, North Africa and South America.^[36]

Hazel

Hazel eyes are due to a combination of Rayleigh scattering and a moderate amount of melanin in the iris' anterior border layer.^[4][34] 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 vice versa.^{[37][38][39][40][41][42][43]} 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 (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.^{[37][39][42][44]}

Around 18% of the US population and 5% of the world population have hazel eyes.^[26] Hazel eyes are more common in Spaniards, North Africa, the Middle East, and Brazil.^[45]

Green

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Green eyes probably result from the interaction of multiple variants within the

Green eyes are most common in Northern, Western and Central Europe.^[47][48] 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.^[49] Among European Americans, green eyes are most common among those of recent Celtic and Germanic ancestry, with about 16%.^[49]

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
• 
  A

  Chinchilla Persian

  with sea-green eyes
• 
  Green-hazel eyes

Blue

There is no blue pigmentation either in the iris or in the

The inheritance pattern followed by blue eyes was previously assumed to be a

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.^[53][54] However, more recent ancient DNA research has identified human remains much older than the Neolithic period with the OCA2 mutation for blue eyes. It is now believed that the OCA2 allele dates back to the recent migration of modern humans out of Africa roughly 50,000 years ago, and entered Europe from western Asia.^[18]

Eiberg and colleagues suggested in a study published in Human Genetics that a mutation in the 86th

Blue eyes are predominant in northern and eastern Europe, particularly around the Baltic Sea. Blue eyes are also found in southern Europe, Central Asia, South Asia, North Africa and West Asia.^[57][58]

• 
  A Birman kitten with distinctive sapphire blue eyes
• 
  Jungle crow
• 
  The first blue-eyed koala known to be born in captivity^[59]
• 
  Sclater's lemur

  , also known as the blue-eyed black lemur

Approximately 8% to 10% of the global population have blue eyes.^[26] 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.^[14] As of 2006^[update], one out of every six Americans, or 16.6% of the total US population, has blue eyes,^[60] including 22.3% of whites. The incidence of blue eyes continues to decline among American children.^[61] Of Slovenes, 56% have blue/green eyes.^[62] In Spain, the iris color distribution among the population is 16.3% blue-gray, 55.2% green-hazel, and 28.5% brown.^[63]

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 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 (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.^[64] Alternatively, it has been suggested that gray and blue eyes might differ in the concentration of melanin at the front of the stroma.^[64]

Gray eyes can also be found among the

Gray is the second-rarest natural eye color after green, with 3% of the world's population having it.[67]

Special cases

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.^[68]

Red and violet

The eyes of people with severe forms of

Although the deep blue eyes of some people such as Elizabeth Taylor can appear purple or violet at certain times, "true" violet-colored eyes occur only due to albinism.^[71][72][73] Eyes that appear red or violet under certain conditions due to albinism are less than 1 percent of the world's population.^[68]

Medical implications

The most important role of melanin in the iris is to protect the eyes from the sun's harmful rays.^[74] People with lighter eye colors, such as blue or green, have lessened protection from the sun, and so need greater protection from the sun's rays than those with darker eye colors.^{[citation needed]}

Those with lighter iris color have been found to have a higher prevalence of age-related macular degeneration (ARMD) than those with darker iris color;^[41] lighter eye color is also associated with an increased risk of ARMD progression.^[75] 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.^[76][77] 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.^[78]

Wilson's disease

Wilson's disease involves a mutation of the gene coding for the enzyme 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.^[79]

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,^[80] and may be symptomatic of liver diseases such as cirrhosis or hepatitis.^[81] A blue coloration of the sclera may also be symptomatic of disease.^[80]

Aniridia

Aniridia is a congenital condition characterized by an extremely underdeveloped iris, which appears absent on superficial examination.^[82]

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

Heterochromia

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Heterochromia (

.

A chimera can have two different colored eyes just like any two siblings can—because each cell has different eye color genes. A 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.^[85] 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

calico cats. Trauma and certain medications, such as some prostaglandin analogues, can also cause increased pigmentation in one eye.^[86]

On occasion, a difference in eye color is caused by blood staining the iris after injury.

Limbal ring

Main article: Limbal ring

The limbal ring is also a feature of the iris contributing to eye color, appearing as a darkened, occasionally black region encircling the iris resulting from a manifestation of the optical properties of the corneal limbus. Limbal rings are not present in all individuals, and their thickness and prominence may correlate with health or youthfulness, and contributes to facial attractiveness.^[87][88]

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.^[89] 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.^[90] 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.^[90] In another study, people with darker eyes performed better at hitting racquetballs.^[91] There are also other studies that challenge these findings.^[92] According to scientists, more study is needed to verify these results.^[89]

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.^[93] Classification systems have ranged from a basic light or dark description to detailed gradings employing photographic standards for comparison.^[93] Others have attempted to set objective standards of color comparison.^[94]

The

Bruno K Schultz in the first half of the 20th century. The scale consists of 20 colors^[95] ranging from light blue to dark brown-black, corresponding to natural eye colors caused by the amount of melanin in the iris:^[96][97]

Normal eye colors range from the darkest shades of brown to the lightest tints of blue.

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

.

Eye color in non-human animals is regulated differently. For example, instead of blue as in humans,

As the perception of color 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.^[100]

See also

• Hair color
• Human skin color
• Iridology
• Xanthophore

References

External links

Wikimedia Commons has media related to Eyes by color.

• genetics.thetech.org
• Eye Color and Human Diseases