Eye color

The brightly colored eyes of many bird species result from the presence of other pigments, such as pteridines, purines, and carotenoids.

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.

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.

[10] The gene OCA2 (OMIM: 203200), 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.

A 1997 study of White Americans found that eye color may be subject to change in infancy, and from adolescence to adulthood.

[28] 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.

Although hazel eyes may contain specks of amber or gold, they usually tend to have many other colors, including green, brown, and orange.

[33] 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.

[35] Many animals such as canines, domestic cats, owls, eagles, pigeons, and fish have amber eyes, whereas in humans this color occurs less frequently.

[36] People with amber-colored eyes are found in Europe, and in fewer numbers in the Middle East, North Africa, and South America.

[37] Hazel eyes are due to a combination of Rayleigh scattering and a moderate amount of melanin in the iris' anterior border layer.

[52] Among European Americans, green eyes are most common among those of recent Celtic and Germanic ancestry, occurring in about 16% of people with those backgrounds.

[55] In 2008, a team of researchers from the University of Copenhagen located a single mutation that causes the phenomenon of blue eyes.

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.

[56][57] 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.

[56][58][59] It has been proposed that blue eyes may have been adaptive to shorter day lengths at higher latitudes, as blue eyes increase intraocular light scattering, which suppresses melatonin release from the pineal gland, perhaps reducing psychological depression (which is linked to the short day length of higher latitudes).

[68] Under magnification, gray eyes exhibit small amounts of yellow and brown color in the iris.

[70] The eyes of people with severe forms of albinism may appear red under certain lighting conditions owing to the extremely low quantities of melanin,[71] allowing the blood vessels to show through.

[73][74][75] Eyes that appear red or violet under certain conditions due to albinism occur in less than 1 percent of the world's population.

[80] 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.

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

[87] 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.

A common cause in females with heterochromia is X-inactivation, which can result in a number of heterochromatic traits, such as calico cats.

Limbal rings are not present in all individuals, and their thickness and prominence may correlate with health or youthfulness, and contributes to facial attractiveness.

[92] 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.

[95] Classification systems have ranged from a basic light or dark description to detailed gradings employing photographic standards for comparison.

[14] 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.