Eye colour is determined by genetics, but no single gene determines what that colour will be. The OCA2 gene often controls the amount of melanin pigment that a body generates, which affects 74 per cent of human eye colour variation, but even that gene is impacted by many other factors in the DNA. Most human eyes are brown or blue, while other colours like hazel and grey are less common. Also, different populations tend to have different eye colours that are more prevalent or rare.
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Green is the most common of the rare eye colours -- it is found throughout the world but only in about two per cent of the total world population. This colour is produced by a large amount of melanin in the iris during development, which is a result of genetics. Like other rare eye colours, green eyes are often weeded out of a family because dominant traits like brown eyes are expressed more often.
Amber-coloured eyes are often called “wolf eyes” because they resemble the pigment of those in wolves. This eye colour consists of a golden or yellow pigment with a touch of copper and rust tint. Lipochrome, the name of the yellow pigment, is extremely rare and, as a result, few people have this eye colour.
Red-coloured eyes are typically found in people with pigment problems, such as albinos. Some rare cases exist in which people without pigment conditions have red eyes -- this pigment might be the result of a small leak of blood into the iris causing a reddish eye.
Eyes can occasionally appear violet. This is typically when there's not enough pigment in the iris to cover blood vessels, and the light that reflects off these vessels makes the eyes appear violet. This can also occur if a person with blue eyes has reddish, bloodshot eyes from irritation, making the eyes appear violet temporarily.
Some people believe that black eyes do not actually exist and that the colour is really just extremely dark brown. There have been a few cases, however, that point to purely black eyes in human beings.
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