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The most common eye-color in the world is brown, the second most common is blue or grey, while green eye color is rarest, observed in only 1-2% of people in the world.

Genetic variants in one small region of chromosome 15 account for about 75% of the overall variation in human eye color.

Let deCODEme introduce you to the genetics of your eye color

Variation in human eye color

Human eye-color exists on a continuum from the lightest shades of blue to the darkest shades of brown or black. The color range is largely genetically determined by a combination of two forms of melanin produced by melanocytes of the iris.

The colored (pigmented) part of our eyes is the iris, which regulates light exposure to the pupil like the aperture of a camera. The muscles of the iris react to more light by contracting the pupil, and to less light by expanding it. The role of pigmentation in the iris is thought to be similar to the role of pigmentation of the skin, where it protects underlying organs from harmful UV-radiation emitted by the sun.

Why does eye color vary among humans?

The color of the iris is determined by the amount and distribution of the pigment melanin, which is usually dark brown and is produced by a special type of cell called the melanocyte. In simple terms, a brown iris contains abundant melanin (more UV-protection), whereas a blue iris contains much less melanin. Albinos have an almost complete lack of melanin. This results in red or pink iris color, due to the greater visibility of blood vessels through the almost transparent iris. Hence, the eyes of albinos are extremely vulnerable to the sun’s UV-radiation.

Eye color is a relatively simple trait that is primarily determined by one gene

For many decades, eye color has been used in introductory texts about genetics as an example of a human trait that is determined in a simple way by only one gene. The story was that brown eye color was dominant to blue. According to this scheme, if both parents had blue eyes, then all their children would also have blue eyes. However, if one or both parents had brown eyes, then their children could have either blue or brown eyes.

The human iris has indeed many characteristic patterns that have not been fully assessed in genetic studies, but are probably also under strong genetic influences, such as colored spots, borders, furrows etc, that can affect the color appearance of an iris and make color classification of the eye challenging.

The genetics of eye color are more complicated than we used to think, and all the more interesting!

Genetic research has now shown that eye-color is determined by a number of genes. As a result, blue eye color can no longer be viewed as a simple recessive trait. In fact, almost any parent-child combination of eye colors can occur. Nonetheless, one region of the genome plays a more important role than others, so predictions based on the old ‘one gene’ model are correct for most families.

Variants from one region of the genome account for about 75% of the variation in eye color

The actual number of genes that contribute to eye color is not yet fully known. However, it seems that genetic variants in one small region of chromosome 15, containing the genes OCA2 and HERC2, account for about 75% of the overall variation in human eye color. Recent studies have identified variants within the HERC2 and OCA2 genes that are very strongly associated with blue versus brown eye color (article 1, 2, and 3).

The deCODEme Genetic Scan identifies the SNP rs12913832 from the HERC2 gene on chromosome 15, which is strongly associated with blue versus brown eye-color and provides an interpretation of the associated likelihood of blue/grey or brown eye color in individuals of European descent.

This content was last reviewed on February 08, 2010.

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