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Eye color is an inherited trait influenced by more than one gene.

Although genetically determined, the appearance of people´s eye color can slightly change with age and with changes in pupil size, when the pigments in the iris are compressed or spread apart.

Drastic eye color changes in adults can be a warning sign of certain diseases.

A number of genes determine the color continuum of the human iris

The colored part of our eyes is 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.

The color or the iris is determined by the pigment melanin

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. Individuals with albinism have an almost complete lack of melanin. This results in a red or pink iris color, due to the greater visibility of blood vessels through the almost transparent iris. Hence, the eyes of these individuals are extremely vulnerable to the sun’s UV-radiation.

Mainly two forms of pigmentation give rise to the color continuum of irises around the world

Normal human eye color can be graded on a continuum from the lightest shades of blue to the darkest shades of brown or black. The color range is determined by a combination of two forms of melanin produced by melanocytes of the iris; eumelanin, which is dark brown in color, and pheomelanin, which has a red-yellowish color.

Research on eye-color classification has revealed that these two substances give rise to only three true colors of the iris; brown, yellow, and grey. Thus, for example, people with green eyes have irises with a lot of yellow (pheomelanin) and some brown (eumelanin), making them appear green. People with blue eyes have irises with a little yellow and almost no brown, making them appear blue. People with grey eyes have irises with a little yellow and no brown, making them appear grey. People with brown eyes have irises with a lot of dark brown eumelanin, making them appear brown in color.

It is known that eye color is an inherited trait influenced by more than one gene. Recent studies (here, here, and here) have identified two SNPs, rs12913832 and rs1129038, within the HERC2 gene on chromosome 15 that are almost perfectly associated with blue versus brown eye color. Eye colors can however range from the most common color, brown, to various shades of blue or grey, to the least common color green. Rare genetic mutations can even lead to unusual eye colors: black, red, or the appearance of violet. The actual number of genes that contribute to eye color is however still unknown.

Eye-color can change with age

Babies of European descent are born mostly with blue eyes that can darken or change in color in their first three years. Darkening occurs if melanin, the brown pigment usually not present at birth, develops with age. Children can have completely different eye colors than either of their parents. But if both parents have brown eyes, it’s most likely that their children also will have brown eyes. Eye color can also change slightly with age in adults. This happens in about 10 to 15 percent of individuals of European descent (generally in people with light eye colors).

The appearance of eye-color can change with pupil size

The iris expands and contracts to control pupil size in response to light. With changes in pupil size, the pigments in the iris are compressed or spread apart, which can change the appearance of people´s eye color. Certain emotions can change pupil size and therefore also appearance of the iris color. This is why in some people, a slight change in eye color may for example be noticed when they are angry.

Eye color changes in adults can be a warning sign of certain diseases

If adult eye color changes pretty dramatically, or if one eye changes from brown to green or blue to brown (called heterochromia), it is important to see an eye doctor since eye color changes can be a warning sign of certain diseases.

This content was last reviewed on February 09, 2010.

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