Eye color in humans varies from very light to very dark. Eye color should more accurately be called iris color, since the iris, the central part of the eye, contains the different colors. While genetics plays the dominant role in eye color, understanding of the genes involved in eye color determination has advanced over the years. Eye color, like many genetic characteristics, is not always as simple as it once seemed and has several causes.
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At the most basic level, the answer to what causes eye color is the pigment melanin. Because melanin isn’t fully present at birth, many babies have blue eyes at birth that will later change to brown or green. Melanin can deposit in either the front or back layer of the iris. The location and amount of melanin in the front layer most affects the eye coloration. Nearly all people have melanin in the back layer of eye. Blue-eyed people have no melanin in the front layer and melanin in the back layer, which appears blue because of light refraction, the Stanford School of Medicine reports. Brown-eyed people have a lot of melanin in the front layers, while people with green or hazel eyes have less than brown-eyed people but more than blue-eyed people.
At least eight different genes control eye color, the Hudson Alpha Institute for Biotechnology reports. The gene seemingly most responsible for determining whether eyes are brown or blue is the OCA2 gene, located on chromosome number 15. The OCA2 gene produces a protein called P-protein that helps create and process melanin. When the OCA2 gene contains certain variants that increase or lessen the amount of melanin produced in the eye, brown or blue eyes result. The combined function of a number of other genes may increase melanin in the eyes to a greater level than in either parent, explaining how parents with light-colored eyes occasionally produce a darker-eyed child. Recent research indicates that the allele, or variant, for blue eyes occurred only in the last 6,000 to 10,000 years.
Certain diseases and medications can cause color changes in the eye. Fuch's heterochromic iridocyclitis, Horner's syndrome or pigmentary glaucoma can all cause color changes and require medical evaluation. Certain glaucoma medications can also cause changes in iris color.
In 10 to 15 percent of Caucasians, eye color changes as they age. Iris color can lighten or darken with age, All About Vision reports.