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eye color happens because of at least two separate genes. The first gene, OCA2, comes in two forms, brown (B) and blue (b). The second gene, called gey, comes in two forms, green (G) and blue (b). Here is a table that shows how these genes are thought to work together to create eye colors.
BB bb Brown
BB Gb Brown
BB GG Brown
Bb bb Brown
Bb Gb Brown
Bb GG Brown
bb GG Green
bb Gb Green
bb bb Blue
The first thing to notice from this table is that whenever there is a B, there are brown eyes. So B is dominant over both G and b. Also, whenever there is a G (but no B), there are green eyes. So G is dominant over b.
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Often, paler newborns have blue eyes, which change to green, hazel, light brown or dark brown, as they grow older. This is probably the origin of the idiom "being blue-eyed" (i.e. na?ve; gullible).
It is thought that exposure to light after birth triggers the production of melanin in the iris of the eye. By three years of age, the eyes produce and store enough melanin to indicate their natural shade. While changes in eye color of infants are more common, even in adults, eye color changes are seen, most often as a result of exposure to the sun. Sunlight triggers melanin production in the eye, as it does to the skin.