Eye colour is a polygenic trait - meaning that it is controlled by the interactions of several genes. Rare genetic mutations can even lead to unusual eye colours like black, red, and violet. Often, paler newborns have blue eyes, which change to green, hazel, light brown or dark brown. 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.
Green eyes are the product of moderate amounts of melanin. Only 1-2% of the world population have Green eyes. They are most often found among people of Northern and Southern European origin. Countries with a high percentage of green eyes are Iceland, Sweden, Denmark, Finland, Norway, Germany, Ireland and the Netherlands. Green eyes are also present in Eastern and Southern Europe and in the Middle East.
In humans, brown eyes contain large amounts of melanin (eumelanin) within the iris stroma, which serves to absorb light, particularly at the shorter wavelengths. Brown is the most common eye colour but less common in countries around the Baltic Sea, such as Finland and Estonia.