The human eye presents a spectrum of colors, from deep browns to striking blues. Green eyes stand out as unique and vibrant. This distinct shade is also relatively uncommon, leading many to wonder about its origins and prevalence.
The Genetics of Eye Color
Eye color is primarily determined by the amount and type of melanin in the iris. Melanin, a pigment also responsible for skin and hair color, is produced by specialized melanocytes within the iris. Higher melanin concentrations result in darker eye colors, while lower concentrations lead to lighter shades.
The distribution and density of melanin within the iris’s stroma, the connective tissue layer, also play a significant role. How light interacts with the stroma influences the perceived color. This phenomenon, known as Rayleigh scattering, involves light waves scattering as they pass through the iris, similar to how the sky appears blue. The interplay between melanin levels and light scattering creates the full range of human eye colors.
What Makes Eyes Green?
Green eyes are not caused by a green pigment, but result from a specific combination of melanin concentration and light scattering. They contain a moderate amount of melanin, more than blue eyes but less than brown. This melanin is primarily eumelanin, which provides brown and black hues, alongside a yellowish pigment called lipochrome.
The perception of green arises when light enters the eye and undergoes Rayleigh scattering in the iris’s stroma, scattering blue wavelengths. This scattered blue light then mixes with the yellowish pigment. The combination of blue light and yellow pigment creates the visual effect of green. Thus, green eyes are an optical phenomenon, where the iris’s structural properties and specific pigments interact with light to produce this distinctive color.
The Rarity and Global Distribution
Green eyes are the rarest eye color globally. Only about 2% of the world’s population has green eyes. In the United States, this percentage is slightly higher, with approximately 9% having green eyes.
The highest concentrations of people with green eyes are found in geographical regions, particularly Northern, Central, and Western Europe. Countries like Ireland, Scotland, and Iceland show a higher prevalence. For instance, in Ireland and Scotland, over three-fourths of the population has either blue or green eyes. This distribution is linked to historical migrations and genetic factors within these populations.