Iris Color Basics
The iris, a muscle controlling pupil size, also holds pigments determining eye color. The primary pigment is melanin, with variations in color stemming from melanin concentration and distribution within the iris stroma.
Genetics of Eye Color
Iris color is polygenic, involving multiple genes. The two main genes are OCA2 and HERC2. Variants of these genes influence melanin production, resulting in the spectrum of eye colors from blue to brown.
Melanin Types Matter
Two types of melanin affect iris color: eumelanin (brown-black) and pheomelanin (red-yellow). Higher eumelanin results in brown eyes, while a dominance of pheomelanin, or a lack of both, results in lighter eyes.
Blue and green eyes don't have blue or green pigments. These colors result from Rayleigh scattering, where light is dispersed by the transparent iris layer, similar to how the sky appears blue.
Influence of Light
Eye color can appear to change under different lighting conditions. This is due to the iris's two layers refracting light differently, which can enhance or mute certain colors.
Age and Eye Color
Melanin production can change over time. A child born with blue eyes might develop brown or green eyes as they age, depending on genetic expression and melanin development.
Rarest Eye Color
The rarest eye color is true amber, which has a strong yellow-copper tint. It's caused by a specific ratio of melanin and lipochrome, and it's estimated to occur in less than 1% of the world's population.