Green eyes are a distinct eye color, setting them apart from more common brown or blue hues. Their appearance sparks curiosity about their origins and the biological processes that create them. This eye color results from specific optical phenomena and genetic factors.
The Unique Optics of Green Eyes
The perception of green eyes arises from a specific interplay of light and a low concentration of melanin in the iris. Melanin, a brown pigment, is also responsible for skin and hair color. In green eyes, a small to moderate amount of eumelanin is present in the iris’s anterior border layer, along with a yellowish pigment called lipochrome.
The green appearance is not due to a green pigment within the eye. Instead, it is a structural color, similar to how the sky appears blue. This occurs through Rayleigh scattering, where shorter wavelengths of light, like blue light, are scattered more effectively by the iris stroma. When this scattered blue light combines with the yellowish tint from lipochrome and underlying light brown pigmentation, the human eye perceives green. Green eyes can therefore appear to shift in shade depending on lighting conditions.
How Green Eye Color is Inherited
Eye color inheritance is a complex, polygenic trait, meaning multiple genes contribute to the final eye color. As many as 16 different genes are thought to influence eye color in humans.
Two genes, OCA2 and HERC2, on chromosome 15, play significant roles in determining eye color. The OCA2 gene influences melanin production, with variations leading to reduced melanin and lighter eye colors like green or blue. The HERC2 gene regulates OCA2, controlling its expression and impacting pigmentation. The combination of specific variants of these and other genes, alongside their interactions, determines the unique green hue.
Global Prevalence of Green Eyes
Green eyes are among the rarest eye colors globally. Only about 2% of the world’s population possesses green eyes. This makes them significantly less common than brown eyes (70-79% of the global population) and blue eyes (8-10%).
The highest concentrations of green eyes are found primarily in Northern and Central European regions. Countries like Ireland and Scotland have a high prevalence, with over 75% of their populations having blue or green eyes. This geographical clustering links to historical migrations and founder effects, where a small group with a genetic trait establishes a new population.
Eye Color Changes Over Time
Many infants are born with lighter eye colors, such as blue or gray, which can later develop into green. This occurs because melanin production is not fully established at birth. Melanocytes, specialized cells that produce melanin, continue to develop and increase activity after birth.
As these cells produce more melanin over the first few months and years of life, eye color can gradually change and darken. Significant changes typically happen within the first 6 to 9 months, though subtle shifts can continue until a child is about 3 years old, and sometimes up to 6 years of age. Eyes born brown usually remain brown due to their significant melanin content.