Eye color is a captivating human trait that often sparks curiosity, particularly when it comes to less common hues like green. The diverse spectrum of eye colors seen across the global population is a result of complex biological processes and genetic inheritance. Understanding how these colors manifest provides insight into the intricate nature of human variation.
Global Prevalence of Green Eyes
Green eyes are among the rarest eye colors globally. Only about 2% of the world’s population has green eyes, making them significantly less common than brown or blue eyes. For comparison, brown is the most prevalent eye color, accounting for approximately 70-79% of people worldwide. Blue eyes are the second most common, found in about 8-10% of the global population.
The highest concentrations of green eyes are found in parts of Northern, Central, and Western Europe. Countries like Ireland, Scotland, and Iceland show higher percentages, with nearly 15-20% of their populations having green eyes.
The Science Behind Eye Color
Eye color is determined by the amount and type of melanin present in the iris, the colored part of the eye. There are two primary types of melanin: eumelanin, which produces brown and black pigments, and pheomelanin, which contributes to red and yellow hues. The concentration and distribution of these pigments within the iris’s stroma, the front layer, dictates the resulting eye color.
Beyond pigmentation, a phenomenon called Rayleigh scattering also plays a significant role in lighter eye colors. In green eyes, a small amount of yellowish pheomelanin combines with the blue light scattered by the stroma, creating the perception of green.
How Green Eye Color is Inherited
The inheritance of eye color is a polygenic trait, meaning it is influenced by multiple genes, not just a single gene. Two genes, OCA2 and HERC2, located on chromosome 15, are considered major contributors to eye color determination.
The OCA2 gene is involved in producing the P protein, which is important for melanin production and storage in the iris. The HERC2 gene acts as a regulator, controlling the activity and expression of the OCA2 gene. Variations in these genes can reduce melanin production, leading to lighter eye colors like blue or green. Green eyes result from a moderate amount of melanin in the iris, less than brown eyes but more than blue eyes, combined with the light-scattering effect.
Factors Influencing Eye Color Development
Eye color can change during infancy, often not reaching its final shade until several months or even years after birth. Many babies are born with blue or gray eyes because melanin production in the iris is not yet fully activated. Exposure to light after birth stimulates melanocytes to begin or increase pigment production.
As melanin develops and stabilizes, a baby’s eye color may deepen or change. While genetic predisposition is the primary determinant of eye color, the process of melanin accumulation can take time. Most significant changes occur between 3 and 9 months of age, though subtle shifts can continue up to three years old.