Eye color is a distinctive human trait, yet the mechanisms that create the spectrum of hues are often misunderstood. Eye color is not determined by a single pigment but is a complex result of the amount of melanin present in the iris and how light interacts with the tissue structure of the eye. This process is governed by a combination of multiple genes that regulate pigment production, which accounts for the scarcity of certain eye colors globally.
Understanding Global Eye Color Prevalence
Green eyes are the rarest naturally occurring eye color globally, appearing in only about 2% of the world’s population. This is significantly less common than brown eyes, which are found in approximately 70% to 80% of individuals worldwide. Brown eyes are dominant across Africa, Asia, and the Americas, reflecting a high concentration of melanin pigment in the iris.
Blue eyes are the second most common light eye color, present in about 8% to 10% of people globally. Green eyes are also less common than hazel eyes, which account for roughly 5% of the global population. The rarity of green eyes is most pronounced outside of Europe, where the color is most concentrated. The highest frequency is found in European populations, particularly those with Celtic and Germanic ancestry, such as in Ireland and Scotland. Even in the United States, only about 9% of the population has green eyes.
The Science Behind Green Coloration
Green eyes are not caused by a specific green pigment. The human iris contains only two types of melanin: eumelanin (brown/black) and pheomelanin (red/yellow). Instead, the color is an optical illusion created by how light scatters and reflects within the iris tissue. Green eyes possess a low to moderate amount of melanin within the stroma, which is the front layer of the iris.
This low melanin concentration allows for Rayleigh or Tyndall scattering to occur. This phenomenon causes shorter, blue wavelengths of light to scatter back out of the stroma. This scattered blue light then mixes with a yellowish or amber pigment, known as pheomelanin, present in the iris. The combination of scattered blue light overlaying the yellowish pigment base creates the perception of green. Because the color is structurally based, green eyes can appear to shift in hue depending on lighting conditions.
Genetic Factors Determining Green Eyes
Eye color inheritance is a complex polygenic trait, influenced by multiple genes working in combination. While variations in at least 16 different genes contribute to the final color, two major genes, OCA2 and HERC2, are primarily responsible for light eye colors, including green. Both are located on chromosome 15.
The OCA2 gene provides instructions for the P protein, which is involved in producing and storing melanin. A fully functional OCA2 gene typically leads to high melanin production and brown eyes. For lighter eyes, the HERC2 gene acts as a switch, controlling OCA2 activity.
A specific variation in HERC2 reduces OCA2 expression, resulting in less melanin production, which is necessary for green and blue eyes. Green eyes require an intermediate level of melanin—more than blue eyes but less than brown eyes. This complex relationship explains why simple dominant and recessive inheritance models do not accurately predict eye color. Because multiple genes are involved, two brown-eyed parents can potentially have a child with green eyes if both carry the genetic variants for reduced melanin production.