Eye color, a diverse human characteristic, is determined by biological mechanisms within the eye. Its variations offer insight into human diversity.
How Eye Color Works
The color of human eyes primarily depends on the amount and type of melanin present in the iris, the colored part surrounding the pupil. The iris contains specialized cells called melanocytes, which produce melanin pigment stored in structures known as melanosomes. There are two main types of melanin: eumelanin, which is brown-black, and pheomelanin, which has a reddish or yellowish hue.
Different concentrations of these pigments lead to distinct eye colors. Brown eyes, the most common worldwide, result from a high concentration of eumelanin in the iris, absorbing most light. In contrast, blue eyes contain very little melanin in the front layer of the iris. Their blue appearance is not due to blue pigment but rather to the scattering of light, specifically shorter wavelengths, by collagen fibers in the iris’s stroma, a phenomenon similar to Rayleigh scattering, which also makes the sky appear blue.
The Science Behind Green Eyes
Green eyes emerge from a unique combination of melanin levels and light interaction within the iris. They contain a low to moderate amount of melanin, typically a mix of both eumelanin and the yellowish pheomelanin. There is no green pigment within the human eye itself.
The characteristic green color is produced when the yellowish pheomelanin in the iris combines with scattered blue light. This blue light arises from Rayleigh scattering of shorter wavelengths within the iris’s stroma, similar to how blue eyes get their color. The presence of the yellowish pigment mixes with this scattered blue light, creating the green appearance. Green eyes generally have slightly more melanin than blue eyes, but significantly less than brown eyes, allowing for this interplay of light and pigment.
The Genetics of Eye Color
Eye color inheritance is a complex trait, influenced by the interaction of multiple genes rather than a single gene. Among the many genes involved, OCA2 and HERC2, located on chromosome 15, play major roles in determining eye color.
The OCA2 gene is responsible for producing the P protein, which is involved in melanin production and storage. Variations in OCA2 can lead to reduced melanin, resulting in lighter eye colors.
The HERC2 gene acts as a regulator for OCA2, influencing its activity and the overall amount of melanin produced. While brown eyes are considered the most prevalent, green eyes are often considered recessive to brown but can be dominant over blue in certain genetic combinations.
Where Green Eyes Are Most Common
Green eyes are considered one of the rarest eye colors globally, accounting for approximately two percent of the world’s population. Their distribution is not uniform, with a concentration in specific regions.
Green eyes are most prevalent in Northern, Central, and Western Europe. Countries such as Ireland, Scotland, and Iceland exhibit high percentages of individuals with green eyes. For example, around 16 percent of European Americans with recent Celtic and Germanic ancestry have green eyes. This geographical pattern suggests a historical and evolutionary concentration of the genetic variations that contribute to this eye color within these populations.