Green eyes, with their unique shades, often spark curiosity. Their distinct color is not due to a green pigment, but rather a fascinating interplay of genetics, light scattering, and the amount of melanin in the iris.
The Genetics of Eye Color
Eye color is determined by variations in a person’s genes. The primary determinant is the amount and distribution of melanin within the iris. Its concentration dictates whether eyes appear brown, blue, or green. High levels of melanin result in brown eyes, while lower levels lead to lighter colors.
Green eyes result from an intermediate amount of melanin in the iris, combined with a yellowish pigment lipochrome. The perception of green also involves Rayleigh scattering. This effect, also responsible for the sky’s blue color, involves light scattering as it enters the eye. The scattered blue light mixes with the yellowish tint from lipochrome and the small amount of brown melanin, creating the green hue.
Two genes, OCA2 and HERC2, are key to eye color. OCA2 is involved in melanin production, while HERC2 influences OCA2’s activity. However, at least 16 different genes contribute to eye color, making it a complex trait. This complexity means eye color inheritance is not always straightforward, resulting in a wide spectrum of shades.
Global Prevalence and Ancestral Origins
Green eyes are most commonly found in populations of Northern, Western, and Central European ancestry. This geographical concentration is linked to historical migrations and the genetic makeup of these populations. The highest percentages of individuals with green eyes are observed in countries such as Ireland and Scotland, where over 75% of the population has blue or green eyes.
Other countries in Northern Europe, including Iceland, Estonia, and Scandinavia, have a higher incidence of green eyes. In Central Europe, countries like Germany and France also have notable percentages of green eyes. For instance, France has an “intermediate” eye color prevalence of up to 44%, including green eyes.
The prevalence of lighter eye colors, including green, decreases from northwest to southeast across Europe. This distribution suggests a strong genetic component tied to the ancestry of people in these regions. While green eyes are predominantly associated with European descent, instances can be found elsewhere, such as in Liqian, China, where many inhabitants have green eyes and blonde hair, possibly due to Roman ancestry. Green eyes have also been found in Siberia, dating back to the Bronze Age, approximately 4,000 years ago.
Why Green Eyes Are Rare
Green eyes are considered the rarest eye color globally, making up approximately 2% of the world’s population. This rarity contrasts with brown eyes, the most common at 70-80%, and blue eyes at 8-10%. The specific genetic combinations for green eyes are simply less widespread than those for brown or blue.
The low global percentage means that while green eyes may seem more common in certain regions, they remain a minority worldwide. The distinct blend of melanin and light scattering needed for green eyes is less frequent than conditions producing brown or blue eyes. This makes green eyes a unique and less common trait.