Green eyes possess a striking quality due to their uncommon presence. This eye color is rare globally, and its development reveals a fascinating interplay of genetics and light.
The Science of Eye Color
Human eye color stems from the amount and type of pigment in the iris and how light interacts with it. The iris, the colored part of the eye, contains cells that produce melanin, the pigment responsible for skin and hair color. Two main types of melanin influence eye color: eumelanin (brown and black hues) and pheomelanin (red and yellow tones).
The concentration and distribution of these pigments within the iris’s front layer, the stroma, determine the perceived color. A high concentration of eumelanin results in brown eyes, the most common eye color worldwide. Conversely, blue eyes contain very little eumelanin, appearing blue due to the scattering of light within the stroma, a phenomenon similar to the sky.
Unraveling Green Eye Color
Green eyes are not colored by a green pigment; instead, their appearance is an optical phenomenon. This unique color arises from a specific combination of melanin types and how light scatters within the eye. Individuals with green eyes have a relatively low amount of eumelanin and a small presence of yellowish pheomelanin in the iris.
When light enters the eye, shorter wavelengths, primarily blue light, are scattered by the collagen fibers in the iris’s stroma, a process known as Rayleigh scattering. This scattered blue light then mixes with the yellowish pheomelanin, creating the perception of green.
Genetic Inheritance of Green Eyes
Eye color inheritance is a complex trait, influenced by multiple genes. While many genes play a role, two primary genes, OCA2 and HERC2, located on chromosome 15, are particularly influential in melanin production and distribution.
The OCA2 gene provides instructions for making the P protein, essential for melanin production and processing. Variations in this gene can lead to different levels of melanin, affecting eye color. The HERC2 gene regulates OCA2 activity; a specific variant can reduce OCA2 expression, leading to less melanin and lighter eye colors like blue or green. The precise combination of these gene variants required for green eyes is less common than for brown or blue eyes.
Global Patterns of Green Eye Rarity
Globally, green eyes are one of the rarest eye colors, found in approximately 2% of the world’s population. Their prevalence is not uniform across all populations, being most commonly observed in individuals of Northern, Central, and Western European descent.
Countries like Ireland and Scotland show higher concentrations of green eyes, with some estimates suggesting up to 75% of people born with green eyes can be found in these two nations. Iceland also has a notable percentage, with surveys indicating 18% to 21% of women and 8% to 10% of men have green eyes. This geographical concentration is influenced by historical migrations and the genetic makeup of these populations.