Human eye color is a complex, polygenic trait. This means it is determined by the interplay of several different genes, rather than being controlled by a single gene as was once a common misconception. This genetic involvement explains the wide spectrum of eye colors observed across individuals.
The Role of Melanin in the Iris
The color of the human iris is primarily determined by the concentration and distribution of a single pigment called melanin. A type of melanin known as eumelanin is responsible for the shades observed in eyes. The human eye does not contain blue or green pigments. Instead, these lighter colors arise from how light interacts with the structures within the eye.
The Genetics of Eye Color
Eye color is influenced by multiple genes, contrasting with older, oversimplified models like the idea that brown eyes are simply dominant over blue eyes through a single gene. Modern research shows this is an inaccurate representation of how eye color is inherited.
The two most significant genes identified in determining eye color are OCA2 and HERC2, both located on chromosome 15. The OCA2 gene provides instructions for making the P protein, which is involved in melanin production within the iris. The HERC2 gene acts as a regulatory switch, influencing the activity of the OCA2 gene and thereby controlling how much melanin is produced. Variations within these two genes largely dictate the amount of melanin present, but scientists have identified as many as 16 other genes that also contribute to the final eye color.
How Different Eye Colors Are Formed
The varying levels of melanin and the way light scatters within the iris create the diverse range of human eye colors. Brown eyes, for instance, result from a high concentration of melanin in the iris. This abundance of pigment absorbs most of the light entering the eye, leading to a dark hue.
Blue eyes, conversely, have a very low concentration of melanin in the iris. Their color is due to a phenomenon called Tyndall scattering or Rayleigh scattering. In this effect, shorter blue wavelengths of light are scattered by the collagen fibers in the iris’s stroma, similar to how the sky appears blue. Longer wavelengths are absorbed by the darker underlying epithelium, allowing the scattered blue light to be perceived.
Green and hazel eyes represent intermediate cases, resulting from moderate amounts of melanin combined with this light-scattering effect. Green eyes have a small trace of melanin, and when light reflects from the iris, it appears green. Hazel eyes are a mix, shifting from brown to green due to a moderate amount of melanin combined with light scattering, displaying flecks of gold or brown.
Changes and Variations in Eye Color
Many infants are born with blue or gray eyes, and their eye color can change over time. This occurs because melanin production is often low at birth, and the melanocytes, the cells that produce melanin, become more active in response to light exposure after birth. This increased melanin production leads to a gradual darkening of the eye color during the first year of life, though subtle shifts can continue until around age three or even six years.
Another unique variation is heterochromia, a condition where an individual has two different colored eyes or multiple colors within a single eye. This rare occurrence can result from genetic variations affecting melanin distribution. In some instances, heterochromia can also be an acquired condition caused by factors such as eye injury, certain diseases, or specific medications.