The color of a person’s eyeball is determined by the amount and quality of melanin present. Melanin is a pigment that gives color to the iris, the colored part of the eye. The amount and type of melanin present in the iris is determined by genetics. Eye color is a polygenic phenotypic character and is determined by two distinct factors: the pigmentation of the eye’s iris and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris. Generally speaking, the more melanin present in the iris, the darker the eye color. Here’s a quick overview of what different eye colors typically indicate about melanin content and genetics:
| Eye Color | Melanin Level | Genetics |
|---|---|---|
| Brown | High levels of eumelanin | Dominant trait |
| Hazel | Moderate amounts of eumelanin | Recessive trait |
| Amber | Low to moderate amounts of eumelanin | Recessive trait |
| Green | Low levels of melanin overall | Recessive trait |
| Blue | Low levels of melanin overall | Recessive trait |
| Gray | Low levels of melanin overall | Recessive trait |
In the rest of this article, we’ll explore the different eye colors in more detail and what they indicate about someone’s genetics and ancestry.
Brown Eyes
Brown eyes contain large amounts of melanin on the front of the iris. In fact, brown eyes contain the largest amount of melanin of all eye colors. The dark brown coloring comes from a melanin called eumelanin. People with brown eyes have very dark brown eumelanin across the entire front surface of the iris. This high concentration of melanin gives brown eyes their distinctive dark color.
Genetically speaking, the ability to produce high levels of eumelanin is dominant. This means that having brown eyes is a dominant genetic trait, and will overpower genes for lighter eye colors if present. Over 75% of the global population has brown eyes. This makes sense evolutionarily, as higher melanin levels help protect the eyes from sun damage. Populations that evolved around the equator, like Africa, Asia, and the Americas have predominantly brown eyes for this reason.
Hazel Eyes
Hazel eyes have a mix of brown, green, and orange. They tend to be medium-tone and contain small amounts of melanin. The brownish-orange melanin concentration is found around the edge of the iris, with very little melanin at the center. This gives hazel eyes their multicolored appearance. The melanin found in hazel eyes is eumelanin, the same dark brown pigment found in brown eyes. However, hazel eyes contain less eumelanin overall compared to brown eyes.
Hazel eye color is unusual genetically, as it requires a combination of variants in eye color genes. It is controlled by multiple alleles, which each code for slightly different versions of traits. To have hazel eyes, a person needs to inherit some genetic codes for brown/green eyes, which results in the eyes having a blended, in-between appearance.
Amber Eyes
Amber eyes are a solid golden or copper color without flecks of blue or green typical of hazel eyes. This eye color results from a yellowish or rufous-colored eumelanin pigment in the iris. The concentration of this melanin pigment is low to moderate. Amber eyes are extremely rare worldwide, occurring in less than 1% of the population. They are however more common in certain parts of Asia and South America.
Like hazel eyes, amber eye color is due to a rare combination of recessive genetic traits. These cause low to moderate production of yellowish-red melanin instead of darker brown eumelanin. People can be born with amber eyes, or develop them later in childhood as melanin concentration changes in the iris. Eyes that appear amber may also change to a more brown color later in life as melanin levels increase.
Green Eyes
Green eyes contain low levels of melanin overall. They have just a small amount of the brown-yellow eumelanin pigment. However, they also have low levels of lipochromes, which are yellow and red pigments. This lack of melanin and lipochromes causes green eyes to appear a brilliant shade of emerald or forest green. The shades can range from blue-green to olive-green.
Only 2% of the world’s population has green eyes. The trait for green eyes is recessive and rare. For someone to have green eyes, they must inherit genetic codes for minimal melanin production from both parents. Populations in Northern and Central Europe have the highest rates of green eyes. Iceland has the highest percentage in the world, with over 89% of people having green eyes.
Blue Eyes
Blue eyes contain low levels of melanin. They have the least amount of melanin of any eye color. Blue eyes get their color from the transparent collagen fibers in the irisscattering light and reflecting back blue wavelengths. There is no blue pigment. The lack of melanin allows light to scatter and reveal the underlying blue color of the collagen.
The genetics of blue eyes are simple – having blue eyes is determined by a recessive gene. It is caused by low melanin levels produced by the body. For someone to have blue eyes, they must inherit two copies of the recessive gene for minimal melanin from both parents. Only 8% of the world population has blue eyes. The highest rates are found in Northern and Eastern Europe.
Gray Eyes
Gray eyes, like blue eyes, have low levels of melanin. However, they also have larger amounts of collagen in the stroma that give them a paler, grayer appearance. The color comes off as a muted, smokey blue and gray. The shade of gray can vary from light to dark charcoal.
Gray eyes are relatively rare, occurring in less than 2% of the population. Like blue eyes, gray eyes are a result of very little melanin and pigmentation. To have gray eyes, a person must inherit two copies of a recessive gene for minimal melanin. Gray eyes are found more commonly in Northern and Eastern Europeans, similar to blue eyes.
Conclusion
In summary, eye color is determined by genetic factors that control melanin production and deposition in the iris. Brown eyes have the most melanin, while blue and gray eyes have the least. Rare colors like hazel, amber and green arise from unusual genetic combinations resulting in modified melanin content. Eye color can offer clues about a person’s ancestry and genetics based on global pigmentation adaptations. But no matter what eye color someone has, their eyes have the same basic function and structure.
