The color of the human eye is determined by pigments found within the iris. The most common eye colors are blue, brown, and green. But what determines whether eyes appear blue, green, or somewhere in between? The answer lies in the specific amounts and combinations of pigments present.
The Role of Melanin
Melanin is a pigment that is found in various parts of the human body, including the eyes. It comes in two forms:
- Eumelanin – This form creates brown and black pigments.
- Pheomelanin – This form creates red and yellow pigments.
Higher amounts of melanin result in darker eye colors. Brown eyes contain a lot of eumelanin, while blue eyes contain very little of either type of melanin. Green eyes fall somewhere in the middle, with a moderate amount of melanin overall.
The Role of Lipochrome
In addition to melanin, another pigment called lipochrome also impacts eye color. Lipochrome contains yellow and red pigments. A high concentration of lipochrome results in amber and green eyes. Lower amounts lead to blue eyes. The combination of melanin and lipochrome is what creates the range of possible eye colors.
Rayleigh Scattering
The light scattering properties of the iris can also influence eye color. This effect is called Rayleigh scattering. It’s the same phenomenon that makes the sky appear blue. The structure of the iris causes shorter wavelengths of light to scatter more than longer wavelengths. In eyes with little melanin, this leads to more blue light being reflected back out. This further enhances the appearance of blue or greenish eyes.
Genetics of Eye Color
Multiple genes influence the pigmentation of the human eye. The main gene is OCA2, which controls the production of melanin. However, there are as many as 16 different genes impacting eye color in humans. This contributes to the continuous range of shades that are possible.
Since several genes are involved, eye color is an example of a polygenic trait. This means multiple genes work together to create subtle variations in eye shades. Each parent contributes a mix of genetic variants that interact to produce the child’s eye color.
| Gene | Effect on Eye Color |
|---|---|
| OCA2 | Main contributor to melanin production |
| HERC2 | Turns OCA2 gene on or off |
| SLC24A4 | Influences melanin type (eumelanin vs pheomelanin) |
| TYR | Converts tyrosine to melanin pigments |
As this table shows, different genes control various aspects of melanin and pigment production that cumulatively result in a final eye color.
The Interplay Between Melanin, Lipochrome, and Rayleigh Scattering
Green eyes result from a combination of factors. On a base level, they contain a moderate amount of melanin. However, they also have substantial deposition of lipochrome in the iris. This introduces yellows and reds. Finally, Rayleigh scattering of light through the irregular iris surface enhances the blue wavelengths.
The combination of a medium amount of melanin, the lipochrome adding yellow/red hues, and Rayleigh scattering adding blue results in eyes that appear distinctly green. The exact shade depends on the specific levels of each component.
In contrast, blue eyes contain very little melanin and lipochrome. Instead, they show primarily the light scattering effects from Rayleigh dispersion. Brown eyes have high melanin contents that override lipochrome and Rayleigh scattering signals.
Distribution of Eye Colors
Eye color distribution varies substantially across different geographic regions and ethnicities. In the United States, the most common eye colors are:
| Eye Color | Frequency |
|---|---|
| Brown | 55% |
| Blue | 27% |
| Hazel/Amber | 10% |
| Green | 8% |
Brown is by far the most common eye shade among Americans of varied ancestry. Blue eyes are rare globally but are somewhat more common among those of European descent. Green eyes are especially uncommon, occurring in only around 2% of the world population.
Changes in Eye Color With Age
While eye color is mostly stable throughout life, some subtle changes can occur with age. Two main factors influence changes later in life:
- Melanin levels may fluctuate slightly over time. Eyes often darken somewhat into the teenage and early adult years as melanin content increases. They may then rebound and lighten a bit later in life as melanin declines again.
- The lens and cornea grow less transparent with age. This can reduce the Rayleigh scattering effect, making older eyes appear a bit darker and less vibrant.
However, these impacts are usually small. The basic eye color someone is born with remains largely the same over their lifetime.
Conclusion
Green eyes derive their color from moderate melanin levels, deposition of lipochrome pigments, and Rayleigh light scattering. The end result is an eye color that is distinctly green rather than blue. While green eyes may technically have some blue components, they are genetically and visually distinct from blue eyes due to their additional pigmentation and structure. The richness of the green color comes precisely from having more colorants than just the scattering of light seen in blue eyes.