Green eyes are one of the rarest eye colors in the world. They are most commonly found in people of Northern European descent but can occur in any ethnicity. The amount of melanin pigment and the density and composition of the stroma in the iris determine eye color. Green eyes get their distinctive appearance from having low to moderate amounts of melanin in the iris combined with a yellowish stroma. The specific requirements that make an eye green versus another color like blue or hazel are quite complex.
What Makes An Eye Green?
The iris is the colored part of the eye. It contains 2 layers – the anterior border layer which has melanocytes that produce melanin, and the stroma which is underneath. Both the amount of melanin and the composition of the stroma affect eye color. Here are the key factors that contribute to green eye color:
– Low to Moderate Melanin: Green eyes have low to moderate amounts of melanin. Melanin is the pigment that gives color to the iris and controls the lightness or darkness of the eye. People with very dark brown eyes have a lot of melanin, while people with blue eyes have very little. Green eyes fall somewhere in between. They don’t have so little melanin that the eye appears blue, but also don’t have so much that they appear brown. The moderate melanin levels result in the greenish-yellowish hue.
– Yellowish Stroma: The stroma underneath the anterior border layer contains collagen and also determines eye color. People with green eyes tend to have a yellowish stroma which enhances the greenish appearance. The yellow hues in the stroma mix with the low to moderate melanin levels to create green.
– Low Melanocyte Density: Green eyes often have fewer melanocytes or less densely packed melanocytes in the anterior border layer compared to brown eyes. This lower melanocyte density also reduces the melanin and allows the yellow stroma to show through more.
– Lipochrome: Lipochrome is a yellow pigment derived from carotenoids that may be deposited in the iris stroma. Higher levels of lipochrome in green eyes also enhance the yellowish quality that mixes with melanin to form green.
Genetics Of Green Eyes
Green eye color is influenced by multiple genes that control the production, type, and amount of melanin in the iris. The key genes involved include:
| Gene | Function |
|---|---|
| OCA2 | Determines melanin production |
| HERC2 | Regulates OCA2 expression |
| SLC24A4 | Modulates melanin type (pheomelanin vs. eumelanin) |
| SLC45A2 | Controls melanin production/transport |
| TYR | Catalyzes melanin synthesis |
The exact variants of these genes present determine how much and what type of melanin is made in the iris, leading to different eye colors. Typically, green eyes are associated with moderate eumelanin production combined with higher levels of pheomelanin and deposits of lipochrome in the stroma. The inheritance patterns of these genetic factors mean that green eyes often run in families.
How Rare Are Green Eyes?
Green is one of the rarest eye colors worldwide. Estimates of the percentage of people with green eyes vary widely but tend to be under 5% of the global population. Here are prevalence statistics on green eyes by geographic ancestry:
| Population | Green Eyed Percentage |
|---|---|
| Northern European (Icelandic, Scottish, Irish) | 14-30% |
| Central/Western European | 2-10% |
| Southern European | Around 2% |
| Eastern European | 2-4% |
| Ashkenazi Jews | 2-5% |
| Asia | Less than 1% |
| Africa | Less than 1% |
This shows that green eyes are by far most common in people of recent Northern European descent. Iceland has the highest known percentage of green-eyed individuals at around 90%. But green eyes can occur in any ethnicity, just much more rarely outside of Europe.
Requirements For Green Eyes
For an eye to be classified as truly green, it must meet these requirements:
– Hue between yellow-green to blue-green
– Moderate levels of melanin pigment in the iris (more than blue eyes but less than brown)
– Yellowish stroma containing lipochrome and collagen
– Low density of melanocytes in the anterior border layer
– Genetic variants linked to reduced melanin production combined with higher pheomelanin
The amount of green in the eye can vary. An eye may appear more yellow-green or blue-green. But the key is having some degree of greenish color that falls between pure blue and hazel/brown eyes due to the specific combination of melanin content, stroma color, and genetics.
How Lighting Affects Green Eyes
The way green eyes look can change quite a bit based on the lighting conditions:
– Natural Daylight – Shows the true green color best
– Warm Tungsten Lighting – Makes green eyes look more brown/amber
– Cool White Fluorescent Light – Enhances light blue tones in green eyes
– Bright Sunlight – Reduces apparent greenness with squinting
Green eyes essentially contain a mix of blue, yellow, brown, and green. Different lighting brings out some of those components more than others, so the balance of the hues can look different. But in normal conditions, quality natural lighting will show the true green color that defines the eye.
Green Eyes Compared To Other Colors
It’s helpful to compare green eyes to other similar eye colors:
| Eye Color | Melanin Level | Stroma Color |
|---|---|---|
| Blue | Very low | Blue/gray |
| Green | Low to moderate | Yellowish |
| Hazel | Moderate | Multi-colored |
| Brown | High | N/A |
While hazel and green eyes may seem similar, hazel has a very different appearance because of the multi-hued stroma compared to the solid yellowish-green stroma in green eyes. Green eyes also have clearly more melanin than blue eyes. So they are distinctly in between blue and hazel/brown.
Eye Conditions Associated With Green Eyes
Certain eye health conditions occur more frequently in people with green eyes compared to other colors:
– Uveitis – This inflammatory eye disease shows 2-3x higher prevalence in green-eyed individuals. The genes linked to green color may also associate with uveitis risk.
– Fuchs Corneal Dystrophy – This disorder affecting the cornea has 5x higher rates in people with green eyes compared to brown eyes. The genetic variation in collagen production that contributes to the yellowish stroma may also lead to corneal abnormalitites.
– Exfoliation Syndrome – Green/blue eyed individuals have significantly higher rates of this age-related eye condition causing vision loss. It may relate to the lack of protective melanin pigment.
While these conditions can occur in anyone, green-eyed people should be aware of the higher risks and get regular dilated eye exams to monitor for potential problems.
Conclusion
Green is one of the most rare and unique eye colors that results from very specific combinations of melanin content, stroma coloration, and genetics that make the eyes appear distinctly greenish. Moderate melanin levels that are higher than blue eyes but lower than brown eyes combined with a yellowish stroma cause the green color. Northern European ethnicities have the highest prevalence, but green eyes can occur in any population. Correct identification of truly green eyes requires observing the eye color in natural lighting to assess the specific hue and properties that meet the definition of true green. While beautiful, green eyes do come with slightly higher risk for some ocular conditions that should be monitored. But ultimately, green eyes are a special variation that exemplifies the wondrous diversity of the human genome expressed through our outward appearance.
