Green eyes are one of the rarest eye colors in the world, found in only about 2% of the global population. But what causes green eyes genetically? The answer lies in the amount of melanin pigment in the iris of the eye.
The Genetics Behind Eye Color
Eye color is determined by the amount and type of melanin pigment in the iris. Melanin comes in two forms: eumelanin which produces brown/black pigment, and pheomelanin which produces reddish/yellow pigment. The combination and concentration of these two melanins produce different eye colors:
- Brown eyes have a high concentration of eumelanin
- Green eyes have a moderate amount of melanin overall, with similar levels of eumelanin and pheomelanin
- Blue eyes have a low concentration of melanin overall, with mostly pheomelanin
- Gray eyes have a very low concentration of melanin overall
So the amount of each type of melanin pigment present determines eye color. But what genes control melanin production?
Genetic Mutations That Reduce Melanin
The main gene involved in eye color is the OCA2 (ocular albinism type 2) gene located on chromosome 15. This gene produces a protein called the P protein that is involved in the production and processing of melanin in the iris.
Several different mutations in the OCA2 gene reduce the amount of P protein, which results in less melanin pigment produced. The two main mutations are:
- rs12913832 – a mutation commonly found in Europeans that reduces eumelanin production. This causes blue eyes or green eyes if combined with other melanin-reducing mutations.
- rs1800407 – reduces overall melanin production. Often found in combination with rs12913832 in people with green eyes.
So green eyes typically involve having both the rs12913832 and rs1800407 mutations in the OCA2 gene, resulting in moderately reduced eumelanin and overall melanin levels.
Other Genes Involved
A few other genes can also influence eye color by regulating melanin production, including:
- TYR – involved in eumelanin production
- SLC24A4 – regulates melanin synthesis
- SLC45A2 – helps produce melanin
- IRF4 – turns on expression of OCA2
Mutations in these genes can interact with the OCA2 mutations to produce less melanin overall, leading to lighter eye colors.
The HERC2 Gene
The HERC2 gene helps regulate expression of the OCA2 gene. A mutation in HERC2 switches off OCA2 expression, resulting in low melanin levels and blue eyes. This mutation is found in almost all people with blue eyes.
Those with green eyes may have a variant form of the HERC2 mutation that only partially reduces OCA2 expression, allowing the intermediate melanin levels seen in green.
Genetic Inheritance of Eye Color
Eye color is passed down through generations via genes. Since multiple genes influence eye color, the inheritance patterns are complex. But in general:
- Darker eyes (brown/black) are dominant over lighter eyes
- Blue eye color is recessive
- Green is somewhere in between
Some possibilities for eye color inheritance include:
| Parents’ Eye Colors | Possible Children’s Eye Colors |
|---|---|
| Brown + Blue | Brown, Blue, or Green |
| Brown + Green | Brown, Green |
| Green + Green | Blue, Green |
As you can see, two parents with brown eyes are very likely to have brown-eyed children, since brown eye color is dominant. Recessive traits like blue or green eyes may suddenly appear after being hidden for generations if both parents carry the recessive mutations.
Is Eye Color Linked to Other Traits?
Eye color on its own does not necessarily reveal anything else genetically about a person. However, due to common ancestry and geographic clustering of certain genetic traits, some associations exist.
In particular, blue and green eyed populations in Europe show increased frequencies of other recessively inherited traits such as:
- Red or blonde hair
- Pale skin that freckles instead of tans
- Left-handedness
But again, these are correlations only – not a hard and fast rule. Genetics is complex with many different genes interacting in ways we don’t fully understand yet.
The Rarity and Uniqueness of Green Eyes
Green is one of the rarest eye colors because it relies on having just the right combinations of melanin-reducing genetic mutations. Most people end up with either brown eyes or blue eyes instead. Some estimates for the prevalence of green eyes include:
- 1-2% globally
- 2-5% of Europeans
- 3-10% of North Americans
Within Europe, countries like Ireland, Scotland and some Nordic regions have the highest percentages of green-eyed individuals. This likely reflects common ancestral origins where the genetic traits for green eyes became concentrated.
The uniqueness of green eyes is partly what makes them so aesthetically striking. When light hits the iris, it reveals a fusion of different colors and hues – yellows, greens, browns and grays. No two pairs of green eyes are exactly alike.
Shades and Variations
Green eyes can come in a wide spectrum of shades and mixes of colors. Some common variations include:
- Emerald green – vivid green, potentially with a bluish tint or dark limbal ring around the edge
- Light green/Mint green – very light, almost turquoise looking shade of green
- Forest green – deep green, may have brown/golden flecks or central heterochromia
- Hazel – mix of light brown and subtle green around the pupil
- Green-gray – muted grayish hue with greenish tones
The exact hue and intensity of green eye color can vary within an individual depending on lighting conditions. Mood, health, and age may also affect the shade.
How Green Eyes Develop and Change
Green eyes are not present at birth. All babies are born with grayish-blue eyes as melanin has not yet begun to accumulate in the iris. Eye color develops and changes during early childhood:
- Around 6 months – some brown pigment visible
- 12-18 months – full eye color established
- 3-5 years – hue stabilizes, shade intensifies
Green eyes may start off more blue or gray and intensify with age. The actual color continues to change into the early teenage years as melanin levels fluctuate.
In old age, eye color often fades and dulls. Green eyes may lose their distinctness and turn into more of a grayish green.
Conclusion
Green eyes arise from moderate levels of melanin pigment in the iris, regulated by several different genes involved in melanin production. They require a specific mix of genetic mutations that reduce, but don’t eliminate, the total melanin content. The rarity of green eyes makes them unusual and special.
While intrinsically captivating, green eyes do not have any other inherent meaning or reveal anything specific about a person genetically beyond eye color. The genetics behind eye color inheritance are complex and involve multiple interacting genes.
Green eyes display stunning variety in shade and hue. Their uniqueness lies in the flecks of different colors and the way the iris changes with lighting conditions. For these reasons, green eyes will likely continue to fascinate and delight.
