Green eyes are one of the rarest eye colors in the world, with only 2% of the global population having green eyes. But what exactly causes green eyes? The answer lies in genetics.
How Eye Color Is Determined
Eye color is determined by the amount of melanin pigment in the iris of the eye. Melanin is produced by melanocytes, specialized cells in the iris. The more melanin present, the darker the eye color. Brown eyes contain a lot of melanin, while blue eyes contain very little melanin. Green eyes fall somewhere in between.
The main gene involved in eye color is the OCA2 gene, which codes for the production of melanin. Different variations in this gene result in different amounts of melanin pigment. The level of melanin present determines whether eyes are brown, green, blue, gray, or hazel.
Genetics of Green Eyes
For an individual to have green eyes, they must inherit genetic variants from both parents that moderate melanin production and result in reduced pigmentation in the iris:
- One variant of the OCA2 gene must be inherited from each parent. This variant reduces the amount of brown melanin (eumelanin) made in the iris.
- The HERC2 gene regulates OCA2 activity. At least one normal copy of HERC2 must be inherited to allow some melanin production.
- Other genes like SLC24A4 and TYR also influence melanin production and can contribute to green eye color.
When these genetic requirements are met, a reduced amount of melanin pigment is made in the iris, creating the distinctive green eye color.
The Role of OCA2
The OCA2 gene plays the biggest role in determining green eye color. This gene is located on chromosome 15 and provides instructions for making the P protein. This protein is essential for melanin production. OCA2 controls the maturation and distribution of melanin pigment within melanocytes.
There are different variants of the OCA2 gene. The dominant versions produce large amounts of P protein, resulting in a lot of melanin and brown eyes. Recessive variants lead to less P protein and melanin, creating blue eyes. Green eyes are caused by moderate OCA2 activity and melanin levels.
Research has identified a specific variant of OCA2 strongly linked to green eye color. This variant decreases the gene’s transcriptional activity, reducing the amount of P protein made and lowering melanin production. When this change in OCA2 is inherited from both parents, green eyes result.
The Role of HERC2
The OCA2 gene requires proper regulation from the HERC2 gene to stimulate melanin synthesis. HERC2 acts like a switch that turns OCA2 on or off. At least one functional copy of HERC2 must be present for OCA2 to be activated and produce melanin pigment.
Those with two mutated copies of HERC2 experience OCA2 inactivity, leading to minimal melanin and blue eyes. By contrast, one normal HERC2 variant allows OCA2 to be turned on and some melanin to be made, creating green or brown eyes depending on the OCA2 variant inherited.
HERC2 variants also directly impact pigmentation by interacting with proteins involved in melanin production pathways. Together, HERC2 and OCA2 coordinate to regulate melanocyte function and melanin synthesis, determining eye color.
Other Genes Involved
While OCA2 and HERC2 play the major roles, other genes make subtler contributions to green eye color by influencing melanin production:
- SLC24A4 – involved in calcium signaling in melanocytes, which impacts melanin synthesis
- TYR – provides instructions for making the enzyme tyrosinase which is required for melanin production
- IRF4 – regulates activity of various pigmentation genes
- MC1R – codes for a protein involved in production of darker eumelanin
Variants in these genes can reduce melanin levels and interact with OCA2/HERC2 to bring about green iris pigmentation when combined through inheritance from both parents.
Green-Eyed Population Frequency
Only 1-2% of the global population has green eyes. However, in certain geographic regions and ethnic groups, the frequency of green eyes is higher:
| Region/Ethnic Group | Green-Eyed Frequency |
|---|---|
| Northern Europe (Iceland, Finland, Scotland) | 10-30% |
| Eastern Europe (Czech Republic, Slovakia) | 5-10% |
| Western Europe (Ireland, France) | 3-5% |
| Southern Europe (Spain, Italy) | 1-2% |
| Slavic ethnicities | 5-10% |
| Celtic ethnicities | 10-25% |
| Germanic ethnicities | 2-4% |
The higher prevalence in northern and eastern Europe is thought to be related to natural selection favoring lighter eye colors to allow for better vitamin D absorption in regions with less sunlight.
Genetic Combinations for Green Eyes
For green eyes to occur, an individual must inherit the following genetic traits from both parents:
- One copy of a recessive variant allele in the OCA2 gene
- At least one dominant variant of the HERC2 gene
- Typically one or more variants in accessory pigmentation genes like SLC24A4, TYR, IRF4, MC1R
When these genetic combinations come together through reproduction, the result is green iris pigmentation. The exact shade of green may vary based on small differences in the genetic sequence.
Without the necessary OCA2 variant from both parents, there will be too much melanin for green eyes. And without at least one normal HERC2 variant from one parent, there will be too little melanin for green eyes.
Example Genotype
Here is an example genotype that could produce green eyes:
- OCA2 gene: One recessive variant from each parent
- HERC2 gene: One dominant variant from one parent, one recessive variant from the other parent
- SLC24A4 gene: One variant from each parent
- IRF4 gene: One variant from each parent
This combination of reduced melanin production from the OCA2 recessive variants, with some melanin generation enabled by the single HERC2 dominant variant, along with contributions from other pigmentation genes, creates the right circumstances for green iris coloration.
How Common is Each Genotype?
Research studies have analyzed the prevalence of different genotype combinations that lead to green eye color in European populations:
| Genotype | Frequency |
|---|---|
| OCA2 (GG) – HERC2 (GA) | 65% |
| OCA2 (GG) – HERC2 (AA) | 15% |
| OCA2 (GT) – HERC2 (GA) | 10% |
| OCA2 (GG) – HERC2 (GG) | 6% |
| OCA2 (GT) – HERC2 (AA) | 4% |
In these abbreviations, G stands for the dominant version of the gene and A/T stands for the recessive version. As you can see, the most common genotype leading to green eyes has recessive OCA2 paired with one dominant HERC2 variant.
Role of Interactions Between Genes
Green eye color arises from complex interactions between multiple genes. Here are some ways genes cooperate to reduce melanin production:
- HERC2 regulates expression of OCA2, directly controlling its melanin synthesis
- IRF4 and SLC24A4 variants enhance the pigment-reducing effects of OCA2 variants
- TYR and MC1R variants reduce quantities of darker eumelanin
- OCA2 binds to other melanosomal proteins, influencing melanin production pathways
These multivariate interactions between allele variants in different genes modulate melanin synthesis to generate the intermediary pigment levels that characterize green eye color.
Non-Genetic Factors Affecting Eye Color
While genetics play the primary role, some other factors can influence green eye color to a smaller degree:
- Lighting conditions – Green eyes can appear grayer in dark conditions when the pupil dilates and more green in bright sunlight when the pupil constricts.
- Age – Amount of melanin sometimes increases slightly with age, making green eyes darker.
- Injury – Heterochromia (two different eye colors) can result from physical trauma to the iris.
- Medications – Certain drugs like prostaglandin analogs can lead to increased pigmentation.
However, these effects are minor and genetics are overwhelmingly the determinant of green eye coloration.
Conclusion
In summary, green eye color arises from reduced melanin in the iris due to an ideal combination of genetic variants passed down from both parents:
- Recessive OCA2 gene variants decrease melanin production
- At least one dominant HERC2 variant allows melanin generation
- Contributions from other pigmentation genes like SLC24A4 and TYR
These genotype combinations lead to intermediate iris melanin levels and the beautiful green eye phenotype seen in a small fraction of the population. While many genes are involved, it is the intricate interplay between OCA2 and HERC2 that makes green eyes possible.
Understanding the genetics behind this rare and striking eye color provides a window into the deeper relationship between our genes and what makes each of us unique.