Green eyes are one of the rarest and most striking eye colors in the world. Only about 2% of the global population has green eyes. But what determines whether someone inherits this trait? Eye color is a complex genetic trait controlled by multiple genes. However, the main gene responsible for green eyes is the OCA2 gene located on chromosome 15. The OCA2 gene contains the genetic instructions for producing a protein involved in pigmentation. Certain variations in this gene reduce the amount of brown melanin pigment formed, allowing the green color from underlying structures in the iris to show through. But is green eye color actually a dominant or recessive trait genetically speaking? Let’s take a closer look at how eye color is inherited.
How Eye Color is Inherited
Eye color is considered a polygenic phenotypic trait, meaning multiple genes are involved in contributing to a person’s eye color. However, there are two main genes that influence eye color: OCA2 and HERC2. The HERC2 gene activates expression of the OCA2 gene, which codes for the actual eye color. Variants of these genes are inherited in different combinations from a person’s parents. The interaction between these genes and their variants produces the final eye color. Each parent contributes one of two possible alleles of the OCA2 and HERC2 genes. The alleles received determine how much brown pigment is produced in the iris. So while multiple genes impact eye color, OCA2 and HERC2 play the most significant roles.
Green Eye Color Alleles and Dominance
For green eyes to develop, a person must inherit special variants in both the OCA2 and HERC2 genes. First, they need to inherit an allele in the HERC2 gene that activates only minimal expression of OCA2. Common variants are the ‘G’ allele or less frequently, the ‘A’ allele of HERC2. This reduces the amount of brown melanin pigment produced. However, they also need to inherit recessive alleles in the OCA2 gene itself. The main variants leading to green eyes are less active ‘GCGT’ and ‘GTCT’ alleles of OCA2. Together, these OCA2 and HERC2 alleles result in very low melanin production, allowing the greenish color of the iris to show through.
The inheritance pattern of these alleles is complicated because there are multiple possible allele combinations. However, in simplified terms, green eye color can be considered a recessive trait. Two copies of the recessive OCA2/HERC2 alleles must be inherited for green eyes to develop. Someone who inherits only one recessive allele from one parent will have brown eyes instead. See the table below for a summary of possible OCA2/HERC2 allele combinations and associated eye colors:
| HERC2 Allele | OCA2 Allele 1 | OCA2 Allele 2 | Eye Color |
|---|---|---|---|
| Active HERC2 variant | Active OCA2 variant | Active OCA2 variant | Brown |
| Active HERC2 variant | Active OCA2 variant | Inactive OCA2 variant | Brown |
| Less active HERC2 variant | Inactive OCA2 variant | Inactive OCA2 variant | Green |
As seen in the table, two copies of the recessive OCA2 and HERC2 alleles are needed to override production of brown melanin pigment and result in green eyes. An individual only requires one dominant, melanin-producing allele to have brown eyes. Therefore, green eye color can be classified as a recessive trait genetically.
Probability of Inheriting Green Eyes
Only inheriting two recessive alleles at both the OCA2 and HERC2 loci will result in green eyes. What are the chances of a child inheriting this combination from their parents? Probability depends on the parents’ eye colors and genotypes. If one parent has green eyes and the other has brown eyes, there is a 50% chance their child will have green eyes and a 50% chance they will have brown eyes. See the Punnett square below:
| Parent 1 gg |
||
|---|---|---|
| Parent 2 GG or Gg |
gG | gG |
| Brown eyes | Green eyes |
Here, ‘G’ represents the dominant melanin-producing alleles, while ‘g’ represents the recessive low melanin alleles at both the OCA2 and HERC2 loci. Since one parent has the genotype ‘gg’ for green eyes, they can only pass on the recessive ‘g’ allele. The other parent has brown eyes, either ‘GG’ or ‘Gg’, so they will pass on either allele. This results in a 50/50 chance of the child inheriting recessive alleles from both parents and having green eyes.
If both parents have green eyes, the child has a 100% chance of also having green eyes, since the parents can only pass on the recessive alleles. If neither parent has green eyes, there is very low chance of having a green-eyed child unless there is recessive allele inheritance from more distant ancestors. Overall, green eye color behaves as a recessive trait, requiring two copies of the alleles to show the phenotype.
Other Genetic Factors
In addition to the major OCA2 and HERC2 genes, other genetic factors also influence eye color to an extent. These include:
- The SLC24A4 gene on chromosome 14
- The SLC24A5 gene on chromosome 15
- The TYR gene on chromosome 11
- The PAX6 gene on chromosome 11
Variants in these genes likely play a secondary role in modifying the amount of melanin produced, leading to subtle variations in eye color and hue. Interactions between genes also occur, adding to the complexity of genetic control over this trait. But OCA2 and HERC2 remain the major determining factors in whether a person develops green eyes or not.
Conclusion
In summary, green eye color is controlled primarily by two genes: OCA2 and HERC2. Recessive alleles in both these genes must be inherited from both parents for green eyes to occur, reducing brown melanin pigment in the iris. Green eye color can therefore be classified as a recessive trait genetically, requiring two copies of the recessive alleles to manifest. The inheritance patterns at these two gene loci account for the low incidence of green eyes globally. But for those unique individuals that inherit the right genetic recipe, spectacular green eyes can sparkle for the world to admire.
