Hazel eyes are a mix of brown, green and gold. They appear different shades depending on lighting conditions. The key factor determining hazel eye color is having both brown and green melanin in the iris.
Melanin and Eye Color
Melanin is the pigment that gives color to the skin, hair and eyes. There are two main types of melanin that control eye color:
- Eumelanin – Brown and black pigments
- Pheomelanin – Red and yellow pigments
The amount and type of melanin present determines someone’s eye color. Higher amounts of brown eumelanin result in brown or black eyes. Higher amounts of pheomelanin lead to hazel, amber and green eyes.
What Causes Hazel Eyes?
Hazel eyes occur when there is a combination of brown eumelanin and yellow/green pheomelanin present in the iris. This results in an eye color that shifts between shades of brown, green, gray and gold.
The amount of melanin isn’t evenly distributed in hazel eyes. There are usually flecks or streaks of brown and green throughout the iris. The overall color depends on the lighting and ratio of melanin types present.
Genetics Behind Hazel Eyes
Multiple genes influence eye color and interact in complex ways. The main genes involved in hazel eyes are:
- HERC2 – Regulates OCA2 expression and melanin production
- OCA2 – Catalyzes brown eumelanin synthesis
- SLC24A4 – Influences ratio of brown to green melanin
The HERC2 gene helps control the OCA2 and SLC24A4 genes. Variants in these genes alter melanin production and the brown/green balance. This results in hazel eye color.
HERC2 Gene
The HERC2 gene provides instructions for making a protein that regulates OCA2 activity. Specific variants of HERC2 reduce the expression of OCA2, limiting melanin production in the iris.
The main variants are:
- rs12913832 – Reduces OCA2 expression and brown melanin
- rs1800407 – Linked to green/hazel eyes
Having one or two copies of these variants reduces brown eye color and allows hazel eyes to develop.
OCA2 Gene
The OCA2 gene provides the blueprint for making the P protein. This protein is crucial for producing brown eumelanin.
The main variants of OCA2 are:
- rs7495174 – Associated with brown eyes
- rs4778138 – Reduces eumelanin production
Having two copies of the rs4778138 variant limits brown melanin. This allows pheomelanin to show through, leading to hazel eyes.
SLC24A4 Gene
The SLC24A4 gene helps regulate calcium levels in melanocytes, the pigment producing cells. It influences the ratio of brown eumelanin to yellow/green pheomelanin.
The main SLC24A4 variants linked to hazel eyes are:
- rs12896399 – Increases green/hazel eye color
- rs1426654 – Associated with light brown/green eyes
These variants shift melanin production in favor of pheomelanin over brown eumelanin. This allows hazel eye color to emerge.
Other Genes Linked to Hazel Eyes
In addition to the main OCA2, HERC2 and SLC24A4 genes, several other genes play a role in hazel eye color:
- TYR – Involved in melanin production
- TYRP1 – Helps convert tyrosine into melanin
- SLC45A2 – Mediates melanin synthesis
- IRF4 – Regulates pigmentation genes
Variants in these genes can shift the activity of melanin production genes. This further modifies the ratio of brown eumelanin to pheomelanin in hazel eyes.
Is Hazel a Dominant or Recessive Trait?
Hazel eye color is an intermediate phenotype between brown and green eyes. It is influenced by multiple genes that interact in a complex manner.
Both dominant and recessive alleles contribute to hazel eyes. However, it is not clearly a dominant or recessive single gene trait like brown or blue eye color.
In general, the variants that reduce brown melanin and allow pheomelanin expression are recessive. The alleles linked to green/hazel eyes require one or more functional copies to show their effects.
Can Two Brown-Eyed Parents Have a Hazel-Eyed Child?
Yes, it is possible for two brown-eyed parents to have a hazel-eyed child. Here’s how:
- Both parents carry recessive alleles for reduced brown eye color
- Each parent passes on one copy of these alleles to the child
- The child ends up with two recessive alleles that limit brown melanin
- This allows pheomelanin to show through, producing hazel eyes
For example, both parents could carry one copy of the rs12913832 or rs1800407 HERC2 variants. The child inherits one from each parent, reducing OCA2 expression and brown melanin levels. This allows hazel eyes to emerge.
Can Hazel Eyes Change Color or Turn Green?
The color of hazel eyes can appear to shift from brown to green. However, the eye color itself doesn’t actually change over time.
Hazel eyes have a mix of melanin pigments that look different under varying light conditions. The balance of brown and green hues varies across the iris.
As children grow, hazel eyes may appear more green or light brown. This is because melanin levels continue to build up until around age 2-3 years old.
Hazel eyes do not turn green permanently. But the mix of melanin can make them look more green/golden brown vs. dark brown when lighting changes.
Conclusion
In summary, hazel eye color results from a combination of brown eumelanin and yellow/green pheomelanin in the iris. Variants in the HERC2, OCA2 and SLC24A4 genes alter the melanin ratio and distribution to produce hazel eyes. The exact shade can vary between green, brown and gold. While hazel eyes may appear to change color somewhat, the effect is an optical illusion based on lighting conditions rather than actual color change.
