Hazel eyes are a fascinating and unique eye color that combines shades of brown, green, and gold. The exact cause of hazel eyes has long been a source of speculation and research. In this article, we’ll explore the genetics behind hazel eyes and how this rare eye color is passed down through generations.
What Causes Hazel Eyes?
Hazel eyes occur when there is a combination of the pigments eumelanin (brown) and pheomelanin (tan/gold) in the front layers of the iris. Most commonly, hazel eyes consist of an amber or light brown ring around the pupil, with specks or streaks of green, gold, or brown around the outer part of the iris.
The amount of melanin pigments present determines the exact hue of hazel eyes. Higher levels of eumelanin lead to eyes appearing more brown, while higher levels of pheomelanin cause eyes to look greener-gold. The unique blending and ratio of melanin pigments produce the many different shades of hazel that can range from pale honey-brown to deep swampy-green.
Genetic Factors Behind Hazel Eyes
Genetically, hazel eyes are determined by multiple genes that control the production, distribution and types of melanin in the iris.
Research has identified some of the key genes involved:
- HERC2 – regulates the expression of the OCA2 gene and helps determine brown/non-brown eye color.
- OCA2 – controls melanin production and iris pigmentation.
- SLC24A4 – regulates melanin type (pheomelanin vs. eumelanin).
- SLC45A2 – helps determine melanin distribution and density.
The exact combinations and variations of these genes produce the range of hazel eye colors. Essentially, hazel eyes occur when there is an intermediate pigmentation caused by mixed expression of the genes related to melanin production and type.
How Are Hazel Eyes Inherited?
Hazel eyes are generally considered to be a genetic trait passed on through dominant gene inheritance. However, the exact genetics are complex since multiple genes influence hazel eye color.
One-Gene Trait
Some research suggests hazel eyes could potentially result from a single gene. The proposed hereditary model is:
In this model, the hazel eye trait (Bb) is dominant over pure brown (BB) or green/blue (bb) eyes. So a child would only need one copy of the hazel allele to inherit this eye color.
Polygenic Trait
However, most experts believe hazel eye color is polygenic, meaning multiple genes on different chromosomes work together to produce hazel eyes.
For polygenic inheritance, parents do not need to have hazel eyes themselves to have a hazel-eyed child. The polygenic model involves these genetic components:
- Multiple alleles that code for melanin production.
- Alleles that regulate eumelanin vs pheomelanin ratios.
- Genes influencing melanin density and distribution.
The combinations and variations of these alleles ultimately determine if a person will have hazel or another eye color. Since many genetic factors are involved, hazel can arise spontaneously in families without prior hazel-eyed relatives.
Epistasis
There is also evidence that hazel eye color may involve epistasis. This refers to a gene at one locus masking or altering the expression of a gene at another locus.
For example, the HERC2 gene seems to have an epistatic relationship with OCA2. Specific variants of HERC2 can inhibit OCA2 expression. When OCA2 is reduced but not completely suppressed, it can produce intermediate pigmentation and hazel eyes.
What Determines Hazel Versus Blue/Green Eyes?
Hazel eyes are genetically distinguishable from pure blue and green eyes in a few key ways:
- More eumelanin pigment present.
- Decreased melanosomal motility in iris cells.
- Modifying genes that regulate OCA2 expression.
Compared to blue and green, hazel eyes have higher eumelanin levels which contributes to the brown/amber hues. Research also shows hazel irises tend to have less melanosome movement within iris cells, which produces areas or patches of color variation.
Furthermore, hazel eyes are associated with different heritage genetic variants that result in intermediate OCA2 activation. Blue/green eye color is linked to almost complete suppression of OCA2.
Prevalence of Hazel Eyes
Worldwide, hazel eyes are relatively rare compared to brown, blue, and green eyes. Estimates of hazel eye prevalence include:
- 5-10% of people worldwide
- Around 5% of European descendants
- 15-20% of Southwest Asian peoples
- 10-25% of North African ethnic groups
In the United States, hazel eyes are found in an estimated 6-10% of the Caucasian population. Hazel eyes are more commonly seen in Middle Eastern, South Asian, North African, and Latino/Hispanic ethnicities as well.
Hazel Eyes and Melanin Levels
Studies show hazel eyes are most prevalent in geographic ancestry groups that fall in the mid-range of melanin levels. Populations with very high melanin amounts overwhelmingly have brown eyes. Conversely, ethnicities with minimal melanin levels usually exhibit blue/grey eyes.
Hazel appears to be commonest in ancestry groups that have an intermediate amount of melanin. This supports the theory that hazel arises from a combination and blending of melanin pigments in the iris.
Does Eye Color Change With Age?
It was once thought that hazel eyes could darken or change color over a person’s lifetime. However, current research indicates hazel-eyed individuals are born with the amount of melanin that will determine their eye color.
Eye color is set at around 9 months of age and does not undergo any major changes aside from a slight lightening with increased age. A hazel-eyed infant will have the same hazel eye color well into adulthood.
That said, the perception or appearance of hazel eyes can shift somewhat due to changes in:
- Iris freckles/patterns becoming more noticeable with age.
- Pupil size altering the hue.
- Less melanin being produced later in life.
Environmental factors like clothing color and lighting conditions may also affect how hazel eyes look. But the underlying color and melanin content remains stable over time.
Conclusion
In summary, hazel eye color arises from a combination of melanin pigments in the iris. It likely involves multiple interacting genes that regulate melanin production and distribution. Hazel appears to be most common in populations with intermediate melanin levels compared to brown and blue/green eyed groups.
Once established in infancy, hazel eye color remains relatively constant over a person’s lifetime. The genetics behind hazel remain complex and the inheritance patterns unclear. But this unique blending of eye colors produces an aesthetically pleasing and rare phenotype passed down through generations.
