Hazel eyes present a unique and captivating appearance, characterized by a blend of colors that often appear to shift. This eye color typically combines brown, green, and gold hues, sometimes including flecks of blue or amber. The interplay of these shades can create a distinct “sunburst” effect, with different tones appearing around the pupil and towards the outer rim of the iris.
The Genetic Recipe for Hazel Eyes
The color of hazel eyes is not due to a specific pigment but rather a complex interplay of melanin levels and light scattering within the iris. Melanin, the same pigment that determines skin and hair color, is the primary substance responsible for eye color. In hazel eyes, there is a moderate amount of melanin, more than in blue or green eyes but less than in brown eyes. This melanin is distributed unevenly throughout the iris, contributing to the multi-tonal appearance.
Two main types of melanin are involved: eumelanin, which produces brown and black tones, and pheomelanin, which contributes to yellow, amber, and red hues. Hazel eyes contain both, with their specific balance creating the unique blend of colors. The greenish tint often seen in hazel eyes results from light scattering (specifically Rayleigh and Tyndall scattering) over a base of light brown pigment, rather than a green pigment itself. As light enters the eye, shorter wavelengths (like blue and green) scatter more easily, while longer wavelengths (like red or brown) are absorbed or reflected differently by the melanin.
Eye color inheritance is a complex process influenced by multiple genes. While brown eyes were once thought to be dominant and blue recessive, current research indicates that up to 16 different genes can influence eye color.
Among these, the OCA2 and HERC2 genes play significant roles in melanin production and distribution. OCA2 produces a protein that controls melanin maturation, while HERC2 regulates the expression of OCA2. Variations in these genes contribute to the different amounts and types of melanin, leading to the diverse range of eye colors, including hazel. This polygenic inheritance explains why hazel eyes can appear in families where neither parent has them, as the specific combination of genetic variants creates the unique hazel phenotype.
The Geographic Journey of Hazel Eyes
The precise geographical origin of hazel eyes is not definitively known, but theories suggest they emerged in regions where populations with different eye colors mixed. Unlike blue eyes, which are thought to have a single ancient origin, hazel eyes likely arose from the intermingling of ancestries that carried genes for both brown and lighter eye colors. This mixing could have occurred in areas such as Europe, the Middle East, and North Africa, which historically served as crossroads for trade, migration, and conquest. The evolution of lighter eye colors, including hazel, is linked to the relaxation of selective pressures for dark pigmentation as humans migrated out of Africa into regions with less intense sunlight.
Early human ancestors are believed to have had dark brown or nearly black eyes, providing protection in sunny equatorial climates. As populations moved into higher latitudes, the need for high melanin levels diminished, allowing for greater diversity in eye color. Hazel eyes are considered a relatively newer eye color compared to brown. While ancient DNA studies have not identified a single hazel eye mutation, they indicate that light eye colors, including potentially hazel, were present in Eurasia by the Bronze Age.
Hazel Eyes Across Populations Today
Today, hazel eyes are found globally, but their prevalence varies significantly by region. Approximately 5% of the world’s population has hazel eyes, making them less common than brown or blue eyes but more prevalent than green eyes. They are most frequently observed in populations with European ancestry, particularly in parts of Europe, the Middle East, and North Africa.
In the United States, the prevalence of hazel eyes is notably higher than the global average, estimated at around 18% of the population. This higher incidence is largely attributed to the country’s diverse ancestry, especially among individuals with Irish, German, Spanish, and Middle Eastern roots. Historical migration patterns, intermarriage, and genetic drift have all influenced the modern-day distribution of hazel eyes.