Human eyes exhibit a remarkable range of colors, from deep browns to striking blues and greens. This diversity has long captivated observers and sparked curiosity about how these distinct hues come to be.
What Makes Eyes Hazel?
Hazel eyes are characterized by a blend of colors, typically incorporating green, brown, and gold, and sometimes even flecks of blue or amber. Unlike eyes with a single, uniform color, hazel eyes present a spectrum of shades that often appear to shift depending on the lighting conditions or surrounding colors. This multi-toned appearance results from a moderate concentration of melanin in the iris, the pigmented structure surrounding the pupil.
The distinct look of hazel eyes is also influenced by how light interacts with the iris. Rayleigh scattering, the same optical phenomenon that makes the sky appear blue, plays a role in how green and blue tones are perceived in lighter eyes. This effect, combined with varying melanin distribution, can create a lighter inner ring near the pupil that transitions into a different color towards the outer edge of the iris. Hazel eyes differ from amber eyes, which have a solid yellow-gold or coppery hue without green tones, and from purely green eyes, which maintain a consistent green.
Prevalence of Hazel Eyes
Hazel eyes are less common than brown or blue eyes globally. Approximately 5% of the world’s population has hazel eyes. This makes them significantly less common than brown eyes, which are the most prevalent, accounting for 70% to 79% of people worldwide. Blue eyes are also more widespread than hazel, found in about 8% to 10% of the global population.
Compared to other lighter eye colors, hazel eyes are generally more common than green eyes, which are considered the rarest, present in only about 2% of people. In some regions, the prevalence of hazel eyes is notably higher; for example, they are estimated to occur in about 18% of the U.S. population. Hazel eyes are most commonly found in areas with diverse ancestries, including North Africa, the Middle East, Brazil, and Spain. This suggests a connection to historical population movements and genetic mixing.
The Genetics of Eye Color
Eye color is a complex trait influenced by multiple genes. This polygenic inheritance means that several genes interact to determine the final eye color. The primary pigment responsible for eye color is melanin, which is produced by specialized cells in the iris. The amount and distribution of two types of melanin, eumelanin (dark brown) and pheomelanin (reddish-yellow), determine the resulting hue.
Key genes influencing eye color include OCA2 and HERC2, both located on chromosome 15. OCA2 plays a significant role in producing melanin, while HERC2 regulates the expression of OCA2. Variations in HERC2 can reduce OCA2 activity, leading to less melanin and lighter eye colors. Hazel eyes typically have moderate levels of melanin, more than blue or green eyes but less than brown eyes, contributing to their unique combination of shades. The complex interplay of these genes and melanin levels explains why hazel eyes are less common than brown eyes but generally more prevalent than green eyes.