Human eye color exhibits a wide range of variations, from the most common browns to less frequent blues and greens. This diversity in appearance stems from complex biological processes within the eye. Among these various hues, hazel eyes present a unique and often changing display of color.
Understanding Hazel Eye Characteristics
Hazel eyes are characterized by a distinctive blend of colors within the iris. They typically feature a combination of brown, green, and gold, and can sometimes include flecks of amber or even blue. Unlike single-colored eyes, hazel eyes are multi-tonal, with color concentration and distribution varying from person to person. This varied pigmentation can lead to a “sunburst” effect, where the color around the pupil differs from the outer rim of the iris.
Hazel eyes can appear to shift color depending on lighting or surroundings. For instance, some hazel eyes might appear more green in certain lights, while others might lean more towards brown. This dynamic quality sets them apart from solid-colored irises.
The Global Prevalence of Hazel Eyes
Hazel eyes are not considered one of the most common eye colors globally. Approximately 5% of the world’s population has hazel eyes. They are less prevalent than brown eyes (70-79% worldwide) and blue eyes (8-10%). Green eyes are the rarest, at roughly 2%.
While hazel eyes are uncommon globally, their prevalence can vary significantly by region. They are more frequent in populations of European descent; about 18% of people in the U.S. have them. This eye color is also more common in parts of Europe, such as Spain, the UK, and Germany, and has a higher incidence in the Middle East, North Africa, and Brazil.
The Genetics Behind Hazel Eye Color
The color of hazel eyes, like all eye colors, is determined by the amount and type of melanin present in the iris, along with how light interacts with this pigment. The iris contains two melanin types: eumelanin (brown/black tones) and pheomelanin (yellow, red, amber, green hues). Hazel eyes contain a moderate amount of melanin, falling between the lower levels in blue eyes and the higher concentrations in brown eyes.
The unique multi-tonal appearance of hazel eyes results from the uneven distribution of melanin within the iris. Different amounts of melanin can exist in various parts of the iris, causing some areas to appear brown while others seem green, gold, or amber. The scattering of light within the iris, known as Rayleigh scattering or the Tyndall effect, also influences how these colors are perceived. This optical phenomenon, also responsible for the sky’s blue appearance, contributes to the dynamic and shifting look of hazel eyes.
Eye color inheritance is a complex polygenic trait, influenced by multiple genes rather than a single one. Over 16 genes contribute to eye color, with major genes like OCA2 and HERC2 playing significant roles in melanin production and distribution. This intricate genetic interplay explains the wide spectrum of hazel eye variations and why predicting eye color is challenging.