The color of a person’s eyes often sparks curiosity, particularly regarding hazel eyes. Known for their blend of hues, hazel eyes frequently prompt questions about their commonality and distinctive appearance. Many seek to understand the scientific basis of this captivating eye color.
What Makes Eyes Hazel
Hazel eyes are characterized by a blend of colors, typically featuring brown and green, and sometimes hints of gold or amber. This combination creates a multi-toned appearance, often with different colors visible in various parts of the iris, such as a brown or gold ring around the pupil that radiates outward into green or gray tones. The exact distribution and intensity of these colors can vary, meaning some hazel eyes may appear predominantly green, while others lean more towards brown.
The appearance of hazel eyes stems from a moderate amount of melanin in the iris, a pigment whose concentration influences eye color. This amount is more than in blue or green eyes but less than in dark brown eyes. Another factor is Rayleigh scattering, the same optical phenomenon that makes the sky appear blue. Light entering the eye scatters off the stromal tissue in the iris, with shorter wavelengths (like blue and green light) scattering more, contributing to the varied hues seen in hazel eyes. This interplay between melanin and light scattering gives hazel eyes their dynamic look, often seemingly changing color depending on the lighting, surrounding colors, or even emotional state.
How Eye Color is Determined
Eye color is a complex trait influenced by multiple genes, rather than a single gene as once believed. The primary determinant is the amount and type of melanin present in the iris. There are two main types of melanin involved: eumelanin, which produces brown and black pigments, and pheomelanin, which contributes to red, yellow, amber, and green tones.
The concentration and distribution of these melanin types within the iris determine the final eye color. For instance, high concentrations of eumelanin result in brown eyes, while lower amounts lead to lighter colors. Genes such as OCA2 and HERC2, located on chromosome 15, play significant roles in regulating melanin production and distribution. The OCA2 gene produces a protein involved in melanin synthesis, and variations affect melanin amount. A specific mutation within the HERC2 gene can influence OCA2 activity, leading to reduced melanin and lighter eye colors. Other genes, including TYR, TYRP1, and SLC24A4, also contribute to the complex inheritance patterns of eye color by affecting melanin pathways.
Commonality of Hazel Eyes
Hazel eyes are relatively uncommon globally, accounting for approximately 5% of the world’s population. This makes them significantly less common than brown eyes, which are the most prevalent, found in about 70% to 79% of people worldwide. Blue eyes are also more common than hazel, with a global prevalence of around 8% to 10%. Green eyes, however, are considered the rarest, present in only about 2% of the global population.
The prevalence of hazel eyes varies by region. For example, in the United States, an estimated 18% of the population has hazel eyes, a significantly higher percentage than the global average. This higher prevalence in the U.S. is attributed to its diverse ancestry, particularly among individuals with mixed European, Middle Eastern, and North African roots. Hazel eyes are also more commonly found in parts of Europe, the Middle East, Brazil, and North Africa, with Western European countries like France, the UK, and Germany showing 8% to 12% prevalence. Conversely, regions with more genetic homogeneity, such as East Asia and sub-Saharan Africa, show very low rates of hazel eyes, with brown eyes being almost universally dominant.