Brown eyes are the most common eye color worldwide. This widespread trait is determined by the presence and concentration of a pigment called melanin within the iris. This article explores how this pigment creates the color, its prevalence across different populations, and the genetic mechanisms responsible for its inheritance.
The Science Behind the Color
Brown eye color results from eumelanin, a type of melanin, in the iris. The amount of eumelanin present directly influences the shade of brown; higher concentrations lead to darker brown eyes, while lower concentrations result in lighter shades.
The color isn’t a brown pigment like paint; it’s how light interacts with the melanin in the iris. Melanin absorbs most wavelengths of light that enter the eye, and the small amount of light that is reflected back appears brown. All human eyes contain melanin, but brown eyes have the highest concentration, particularly in the front layers of the iris.
Prevalence and Variations
Approximately 70% to 80% of the global population has brown eyes. This prevalence is especially notable in regions with higher sun exposure, such as Africa, Asia, and parts of the Americas, where the increased melanin offers protection against UV damage. Research suggests that around 10,000 years ago, brown was the universal eye color before genetic variations led to the emergence of other colors.
The spectrum of brown eyes is broad, ranging from very light brown, sometimes appearing golden or amber, to very dark brown, which can appear almost black. Variations like hazel eyes are often considered within the brown eye spectrum, characterized by a mix of brown, gold, and green hues. Hazel eyes contain less melanin than brown eyes, allowing for some light scattering that reveals other colors, often appearing to shift in shade depending on the lighting.
How Eye Color is Inherited
Eye color inheritance is a complex trait influenced by multiple genes, rather than a single gene as once believed. At least 16 genes are associated with eye color, with major genes like OCA2 and HERC2 playing significant roles in melanin production and distribution within the iris.
While the inheritance pattern is intricate, brown eye color is generally considered dominant over lighter colors like blue or green. This means that if an individual inherits a gene variant for brown eyes, they are likely to have brown eyes even if they also inherit a variant for a lighter color. However, because multiple genes are involved, it is possible for two brown-eyed parents to have a child with lighter eyes, demonstrating the nuanced nature of eye color genetics.