Hazel eyes are a captivating human trait, representing a unique intersection of genetics and physics. Understanding what causes the distinctive golden-green blend requires looking beyond simple color categories into the biology of pigment and the interaction of light. Hazel eyes result from moderate pigment levels and a subtle play of light scattering that creates a dynamic and less common visual effect.
Defining the Unique Look of Hazel Eyes
Hazel is not a single, solid color but a multi-tonal combination that creates a highly variable appearance. The iris typically features a blend of brown, gold, or amber tones concentrated near the pupil, transitioning into a greenish, blue, or light brown hue toward the outer rim.
This uneven distribution gives hazel eyes their dynamic quality, often causing them to appear to shift hues. This phenomenon is not a biological change in pigment but an optical illusion known as the “chameleon effect.” The perceived color changes based on the angle and brightness of ambient light, clothing colors, or even pupil size.
The Physics of Light and Pigment
The physical color of the eye is determined by the amount of melanin pigment present and how light interacts with the iris structure. Melanin, a dark brown pigment, is the only pigment found in the human iris, and its concentration dictates eye color. Hazel eyes have a moderate amount of melanin, more than blue or green eyes but less than dark brown eyes.
This moderate melanin is distributed unevenly within the iris stroma, the delicate, front-facing layer of tissue. The brown and gold flecks are areas where melanin is most concentrated, absorbing most of the light. The perceived green or blue color, however, is a structural color, meaning no actual green or blue pigment exists.
This structural effect is caused by Rayleigh scattering, the same phenomenon that makes the sky appear blue. When light enters the less pigmented areas of the stroma, its shorter wavelengths, such as blue and green, are scattered back out. This scattered light mixes with light reflected from the moderate brown melanin, resulting in the characteristic green or golden-green hue.
Genetic Inheritance of Hazel Eye Color
The concentration and distribution of melanin that creates hazel eyes are dictated by a complex polygenic inheritance pattern involving multiple genes. Scientists have identified over 16 genes that contribute to the final eye color phenotype, contrary to earlier models suggesting simple dominant or recessive control.
Two genes, \(OCA2\) and \(HERC2\), located on chromosome 15, are the most significant determinants of eye color variation. The \(OCA2\) gene provides instructions for creating the P protein, which is directly involved in the production and maturation of melanin within the iris. Higher activity of \(OCA2\) leads to greater melanin production and darker eyes.
The \(HERC2\) gene acts as a regulatory switch that controls the expression of \(OCA2\). A specific genetic variant in \(HERC2\) can reduce \(OCA2\) activity, resulting in less P protein and less melanin in the iris. The precise combination of variants in these and other contributing genes determines the intermediate melanin levels and uneven distribution seen in hazel eyes.
A single nucleotide polymorphism (SNP) called rs1800407 in the \(OCA2\) region is specifically associated with the development of green and hazel eye colors. Hazel eyes represent a middle ground in the genetic spectrum, arising from gene variants that permit a moderate, non-uniform amount of pigment. This complexity explains why two brown-eyed parents can potentially have a child with hazel eyes, as the trait depends on many genetic factors.
How Rare Are Hazel Eyes Globally?
Hazel eyes are considered one of the less common eye colors worldwide. According to global population estimates, approximately 5% to 8% of the world’s population has hazel eyes. This makes them significantly rarer than brown eyes, which are the most common, accounting for 70% to 79% of people globally.
The prevalence of hazel eyes is similar to amber eyes and more common than green eyes, which occur in only about 2% of the population. However, the frequency varies dramatically depending on geography and ancestry. Hazel eyes are most frequently found in populations with European heritage, particularly those with a blend of Northern European, Southern European, and Middle Eastern roots.
In the United States, the percentage is notably higher, estimated to be around 18% of the population, due to the diverse mix of ancestries. High concentrations are also reported in parts of North Africa, the Middle East, and Brazil.