Human eye colors range across a spectrum of shades, sparking curiosity about how these colors arise and which are less common. Eye color results from a complex interplay of genetic factors and how light interacts with the eye’s structure.
The Rarest Eye Color
The rarest eye colors are red or violet, almost exclusively linked to albinism. Individuals with severe albinism have a significant reduction or complete absence of melanin, the pigment responsible for eye, skin, and hair color. Without sufficient pigment, the iris cannot obscure blood vessels at the back of the eye, causing light to reflect off them and create a reddish or pinkish hue.
True violet eyes are also exceedingly rare, appearing in individuals with albinism due to very low melanin and light scattering from underlying blood vessels. While albinism affects approximately 1 in 18,000 to 20,000 people globally, not all individuals with the condition exhibit red or violet eyes; some may have light blue or even hazel eyes. These specific eye appearances are exceptionally uncommon.
The Science of Eye Color Determination
Eye color is determined by the amount and type of melanin in the iris, and how light scatters within its structure. The iris, the eye’s colored part, contains cells that produce melanin. Two primary types of melanin influence eye color: eumelanin, which produces brown or black tones, and pheomelanin (lipochrome), which contributes to red and yellow hues.
The concentration and distribution of these melanin types within the iris’s stroma, the front layer, play a significant role. Brown eyes, for instance, contain high concentrations of eumelanin, absorbing most light and leading to a darker appearance. Blue eyes, in contrast, have very low melanin content in the stroma, allowing light to scatter as it enters the eye. This scattering, similar to how the sky appears blue, reflects shorter wavelengths of light (blue) more than longer ones, creating the perception of blue color.
Eye color inheritance is polygenic, meaning multiple genes contribute. Among over 16 identified genes, OCA2 and HERC2 are primary influencers. The OCA2 gene regulates melanin production, while the HERC2 gene acts as a switch, influencing OCA2 expression. Variations in these genes can lead to reduced melanin production, resulting in lighter eye colors like blue or green.
Exploring Other Uncommon Eye Hues
Beyond the rarest red and violet eyes, other uncommon eye colors include amber and gray. Amber eyes, often golden or coppery, are characterized by a higher concentration of pheomelanin (lipochrome) and some eumelanin, giving them a yellowish-orange or reddish-brown tint. This eye color is relatively rare, present in about 5% of the world’s population, and more common in specific regions and ethnic groups.
Gray eyes, sometimes mistaken for blue, possess even less melanin than blue eyes and a greater density of collagen in the iris stroma. Their unique structure and low melanin levels result in a different light scattering effect, contributing to their distinct appearance. Gray eyes are also quite uncommon, found in less than 1% to 3% of the global population.
Another uncommon eye variation is heterochromia, a condition where an individual has irises of different colors, or multiple colors within a single iris. It can manifest as complete heterochromia (each eye a different color) or segmental heterochromia (a portion of one iris differs in color). Heterochromia can be present from birth, often due to a harmless genetic mutation affecting melanin distribution, or acquired later as a result of eye injury, disease, or certain medications. It affects less than 1% of the world’s population.