Grey and blue eyes spark curiosity about their prevalence and the science behind their unique shades. Eye color is determined by a complex interplay of biology, physics, and genetics. Understanding these factors provides insight into why certain eye colors are more common and what distinguishes variations like grey from blue.
The Rarity of Grey and Blue Eyes
Brown is the most common eye color globally, accounting for approximately 70-80% of the world’s population. Blue eyes are significantly less common, found in about 8-10% of people worldwide. While blue eyes are not as rare as green eyes, which occur in about 2% of the global population, they are still considered uncommon. Blue eyes are most prevalent in Northern and Eastern Europe, with some Scandinavian and Baltic countries having blue-eyed populations as high as 89-99%.
Grey eyes are even rarer than blue eyes, estimated to be present in only about 3% of the global population. In the United States, less than 1% of the population is estimated to have grey eyes. This makes grey eyes among the least common eye colors worldwide, predominantly found in Eastern and Northern European populations.
The Science Behind Eye Color
Human eye color primarily depends on the amount and type of melanin present in the iris, the colored part of the eye. The iris has two main layers: the front layer called the stroma and the back layer known as the iris pigment epithelium. Most people, even those with blue eyes, have a brown pigment called melanin in the back layer of the iris.
The color we perceive is not due to blue or grey pigments, as these do not exist. Instead, eye color is structural, influenced by how light interacts with melanin and collagen fibers within the iris stroma. Melanin comes in two main types: dark brown eumelanin and reddish-yellow pheomelanin. Eumelanin amount primarily determines eye color; higher concentrations result in darker eyes.
Lighter eye colors, such as blue and grey, occur due to very low concentrations of melanin in the front layer of the iris. When light enters the eye, shorter wavelengths are scattered by collagen fibers in the stroma, similar to Rayleigh scattering that makes the sky appear blue. This scattered blue light reflects back, making eyes appear blue.
Distinguishing Grey from Blue Eyes
While both blue and grey eyes have low melanin concentrations in the iris, subtle differences in their composition lead to their distinct appearances. The primary factor differentiating them lies in the amount of melanin and the density and arrangement of collagen fibers within the iris stroma. Blue eyes generally have very little melanin in the front layer of the iris, allowing more blue light to be scattered and reflected.
Grey eyes, in contrast, have even less melanin than blue eyes, or a different distribution. Additionally, grey eyes may have a higher concentration of collagen in the stroma. This increased collagen density can cause light to scatter more evenly across the spectrum, resulting in a more uniform, often “smoky” or “silvery” grey hue, distinct from the blue reflection seen in blue eyes. The appearance of grey eyes can also vary with lighting conditions, sometimes appearing to shift towards blue or green.
Genetic Factors in Eye Color
Eye color is a polygenic trait, influenced by multiple genes rather than a single one. This complex inheritance explains the wide spectrum of eye colors. While many genes contribute to eye color, OCA2 and HERC2 on chromosome 15 are primary influencers.
The OCA2 gene plays a significant role in producing P protein, involved in melanin creation and processing. Variations in OCA2 can reduce the amount of melanin produced, leading to lighter eye colors. The HERC2 gene regulates OCA2, controlling its expression. A specific variation within HERC2 can reduce OCA2 activity, resulting in lower melanin production and lighter eye colors like blue and grey.
The combined effects of different alleles (versions of these genes) and other minor genes determine final eye color by influencing melanin amount and distribution in the iris. This complex genetic interplay allows for the wide range of eye colors, including less common blue and grey shades.