The belief that all infants are born with blue eyes is inaccurate, though many newborns, particularly those of European descent, display light-colored eyes, often blue or gray, at birth. Worldwide, most newborns, especially those of African, Asian, and Hispanic ancestry, are born with brown eyes that remain brown. The initial eye color is a temporary snapshot of developing biology, setting the stage for color maturation driven by genetics and pigment production.
The Science Behind Initial Blue Eyes
The temporary blue or gray appearance in many newborns’ eyes is a purely optical phenomenon, not an indication of blue pigment. The color of the iris is determined by melanin, the pigment that colors skin and hair. At birth, specialized cells in the iris, called melanocytes, have not yet begun to actively produce and deposit significant amounts of melanin in the stroma, the front layer of the iris.
Because the stroma lacks this dark pigment, it is translucent. When light enters the eye, it is scattered by the colorless fibers within the stroma. This scattering effect, similar to Rayleigh scattering that makes the sky appear blue, causes blue wavelengths of light to reflect back out. The resulting blue hue is a structural color, dependent on how light interacts with the tissue.
The Dynamic Process of Eye Color Change
The transition from a light initial color to the final adult shade is tied to the gradual activation of melanocytes after birth. Exposure to light triggers these cells to produce melanin, which accumulates within the iris stroma. The amount of pigment produced determines the final eye color: low melanin results in blue eyes, moderate amounts create green or hazel, and high concentrations lead to brown eyes.
This process typically unfolds over the first few years of life. While the most noticeable shifts occur between six and twelve months, the color may continue to change subtly until the child is around three years old, when the color generally stabilizes.
How Genetics Determines Final Eye Color
While the initial appearance is due to light scattering, the final, permanent eye color is predetermined by a complex genetic inheritance pattern. Eye color is a polygenic trait, meaning it is influenced by the interaction of multiple genes, not just a single gene. The two genes with the largest known influence are \(OCA2\) and \(HERC2\), both located on chromosome 15.
The \(OCA2\) gene provides instructions for creating the P protein, which plays a role in the production and storage of melanin. The adjacent \(HERC2\) gene contains a regulatory region that acts as a switch, controlling the activity of the \(OCA2\) gene. Variations in this \(HERC2\) region can reduce \(OCA2\) expression, leading to decreased melanin production and lighter eye colors.
This multi-gene control explains why eye color exists on a spectrum, rather than simply being brown or blue. It also shows why the old model of brown being strictly dominant over blue is too simplistic. Brown eyes are the most common globally and carry higher amounts of melanin. Predicting a child’s eye color based on parental color is a matter of probability, not certainty. Even two parents with blue eyes can, in rare cases, have a child with brown eyes due to the complex interplay of the many genes involved.