New parents often wonder about their baby’s eye color. Eye color is determined by the interaction between genetics and physics. The final hue results from pigment concentration and how light interacts with the eye’s structure. Understanding the science provides strong clues for prediction, even though the color seen at birth may not be permanent.
The Science of Eye Color Pigmentation
The color displayed in the iris is determined by the amount of melanin pigment present in its front layer, the stroma. Brown eyes, the most common color globally, have a high concentration of melanin which absorbs most of the light entering the eye. Higher melanin concentration results in darker eye colors, ranging from light brown to nearly black.
Lighter eye colors, such as blue, green, and gray, are not caused by specific pigments, as none exist in the human iris. Instead, these colors are structural, created by a low concentration of melanin. When light enters an iris with little melanin, it scatters off the fibers of the stroma.
This light-scattering effect, known as Rayleigh scattering, causes shorter, blue wavelengths of light to reflect back, making the iris appear blue, similar to how the sky appears blue. Green and hazel eyes result from a moderate amount of melanin combined with this light scattering. A small amount of yellowish-brown pigment combines with the scattered blue light to create the perceived green or shifting hazel color.
Genetic Inheritance and Prediction
The determination of eye color is a polygenic trait, meaning it is controlled by multiple genes. Although at least 16 different genes influence eye color, two genes on chromosome 15—\(OCA2\) and \(HERC2\)—are the primary factors. The \(OCA2\) gene provides instructions for creating the P protein, which is involved in melanin production.
The \(HERC2\) gene acts as a regulator for \(OCA2\), influencing how much melanin is produced and distributed in the iris. A specific variation in the \(HERC2\) gene can switch off the full expression of \(OCA2\), leading to a minimal amount of melanin and resulting in blue eyes. This interaction makes the inheritance pattern more complex than the traditional schoolhouse model of a simple dominant brown and recessive blue trait.
Brown eye color is considered dominant because it is associated with high melanin production, while blue is associated with low production. If both parents have blue eyes, the probability of their child also having blue eyes is very high, often cited as around 99 percent. This occurs because both parents are likely contributing low-melanin gene variants.
If two parents have brown eyes, they can still have a child with blue or green eyes because they may each carry the genetic variants for lower melanin production. If one parent has brown eyes and the other has blue, the child has a higher probability of inheriting the brown-eye variants, but the chance of a lighter color remains significant. The presence of lighter eye colors in grandparents or extended family also increases the likelihood of inheriting a non-dominant color.
The Timeline of Color Change
Many babies, particularly those of European descent, are born with blue or slate-gray eyes. This initial light color occurs because the melanocytes, the specialized cells that produce melanin in the iris, are not fully activated at birth. Since the fetus develops in the dark womb, these cells have not yet been exposed to light, which stimulates melanin production.
Once the baby is exposed to light, the melanocytes begin increasing melanin production. If the child is genetically programmed for a darker color, this pigment gradually accumulates in the iris. This causes the eyes to darken from blue or gray to green, hazel, or brown. The final eye color reflects the maximum amount of melanin the child’s genes instruct the cells to produce.
The most significant changes in eye color typically occur within the first six to nine months of life. While the color may appear set by the first birthday, subtle shifts can continue throughout toddlerhood, often stabilizing around three years of age. Light exposure stimulates the melanin production process, but it does not change the genetically predetermined color; it merely triggers the expression of the color coded in the child’s DNA.