Many new parents observe a striking phenomenon: their newborn baby’s eyes are a shade of blue or slate gray, regardless of the parents’ permanent eye color. This temporary light color is prevalent in infants, but the initial hue is not a prediction of the final color. This universal starting point is rooted in the biology of pigment production and the physics of light.
The Science Behind Eye Color Determination
Eye color is determined by the amount of melanin present in the iris, specifically within the tissue layer known as the stroma. Eyes with a high concentration of melanin absorb most incoming light, resulting in brown or darker shades. Conversely, eyes that contain very little melanin allow most light to pass through the stroma.
When light enters an iris with low melanin levels, a physical effect known as Rayleigh or Tyndall scattering occurs. Short-wavelength blue light scatters back toward the viewer more effectively than longer wavelengths. This light scattering creates the illusion of blue color, as there is no actual blue pigment present. Green or hazel eyes represent an intermediate stage, combining a moderate amount of melanin with this light scattering effect.
Why Newborn Eyes Lack Pigment
The initial blue or gray color in newborns results from the immaturity of the pigment-producing system. The specialized cells responsible for creating melanin, called melanocytes, are present in the iris stroma at birth. However, these cells have not yet been activated to produce significant amounts of pigment because the baby spent nine months in the dark environment of the womb.
Melanocyte activity is largely stimulated by exposure to external light, a process that begins only after birth. This delay means the iris starts with minimal melanin, leading to the blue light-scattering appearance. Once exposed to the world, the melanocytes begin slowly depositing melanin into the iris tissue. The rate and final extent of this pigment production determine the adult eye color.
If melanocytes secrete a small amount of melanin, the eyes remain blue. Higher secretion leads to green or hazel eyes, while robust production results in brown eyes. This postnatal ramp-up in pigment production causes the common color change observed during infancy.
When and How Eye Color Changes
The process of color change begins quickly, but the most noticeable shift typically occurs between six and twelve months of age. During this time, the gradual accumulation of melanin starts to mask the initial light-scattering effect. As pigment concentration increases, the true, genetically determined color emerges.
The speed and extent of the color change relate directly to the final color the baby is destined to have. If a baby is genetically predisposed to brown eyes, the melanocytes will be highly active, and the eyes will usually darken quickly within the first year. For babies whose final color will be green or hazel, the change may be slower and more subtle, as less total melanin is deposited. While most change is complete by the first birthday, minor color shifts can continue until a child is two or three years old.
The Role of Genetics in Final Eye Color
Although the developmental process of pigmentation is universal, the final eye color is strictly dictated by the baby’s genetic inheritance. Eye color is considered a polygenic trait, meaning it is controlled by the complex interaction of multiple genes. The simple Mendelian model, where brown is dominant over blue, is now understood to be an oversimplification.
The total amount of melanin the melanocytes are capable of producing is set by this genetic blueprint. Two genes, OCA2 and HERC2, located on chromosome 15, are recognized as having the largest influence on eye color variation. The HERC2 gene acts as a regulatory switch that controls the expression of the OCA2 gene, which determines the amount of melanin produced. The specific variants of these and other genes inherited from both parents determine whether the melanocytes will be programmed for low, moderate, or high melanin output, thus establishing the permanent eye color.