Many infants, particularly those of European descent, are born with light eyes that appear blue or gray, but this initial shade is often temporary. The final, stable eye color develops gradually over the first few years of life. This developmental process is driven by the production of pigment within the iris and is ultimately dictated by the child’s genetic blueprint.
How Melanin Production Influences Initial Eye Color
A baby’s eye color is directly linked to the presence of melanin, the pigment responsible for coloring the skin, hair, and the iris. At birth, the specialized cells in the iris, known as melanocytes, have not yet been fully activated and have produced very little melanin.
Because the iris stroma—the front layer of the iris—lacks significant pigment, the initial color is not due to a blue pigment. Instead, the blue or gray appearance is an optical effect called Rayleigh scattering. This phenomenon occurs when light enters the stroma and is scattered by the tissue, reflecting shorter blue wavelengths back to the observer.
As the baby is exposed to light after birth, the melanocytes begin to stimulate melanin production. The accumulation of this brownish pigment in the iris changes how light is absorbed and scattered. This increase in melanin causes the initial light eye color to darken or shift toward green, hazel, or brown over time.
The Developmental Timeline for Final Color Stabilization
Eye color changes begin shortly after birth, though the most noticeable shifts occur within the first year. The primary period of melanin development is observed between three and nine months of age. During this time, parents may notice the eyes showing flecks of color or a gradual deepening of the initial shade.
By the time a child reaches their first birthday, the majority of the permanent eye color is established. The process of stabilization often continues, and the color is considered stable by 18 months to two years of age. For children developing lighter eye colors like green or hazel, subtle changes can continue up until age three.
Eye color changes progress from lighter to darker as melanin production increases. Therefore, a baby born with dark brown eyes will experience little to no change, as their melanocytes are already producing high levels of pigment. While the color is set by the toddler years, some individuals may experience subtle changes that continue into adolescence or even adulthood. These shifts are due to ongoing alterations in the amount and distribution of melanin within the iris.
Understanding the Genetics of Eye Color
The ultimate color a baby’s eyes will stabilize at is predetermined by the genetic information inherited from both parents. Eye color inheritance is not based on a simple dominant or recessive model, as was once thought. Modern understanding recognizes eye color as a polygenic trait, meaning it is influenced by the complex interaction of multiple genes.
A particular region on chromosome 15 contains two genes, OCA2 and HERC2, which are considered the major players in determining eye color variation. The OCA2 gene is responsible for producing the P protein, which is directly involved in the production and maturation of melanin. The HERC2 gene acts as a regulatory switch, controlling the activity and expression of the OCA2 gene.
Variations in these genes dictate melanin production. High levels of melanin production, determined by specific gene variations, result in darker colors like brown. Conversely, variations that reduce the expression of OCA2 lead to less melanin being produced in the iris, resulting in lighter colors such as blue or green. While the combination of parental eye colors can offer a probability, the involvement of multiple genes means that unexpected color outcomes are possible.