Many infants seem to enter the world with bright blue eyes. This initial eye color is a temporary stage in human development, influenced by several biological factors.
The Science Behind Newborn Eye Color
The perception of blue eyes in newborns is not due to blue pigment. Instead, it results from the absence of significant melanin in the iris’s front layers, called the stroma, at birth. While the iris contains specialized cells called melanocytes, these cells have not yet fully activated to produce large quantities of melanin in newborns.
When light enters an infant’s eye with low stromal melanin, it interacts with collagen fibers. Shorter blue wavelengths scatter back out, while longer wavelengths are absorbed by the dark pigment epithelium at the back of the iris, making the eyes appear blue, similar to how the sky appears blue. Not all babies are born with blue eyes; infants with darker complexions often have more melanin at birth, resulting in brown or other darker eye colors.
How Eye Color Changes Over Time
After birth, iris melanocytes activate in response to light, increasing melanin production. This process gradually leads to a change in eye color as more pigment accumulates in the stroma. The amount and type of melanin produced determines the eventual eye color.
As melanin production increases, the initial blue hue can transition to green, hazel, or brown. Eye color changes become apparent around 6 to 12 months, usually stabilizing by one year old. Some children may experience subtle shifts up to three years of age, and in a small percentage, changes can continue into adulthood.
The Genetics of Permanent Eye Color
While the initial eye color in infants is related to the developing melanin, the final, permanent eye color is primarily determined by genetic inheritance. Eye color is a polygenic trait, influenced by multiple genes, not just one. Researchers have identified several genes involved in eye color, with OCA2 and HERC2 playing significant roles.
The OCA2 gene provides instructions for a protein involved in melanin production. The HERC2 gene regulates OCA2 activity, affecting the amount of melanin produced in the iris.
Different combinations of these genes and others contribute to varying levels of two main types of melanin: eumelanin, which produces brown and black pigments, and pheomelanin, which results in red and yellow hues. High concentrations of eumelanin lead to brown eyes, while very low eumelanin results in blue eyes due to light scattering. Green eyes typically have low eumelanin combined with some pheomelanin. This complex genetic interplay ultimately sets the potential for a person’s eye color.