Many newborns may present with blue or grayish eyes, yet a significant number are born with brown eyes. The initial eye color observed at birth is not always the final hue a child will have. Eye color is a complex trait influenced by biological factors that develop over time.
The Science Behind Eye Color
The color of human eyes depends on the amount and type of a pigment called melanin within the iris. Melanocytes produce this melanin. There are two main types of melanin: eumelanin, which is responsible for brown and black hues, and pheomelanin, which contributes to red and yellow tones, often seen in green and hazel eyes. The concentration and distribution of these pigments in the iris determine the perceived eye color.
When melanin levels are low, as in blue eyes, light entering the iris is scattered, reflecting blue wavelengths. Higher concentrations of melanin absorb more light, resulting in darker colors like brown. Green and hazel eyes contain moderate amounts of melanin, with a combination of eumelanin and pheomelanin, which interact with scattered light to produce their distinct shades. The intricate interplay of pigment and light scattering creates the wide spectrum of eye colors observed.
Why Many Newborns Have Blue Eyes
Many newborns appear to have blue or gray eyes because their melanocytes, the cells that produce melanin, are not yet fully active at birth. Inside the womb, the eyes are not exposed to light, which is a stimulus for melanin production. Consequently, there is minimal melanin present in the iris at birth.
With lower melanin levels, the light scattering effect within the iris becomes more prominent. This initial blue or slate-gray appearance is often temporary. As the baby is exposed to natural light, the melanocytes begin to mature and increase their melanin production, leading to potential changes in eye color.
When Eye Color Settles
A baby’s eye color changes as melanocytes continue to develop and produce more melanin in response to light exposure. The most noticeable shifts in eye color occur between three and six months of age, as the iris accumulates more pigment. While some changes can happen within the first year, eye color may continue to evolve until a child is around three years old.
Eye color can continue to change, even into adolescence or adulthood, particularly for lighter eye colors. If a baby is born with dark eyes, such as brown, they are more likely to retain that color, though the shade might deepen. Changes progress from lighter to darker hues, as melanin production increases over time.
What Determines a Baby’s Final Eye Color
A baby’s final eye color is determined by genetic inheritance from both parents. Eye color is a polygenic trait, meaning it is influenced by multiple genes rather than a single one. While traditional models suggested a simple dominant-recessive pattern, current understanding reveals a more complex genetic interplay.
Several genes contribute to the final eye color by influencing melanin production in the iris. The OCA2 and HERC2 genes play significant roles. The OCA2 gene affects the amount and quality of melanin produced, while the HERC2 gene regulates the expression of OCA2, impacting melanin levels. The combination of these and other genes dictates the eventual amount of melanin, establishing the baby’s permanent eye color.