Many new parents observe their newborn gazing into light blue or grayish irises. This common initial observation has led to the widespread belief that all infants are born with blue eyes. However, a baby’s eye color is one of the last physical characteristics to become permanently established, making the initial shade a temporary placeholder. The eventual color is determined by a complex interplay of biology and genetics that unfolds over the first few years of life.
The Initial Statistic: Prevalence of Blue Eyes at Birth
The idea that every baby enters the world with blue eyes is an oversimplification, particularly when considering global populations. A study conducted on newborns in the United States found that approximately 20.8% of the infants had blue eyes at birth. By contrast, 63% were born with brown eyes, illustrating the diversity present from day one.
This percentage varies heavily based on ancestry, which is a major factor in determining initial pigmentation levels. Infants of African, East Asian, and Hispanic descent are overwhelmingly born with eyes that are already dark brown and typically remain that color. The highest prevalence of blue or gray eyes at birth is observed among infants of European ancestry. For many of these babies, the light initial hue represents a developmental stage rather than a final outcome.
The Science Behind Temporary Blue Eyes
Many newborns display blue or slate-gray eyes, but this is not due to the presence of a blue pigment, as the human iris contains none. Instead, the initial color is a result of low melanin production in the iris at the time of birth. Melanin is the pigment responsible for coloring the skin, hair, and eyes, and it is produced by specialized cells called melanocytes.
The color we perceive is a result of light scattering within the iris’s stroma, the front layer of the tissue. When very little melanin is present, the tissue absorbs longer light wavelengths and scatters the shorter, blue wavelengths back out. This optical phenomenon, known as Rayleigh scattering, makes the eyes appear blue because the underlying brown pigment is not yet concentrated enough to absorb all the light.
The Timeline of Eye Color Change
The transition from a light initial hue to the permanent color begins as the melanocytes in the iris become more active following birth. Melanin production is gradually activated by exposure to light, which was limited during the months spent in the womb. As the infant is exposed to the outside world, the melanocytes begin slowly depositing pigment into the iris.
Parents may notice the first subtle color shifts around three to six months of age, with more noticeable changes occurring closer to six to nine months. If melanocytes produce only a small amount of melanin, the eyes may settle on blue or green. A moderate increase in pigment typically results in hazel or green eyes, while a large accumulation of melanin leads to brown eyes. Although the most significant changes happen in the first year, eye color is not considered fully stable until a child is between one and three years old.
Genetic Factors Determining Final Eye Color
The final, permanent eye color is dictated by a complex genetic blueprint inherited from both parents. The older, simple model suggesting that brown is dominant over blue, determined by a single gene, has been proven inaccurate. Eye color is a polygenic trait, meaning it is controlled by the interaction of multiple genes, with up to 16 genes potentially contributing to the final shade.
Two genes, OCA2 and HERC2, located on chromosome 15, are considered the major determinants of eye color variation. The OCA2 gene provides instructions for creating a protein involved in melanin production within the iris. The HERC2 gene acts as a regulatory element, controlling how much the OCA2 gene is expressed. Variations in this region can reduce the expression of OCA2, leading to less melanin in the iris and resulting in lighter colors like blue or green.