The desire to know a baby’s eye color is a common point of excitement for expectant parents. Eye color is determined by the amount of melanin pigment present in the iris and is one of the most recognizable traits passed down through a family. While eye color inheritance is more complicated than previously thought, it still follows general rules determined by the genetic material contributed by each parent.
The Genetic Foundation of Eye Color
The coloring of the iris is directly proportional to the concentration of melanin. High amounts of melanin result in dark brown eyes, while very little melanin results in light blue eyes due to light scattering within the iris structure. This pigment production is controlled by multiple genes, making eye color a polygenic trait, though a few genes have a much larger influence than others.
The two primary genes determining eye color are OCA2 and HERC2, located on chromosome 15. The OCA2 gene provides instructions for making the P protein, which aids in the production and storage of melanin. The HERC2 gene acts as a “switch” that regulates the activity of OCA2, controlling how much P protein is produced. A variation in HERC2 can reduce OCA2 expression, leading to less melanin and, consequently, lighter eyes.
Historically, eye color was taught as a simple dominant/recessive trait, with brown dominant over blue. However, the reality involves a complex interplay of multiple genes. Brown eyes are considered the most dominant due to the high melanin required. Lighter colors, such as blue and green, are recessive because they result from genetic variations that limit melanin production.
Predicting Outcomes for Hazel and Brown Parents
The most likely outcome for a baby with one brown-eyed parent and one hazel-eyed parent is brown eyes. Brown is the most common eye color globally and is visually dominant, meaning it requires only one copy of a highly-pigmented gene variant to be expressed. Even if the brown-eyed parent carries a gene for a lighter color, the brown trait is still the most probable result.
Hazel eyes, which are a mix of brown, green, and amber shades, complicate the prediction because the hazel-eyed parent possesses a more complex set of genes. Hazel eyes contain less melanin than brown eyes, but more than blue or green eyes. Because the hazel color is produced by a unique combination of gene variants, the hazel-eyed parent is likely carrying recessive genes for lighter colors that may be passed on.
Based on simplified models of genetic inheritance, your baby has a high probability of inheriting brown eyes, often ranging from 50% to over 70%. The chance of the baby having hazel or green eyes is moderate, as the hazel-eyed parent has the necessary gene combinations to contribute to these mid-range colors. The probability of the baby having blue eyes is the lowest, but still possible. The overall probability is heavily influenced by the eye colors of the grandparents, which can reveal the recessive genes each parent carries.
Why Unexpected Colors Like Blue May Appear
The appearance of an unexpected color, such as blue, occurs because both parents may be “carriers” for recessive genes, even if they have darker eyes. The brown-eyed parent, despite having the dominant trait expressed, can genetically carry a recessive gene for blue or green eyes that is masked by their dominant brown gene. Similarly, the hazel-eyed parent, whose eye color is already a complex intermediate, possesses a wide range of underlying recessive genes.
When the two parents contribute their genetic material, the baby may inherit a recessive gene for a lighter eye color from both, resulting in an eye color that neither parent visibly has. For a baby to have blue eyes, they must inherit the specific low-melanin gene variant from both the brown-eyed and the hazel-eyed parent. This combination of two recessive genes leads to the expression of the lighter color.
How Infant Eye Color Changes Over Time
The eye color a baby is born with is not necessarily their permanent color, as the process of melanin production in the iris is not fully complete at birth. Many babies, particularly those of European descent, are born with eyes that appear blue or grayish due to the initial lack of melanin in the front layers of the iris. Melanocytes, the cells responsible for producing melanin, begin to work more actively in the months following birth as the eyes are exposed to light.
As melanin production increases, the baby’s eyes may gradually darken from blue or gray to green, hazel, or brown. The final, stable eye color typically sets in between six and nine months of age, though it can continue to change subtly up to the first birthday and, in rare cases, even into childhood. A child whose eyes remain brown from birth is highly likely to keep that color, as brown eyes already indicate a high and consistent level of melanin production.