Human eye colors, from deep browns to striking blues and greens, display genetic diversity. Eye color is a complex trait determined by genetic information passed down through generations.
What Determines Eye Color
Eye color is primarily determined by the amount and type of melanin present in the iris, the colored part of the eye. Melanin is a pigment also responsible for skin and hair color. The iris contains specialized cells called melanocytes that produce two main types of melanin: eumelanin (dark brown) and pheomelanin (reddish-yellow). The concentration and distribution of these pigments influence the final appearance of eye color.
Beyond pigment, the way light interacts with the iris also plays a significant role. Blue and green eyes do not contain blue or green pigments; their appearance results from Rayleigh scattering, the same effect that makes the sky appear blue. In eyes with lower melanin content, light entering the iris is scattered, reflecting specific wavelengths and creating the perception of blue or green.
Brown eyes have high concentrations of eumelanin, which absorbs most light. Blue eyes contain minimal melanin, allowing extensive light scattering. Green eyes have intermediate levels of melanin, often a combination of eumelanin and pheomelanin, which, along with light scattering, produces their distinct shade.
How Eye Color is Inherited
Eye color is an inherited trait, passed from parents to their children through genes. Each person receives two copies of most genes, one from each parent. These gene variants, known as alleles, contribute to the traits an individual expresses.
While once simplified as a single gene trait, current understanding reveals eye color inheritance is more intricate, involving multiple genes. A general hierarchy of dominance is often observed: brown is typically most common and dominant over lighter colors. Green eyes are generally recessive to brown but dominant over blue.
Predicting Eye Color for Green and Brown-Eyed Parents
When one parent has brown eyes and the other has green eyes, predicting a baby’s eye color becomes a matter of probability influenced by each parent’s specific genetic makeup.
A brown-eyed parent might carry two dominant brown alleles or one dominant brown allele and one recessive allele for a lighter color, such as green or blue. A green-eyed parent typically carries at least one allele for green eyes and often a recessive allele for blue eyes, given that green is generally dominant over blue but recessive to brown.
If the brown-eyed parent carries only dominant brown alleles, the child will most likely have brown eyes, regardless of the green-eyed parent’s genetic contribution. However, if the brown-eyed parent carries a recessive allele for green or blue eyes, and the green-eyed parent contributes their green allele, the child could potentially have green eyes. If both parents contribute their recessive blue alleles, the child could even have blue eyes. The precise likelihood of each outcome depends on the specific allele combinations passed down from both parents.
Other Influences on Eye Color
Eye color is influenced by multiple genes, a concept known as polygenic inheritance. This intricate genetic interplay can sometimes lead to outcomes that deviate from simpler inheritance models. For example, two parents with blue or green eyes can, in rare instances, have a child with brown eyes due to complex gene interactions.
A baby’s eye color at birth is not always their permanent color. Many infants, particularly those of Caucasian descent, are born with blue-gray eyes due to lower melanin levels in their irises. As a baby grows, melanocytes in the iris begin to produce more melanin, which can cause the eye color to darken. This change typically occurs between 3 and 9 months of age, but the final eye color may not fully emerge until a child is between one and six years old.