Have you ever met a blue-eyed child with two biologically brown-eyed parents? Or have you ever wondered if the eye color of an expected child can be predicted?
What Determines Eye Color?
The color of our eyes is determined by how much melanin (pigment) is present in our irises; the more pigment present, the darker the eye color. Many babies are actually born with blue eyes that change to brown because melanin production has not begun by birth. As they begin to produce melanin, babies’ eye color may change. Interestingly, the eyes do not have green or blue pigments. These lighter colors appear due to Rayleigh scattering: the effect of light scattering in the layer of the iris called the stroma. This is similar to why the sky appears blue.
While family members often share similarly-colored irises, it’s impossible to predict exactly what a child’s eye color might be. Many of us learned about dominant and recessive genes in school, but it’s not always so simple as making a chart to predict the hue of children’s theoretical irises. The probability of eye color can be determined, but since it is influenced by different genes, it’s possible for a brown-eyed couple to have a blue-eyed child – and vice versa! Scientists have found evidence that as many as 16 genes can work to influence eye color. However, we tend to focus on just two: HERC2 and OCA2. These two genes come in two alleles, or versions. If these two alleles differ, only the dominant allele is expressed; the unexpressed allele is called recessive. It’s sort of like rock, paper, scissors: brown eyes are dominant to green, and green eyes are dominant to blue. Blue eyes will only occur when ALL alleles are blue.
Predicting Your Child’s Eye Color
One way of trying to predict the color of a child’s eyes is to create a graph called a Punnett square and enter the eye colors of the parents. Using this graph to chart the combinations of alleles allows parents to calculate eye color possibilities and their odds. If parents both have brown eyes, for example, but each carries a blue gene, the child has a one-in-four chance of having blue eyes. Odds are good, of course, that their child(ren) will have brown eyes just like mom and dad.
But what about hazel, violet, or gray eyes? Or those who have one brown eye and one blue? Red, pink, or violet eyes are a simple answer, as they are due to albinism, in which the body cannot produce or distribute melanin. The pink or red color is the retina showing through the clear iris. The condition of a person’s eyes being different colors is called heterochromia, in which the body distributes melanin unevenly due to injury or genetics. Other anomalies are due to other eye color genes, mutations, modifier genes, and other factors come in, as any combination of these can lead to varied eye colors. This is one of those details of the complex human body that retains a bit of mystery.
One of the most expressive parts of our bodies, our eyes are an important part of our personal identities, and eye color is one of our distinguishing characteristics. And though parents’ genes certainly play a part in what eye color one has, the exact combination that goes into this hue is as personal as our fingerprints.