The colored part of our eyes is called the iris. It can be brown, blue, green, or numerous shades in between.
Most of us have learned in biology class that the brown color is dominant and blue recessive, which makes the eye color of offsprings between the two groups predictable. In other words: if both parents have brown irises, then according to that theory they are guaranteed to have children with brown irises.
However, it has been shown that it is not so easy to predict the color of a baby’s eyes based on those of the (grand) parents. The color of the irises is indeed a hereditary factor, but there is more than one gene involved. It is the result of an interplay of different genes, making predictions harder than they seem.
Indeed, the color of the eyes depends on:
- the amount of melanin (pigment) present in the iris
- The distribution of said melanin over the iris
Brown eyes have more melanin than blue eyes and all shades in between such as hazelnut, gray, blue-green. The brown eye color is therefore more common in the continents where a warmer climate prevails.
Blue irises do not have melanin in the anterior layer (stroma). The light that falls into it is scattered, and that gives the irises a blue shine.
Because the melanin in their stroma is not yet sufficiently developed, babies tend to also have blue eyes. As soon as that melamine formation is completely on point, a toddler gets their final eye color.
Green eyes are the rarest and only occur in about 2% of the population. The green color is also formed by the combination of the limited amount of melanin and the scattering of light when it falls into the iris.
People with albinism have no or very little melanin and therefore their eyes are colored very light blue or in rare cases even pink or red.
Brown irises do indeed tend to be dominant, but they don’t always win! Parents who both have brown eyes are therefore more likely to have a child with brown eyes, but it is not a guarantee. And parents whose one has brown eyes and the other blue eyes, or something in between, won’t always have children with brown eyes either.