Blue, brown, green, grey. Everyone’s eye colour is a unique marker like a fingerprint. We all have our favourites and some of us even change our eye colour with coloured contact lenses. So how is the colour of our eyes determined?

The coloured part of the eye is called the iris, which has pigmentation that determines our eye colour. Human eye colour originates with three genes, two of which are well understood: brown and blue. The third gene: green is not well understood currently. Other colours, such as gray, hazel and multiple combinations are not fully understood or explainable at this time.

Most babies are born with blue eyes that can darken in their first three years. Darkening occurs if melanin, a brown pigment usually not present at birth, develops with age.

Children can have completely different eye colours than either of their parents. But if both parents have brown eyes, it’s most likely that their children also will have brown eyes. How eye colour is inherited is far more complicated than what was thought back in the days when simple charts were created that supposedly could predict the eye colour of children based on the eye colour of their parents. Generally, though, it’s far more likely for two brown-eyed parents to have a blue-eyed child than for two blue-eyed parents to have a brown-eyed child. This is because the generally less dominant blue-eyed trait can be passed along by brown-eyed people until the genes for the lighter eye colour happen to match up, possibly many generations later. Two blue-eyed parents, on the other hand, are much less likely to have darker-eyed children. This is because darker eyes generally are so much more dominant that the genetic trait, when present, ordinarily would first show up in the parent, who then wouldn’t be blue-eyed at all.

Eye colour can change with age. This happens in 10 to 15 percent of the population. Note that if your adult eye colour changes significantly, or if one eye changes from brown to green or blue to brown (called heterochromia), it’s important to see your optometrist. Eye colour changes can be a warning sign of certain eye diseases.