How Eye Colour Is Determined

Most babies are born with grey, blue or brown eyes at birth, but the colour of their eyes can change as they get older. So what determines your child’s eventual eye colour?

Eye colour is a predominantly inherited trait and often a child’s eye colour is determined by the eye colour of their parents – but not always! It can be more complex than just a replica or mixture of the parents’ eye colour. A grandparent’s genes may have an effect, and, as multiple genes are involved, genetic variations can throw up some unforeseen results.

The science of eye colour

The colour of the iris (the muscular ring around the black pupil that controls how much light enters the eye), just like hair and skin, depends on a protein called melanin and we have specialised cells in our bodies called melanocytes whose job it is to go around secreting melanin where it’s needed, including in the iris.

Melanin is a protein and like other proteins, the amount and type you get is coded in your genes. Irises containing a large amount of melanin appear black or brown. Less melanin produces green, grey or light brown eyes. If your eyes contain very small amounts of melanin, they will appear blue or light grey. Melanin production generally increases during the first year of a baby’s life, leading to a deepening of eye colour and this is often stable by about six months of age, when eye colour becomes apparent.

It’s all in the genes

Genes play a fundamental role in determining eye colour. Scientists are constantly updating their understanding of the roles that genes play in our body and the deterimination of eye colour is no different. There are several genes that play a part and some have different versions. For instance, a father may have an eye colour gene that carries two versions, green and blue. Whilst he may have the green version of the gene, his child may inherit either the green or blue version. Different versions may be “hidden” for several generations until the combination with a parent who also has the hidden gene results in an unexpected eye colour in their child. The mixture of these genes, their versions, and variations, and how they interact with each other makes the science of eye colour rather complicated!

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