Eye colour remains underdeveloped at birth and most babies are born with grey or blue eyes. 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.
Genes play a fundamental role in determining eye colour. We all have the same two eye colour genes and what gives us different eye colours are which variations of these genes we have. One of these genes is called HERC2 and it comes in two variations – brown and blue. The other gene, called gey (or chromosome 19), also comes in two versions – green and blue. Eye colour depends on which combination of these versions you have as shown below:
If you have a brown version of HERC2, you’ll have brown eyes no matter what the gey gene is. If you have only blue versions of HERC2, then which version of gey you have determines your eye colour. So if your HERC2 is blue and your gey is green, you have green eyes. And if you only have blue versions of both genes, then you have blue eyes. (It’s important to note that in genetics, brown is dominant over blue and green. And that green is recessive to brown but dominant over blue. Blue is recessive to both.)