What is eye color determined by?

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Out of our five senses, eyes play such a vital role in our daily lives: we use them to read a captivating book, learn new things at work or look into the eyes of a loved one. You can tell a lot by looking into someone’s eyes: their personality, their mood or perhaps remember that time you met at that party. 

As with fingerprints, the eyes of each person represent unique characteristics – observed in full through technologies such as iris scanners. But no one really knows what evolutionary factors have contributed to giving them such interesting colours and textures. Talking to Vision Direct’s team of experienced eye doctors, we get some valuable  insights into what our eye colour is determined by.

Why are our eyes coloured?

The biological culprit behind eyes having a colour is the iris. Deriving from the Latin word for ‘rainbow’, the iris is a thin, ocular membrane that wraps around the pupil and regulates the amount of light that reaches the retina. But the iris mainly operates in relation to melanin, a pigmented tissue that gives the eye a darker or lighter colour, depending on its quantity. For example, if the iris contains a low concentration of melanin, the eyes will be blue in colour, while an average quantity will result in green colour and with a high quantity, brown.

Some people have multiple shades in their eyes, or specs of other colours here and there. This variation is known as somatic mosaicism, and is caused by a non-uniform concentration of melanin in the iris. An uneven concentration of melanin can also result in a clear difference in colour between one eye and the other, a phenomenon known as heterochromia – the reason behind David Bowie’s iconic green and blue coloured eyes. Heterochromia is often the symptom of a possible ocular pathology among humans, but it’s prominent among animals, such as dogs, cats or horses.

Why are most children born with blue eyes?

Have you ever noticed that almost all newborns have beautiful blue eyes, often unlike their parents? This is because the production of melanin inside the iris doesn’t start until a few months into someone’s life. Melanin certainly takes its time to develop and is affected by external factors, hence why it can’t get a head-start while the fetus is in the uterus. In fact, its development starts showing around seven months post-birth, reaching its final form within a child’s first birthday. The amount of melanin that ends up being produced is entirely dependent on genetic heritage, often spanning across two generations (parents and grandparents).

It’s all in the genes

Some love the captivating look of green eyes, others prefer looking into ice blue eyes; and there are those that enjoy getting lost in dark brown eyes. But who decided what colour your eyes are? The answer is: genes

Our cells are made of 46 chromosomes and during conception, we inherit one from each parent. Chromosomes are characterised by genes, who are behind many of our physical characteristics, such as hair colour. So when it comes to our eye colour, it’s down to these two genes to trigger the production and amount of melanin in the iris. However, one can be dominant over the other.

When it comes to gene 1, it can either be brown or blue. The brown version of gene 1 is dominant over the blue, meaning that if at least 1 of your two copies is brown, it will overpower the blue. The way this is represented through genetics is with two different versions of this eye colour: B for brown and b for blue (the capital letter symbolising the dominant and lowercase the recessive). For gene 2, there are two possibilities: green or blue. Green is dominant over blue and so G usually represents green and b is for blue.


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