The Genetics of Eye Colour

Caucasian children are usually born with un-pigmented, blue eyes. As the child develops, the cells begin to produce melanin which eventually determines the final colour of the eyes as well as the skin and hair. Eye colours range from brown, which is the most common, to green, the rarest, with shades of blue, amber, hazel and grey somewhere in between. Eye colour is generally determined by the amount of pigment in the eye and this is decided by our genetic makeup long before we are born! There are several genes which influence the colour of a person’s eyes. As a general rule, brown eyed genes are often dominant and blue-eyed genes tend to be recessive. These factors of genetic dominance are added to the complex genetic equation that our bodies use to determine our eye colour. Our genes are made up of two alleles. We receive one allele from our mother and a second from our father. But it’s not that simple…multiple genes play a role in determining eye colour. To further complicate the issue, both genetic and environmental influences also affect the eye colour to a degree too. Pregnancy, puberty and trauma can also see a change in the colour of a person’s eyes. Eye colour is complicated.

The Genetics of Eye Colour 1

Blue eyes have become increasingly rare in American children in just the last few decades. As blue eyes are recessive, it needed parents of English, Irish and North
European descent to pass on these traits. Increasingly, immigration has brought a wider pool of genes which are more dominant, with the resulting decline in blue eyes. Thirty years ago about 30% of American babies had blue eyes; now that statistic has changed to about 1 in 6. Researchers believe that all blue-eyed people share one common ancestor. It is believed that the mutation that caused blue eyes occurred sometime between 6,000-10,000 years ago. Isn’t it strange to think that all blue-eyed people are somehow related?

The Genetics of Eye Colour 2

Brown eyes are predominant in humans and in many populations, it is the only iris colour. More than half the world’s population has brown eyes and 90% of the world has brown, hazel or amber eyes which are all variants of brown eyes. Dark brown eyes are prominent in East Asia and contain large amounts of melanin within the iris. Although brown eyes are predominant and are the main eye colour worldwide, in some parts of the world they are very rare…in Iceland, 80% of the population has green or blue eyes.

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Grey eyes are darker than true blue eyes and have less melanin than blue eyes. Grey eyes are predominant in Russia, Finland, Estonia, Latvia and Lithuania. The grey eye colour is determined by the density of proteins and the amount of melanin in the iris. Grey eyes are influenced by the light and may appear to change colour in different lighting or to reflect makeup and clothing colours.

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Green eyes have moderate or low amounts of melanin and are often associated with red hair. Only 2% of the world has green eyes and strangely they are more prevalent in women. They are common in the Icelandic population and those with Celtic and German ancestors.

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Hazel eyes are defined as being the colour midway between the lightest blue and the darkest brown eyes. Hazel eyes have a large amount of melanin in the anterior border of the iris. In different lights, they may appear to change from light brown to medium gold or even dark green. Sometimes they have a multi-coloured iris, lightest in the centre of the iris and dark brown or green on the outer part.