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Blue-eyed humans have a single, common ancestor

Updated on 14 March 2020
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Curious Mind
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Updated on 14 March 2020

New research shows that people with blue eyes have a single, common ancestor. A team at the University of Copenhagen have tracked down a genetic mutation which took place 6-10,000 years ago and is the cause of the eye colour of all blue-eyed humans alive on the planet today.


Originally, we all had brown eyes”, said Professor Eiberg from the Department of Cellular and Molecular Biology. “But a genetic mutation affecting the OCA2 gene in our chromosomes resulted in the creation of a “switch”, which literally “turned off” the ability to produce brown eyes.

The OCA2 gene codes for the so-called P protein, which is involved in the production of melanin, the pigment that gives colour to our hair, eyes and skin. The “switch”, which is located in the gene adjacent to OCA2 does not, however, turn off the gene entirely, but rather limits its action to reducing the production of melanin in the iris – effectively “diluting” brown eyes to blue.


The switch’s effect on OCA2 is very specific. If the OCA2 gene had been completely destroyed or turned off, human beings would be without melanin in their hair, eyes or skin colour – a condition known as albinism.


Limited Genetic Variation

Variation in the colour of the eyes from brown to green can all be explained by the amount of melanin in the iris, but blue-eyed individuals only have a small degree of variation in the amount of melanin in their eyes. “From this we can conclude that all blue-eyed individuals are linked to the same ancestor,” says Professor Eiberg. “They have all inherited the same switch at exactly the same spot in their DNA.” Brown-eyed individuals, by contrast, have considerable individual variation in the area of their DNA that controls melanin production.

Professor Eiberg and his team examined mitochondrial DNA and compared the eye colour of blue-eyed individuals in countries as diverse as Jordan, Denmark and Turkey. His findings are the latest in a decade of genetic research, which began in 1996, when Professor Eiberg first implicated the OCA2 gene as being responsible for eye colour.


Nature shuffles our genes

The mutation of brown eyes to blue represents neither a positive nor a negative mutation. It is one of several mutations such as hair colour, baldness, freckles and beauty spots, which neither increases nor reduces a human’s chance of survival. As Professor Eiberg says, “it simply shows that nature is constantly shuffling the human genome, creating a genetic cocktail of human chromosomes and trying out different changes as it does so.”


Blue eyes don't have blue pigment

As mentioned above, blue eye color is determined by something called melanin. Melanin is a brown pigment that controls the color of our skin, eyes and hair.

The color of our eyes depends on how much melanin is present in the iris. There's only brown pigment in the eye — there is no hazel pigment or green pigment or blue pigment.


Brown eyes have the highest amount of melanin in the iris, and blue eyes have the least.


You can't predict the color of your child's eyes

At one time, it was believed that eye color — including blue eyes — was a simple genetic trait, and therefore you could predict a child's eye color if you knew the color of the parents' eyes and perhaps the color of the grandparents' eyes.


But geneticists now know that eye color is influenced by as many as 16 different genes to some degree — not just one or two genes as once thought. Also, the anatomic structure of the iris can affect eye color to some degree.


So it's impossible to know for sure if your children will have blue eyes. Even if you and your mate both have blue eyes, that's no guarantee your child's eyes will also be blue.

(Here's a royal example of the unpredictability of eye color: Princess Charlotte, the young daughter of blue-eyed Prince William and green-eyed Kate Middleton, has blue eyes. But her brother, Prince George, has very brown eyes.)


Blue eyes at birth doesn't mean blue eyes for life

The human eye does not have its full adult amount of pigment at birth. Because of this, many babies have blue eyes, but their eye color changes as the eye develops during early childhood and more melanin is produced in the iris.

So don't be concerned if your child begins to lose that "baby blue" eye color and her eyes become green or hazel or brown as she gets a little older.


Source: University of Copenhagen


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