Blue-eyed individuals linked to a single, common ancestor

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Washington, Jan 31 : A new research at the University of Copenhagen has shown that people with blue eyes have a single, common ancestor.

The researchers arrived at the conclusion by tracking down a genetic mutation that took place 6000 to 10,000 years ago, which is the cause of the eye colour of all blue-eyed humans alive on the planet today.

For the research, Professor Eiberg and his team from the University of Copenhagen examined mitochondrial DNA and compared the eye colour of blue-eyed individuals in countries as diverse as Jordan, Denmark and Turkey.

"Originally, we all had brown eyes", said Professor Eiberg from the Department of Cellular and Molecular Medicine. "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," he added.

The OCA2 gene codes are 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.

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," said Professor Eiberg. "They have all inherited the same switch at exactly the same spot in their DNA," he added.

Brown-eyed individuals, by contrast, have considerable individual variation in the area of their DNA that controls melanin production.

According to the research, 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.

"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," said Professor Eiberg.

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