All Humans With Blue Eyes Share A Common Ancestor

blue eyes What color are your eyes? We know that the color of your eyes depends on what genes you inherited from each of your parents. We know that blue eyes are a recessive genetic combination, and much less common in humans than brown eyes, the dominant phenotype. However, you might be unaware that a study has revealed that everyone who has blue eyes shares a common ancestor.

Blue eyes are a recessive trait. You may have studied Punnett Squares in your high school biology class. Punnett squares are a simple diagram that can be used to predict the probability that an offspring of two particular parents will inherit a specific gene combination. If both parents have blue eyes, there is a 100% probability that their children will all have blue eyes. This is because each of these parents only has the genes for blue eyes. It takes two of these recessive genes for blue eyes in order for the child to have blue eyes. It only takes one of the dominant genes for brown eyes for their offspring to have brown eyes (even if that offspring is carrying a recessive gene for blue eyes that is not expressed).

If both parents have brown eyes, things get a bit trickier. If both parents each have two genes for brown eyes, then there is a 100% possibility that their children will all have brown eyes. However, if each of the parents is carrying one dominant gene for brown eyes, and one recessive gene for blue eyes, it means that there is only a 25% probability that they will produce a child with blue eyes.

A study from a team at the University of Copenhagen reveals that there was a genetic mutation that occurred somewhere between 6,000 to 10,000 years ago. Before this mutation, all humans had brown eyes. This mutation is what caused the eye color in humans to change from brown to blue. This means that all people who have blue eyes have an ancestor in common: the ancestor who first developed this specific mutation.

The genetic mutation affected the OCA2 gene in the chromosomes of humans. The OCA2 gene holds the genetic instructions for something referred to as the P-protein. It is this P-protein that produces melanin. Melanin is what gives pigment, or color, to our skin, hair, and eyes. The mutation basically caused the P-protein in the gene next to the OCA2 gene to be turned off, but not all the way. In short, it causes the pigment in the eyes to be diluted. This is what causes the blue color in eyes.

This switch does not affect the amount if pigment in a blue-eyed person’s skin or hair, however. A person who has hazel or green eyes may also have the “switch” for the melanin in their eyes turned off, or lessened. This can happen in several places along that particular gene. The interesting thing about the study is that it found that all people who have blue eyes have this “switch” turned off in the exact same place on their chromosomes. This is not the case for people who have hazel or green eyes. So, if you happen to have blue eyes, take a look around you. All those other people with blue eyes are your long lost relatives.

Image by Ali T on Flickr