How Reese Witherspoon Has Blue Eyes

 
 
Geneticists now believe blue eyes are a relatively recent physical trait. While the earth was once filled entirely with brown-eyed people, a genetic mutation occurring within the last 10,000 years resulted in a single, blue-eyed ancestor. Every blue-eyed person since, including femme fatale Reese Witherspoon, is believed to be genetically linked to that common ancestor.

For those unfamiliar with genetics, your physical appearance is a reflection of your genes, which are inherited from your biological parents. Small genomic differences can disproportionately translate to large physiological differences. Gene expression and regulation go a long way toward explaining why we drastically differ in appearance from apes despite sharing around 98%+ of their DNA makeup. Changes in a genomic sequence, known as mutations, can lead to new physical traits.

The mutation resulting in blue-eyed humans occurred in a gene dubbed OCA2, which regulates the level of melanin in your iris. This alteration reduces melanin levels substantially, shifting iris pigmentation from brown to blue. By sampling DNA from 800 living, blue-eyed individuals and tracing maternal lineages back to older and older DNA data, geneticists were able to conclude that all blue-eyed humans descend from a single, maternal ancestor who lived somewhere between 6 and 10,000 years ago. A larger population of ancient DNA samples would shrink the estimate’s range substantially, but DNA is unfortunately hard to come by from that long ago due to organic decomposition.

Are men or women more likely to have blue eyes?

The answer to this is mostly indeterminable because first, iris color is an autosomal physical trait. It is not sex-linked to an X or Y chromosome like color blindness or male pattern baldness, which are primarily X-linked traits. Thus, both sexes have roughly the same probability of inheritance. Secondly, iris color is regulated by more than just the OCA2 gene, and we currently do not know how many or which genes contribute to iris color. This is also the case for many other inherited traits, such as male pattern baldness listed above. Few traits are exclusively sex-linked; often, other genes are involved but are simply less contributive. It’s possible to estimate probabilities of blue-eyed children given a specific set of genes, but as gene variation grows, the accuracy of these estimates diminish.

What’s surprising to geneticists is the abundance of this mutation in present day. An estimated 20-40% of modern Europeans have blue eyes, a surprisingly high figure for a relatively brief period of time.

My take? Blue-eyed people get laid more easily! Unfair? It’s science.

External Link: Genetic mutation makes those brown eyes blue