A One-Letter Blond

If you're tempted to make a dumb blond joke, don't, because they're obsolete! Turns out, blond hair is determined by a single letter in the three billion letters of the human genetic code, and affects no other trait - not intelligence nor the capacity for fun.

This new discovery came from studies on Sticklebacks, a small fish in various habitats around the world, each group having evolved its own distinct skin color. A gene called the Kit ligand is responsible for the fish's skin pigment which in an earlier study, scientists found also affects skin color in humans, and possibly hair color too.

However, Kit ligand plays other important roles in humans, affecting blood cells and sperm cells. It's actually a mutation in an area affecting the Kit ligand that causes blond hair in Northern Europeans. So how did evolutionary changes occur without affecting Kit ligand's other important functions?

Researchers in the new study decided to isolate this regulatory element and determine how it was different in blonds. Using engineered mice, they were able to identify the single letter change from A to G that creates blonds, while affecting nothing else.

This isn't turning a gene on or off, but rather it shows how a gene can be used slightly differently in just one part of the body. Subtle changes like this one reveal how exquisitely controlled the utilization of genes is regulated and how changes to this system contributes to human diversity.