For thousands of years, people have both prized and mocked blond hair. Now, a new study shows that many can thank a tiny genetic mutationa single letter change from an A to a G among the 3 billion letters in the book of human DNAfor their golden locks.

The mutation "is the biological mechanism that helps create that [blond] color naturally," said David Kingsley, a professor of developmental biology at Stanford University and a Howard Hughes Medical Institute investigator, who led the research. "This is a great biological example of how traits can be controlled, and what a superficial difference blond hair color really is."

Kingsley, a brunet, said the study, published today in Nature Genetics, also offers a powerful insight into the workings of the human genome. The mutation doesn't alter the protein production of any of the 20,000 genes in the human genome, he said. Instead, in people of European ancestry, it causes blond hair through a 20 percent "turn of the thermostat dial" that regulates a signaling gene in the hair follicles of the skin.

Elsewhere in the body, that signaling gene is involved in the formation of blood, egg, sperm, and stem cells. Turning such a gene entirely on or off could be devastating. But a tiny mutation that tweaks the gene's activity in only one areain this case the skinallows for harmless changes, he said.

Pardis Sabeti, a computational biologist at Harvard University and Broad Institute who was not involved in the research, said the study is a "beautiful demonstration" of this kind of tweaking, which has previously been poorly understood. To find a single letter change and prove that it is a big driver of blond hair is a major scientific accomplishment, she said.

Blond hair, like this young girl's, is caused by a single DNA base pair change.

Photograph by Martin Schoeller, National Geographic Creative

A Subtle Change With Big Results

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Gene Study Shows Blond Hair Color Is Just Skin Deep

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