Jun 02, 2014 02:35 AM EDT By Staff Reporter, UniversityHerald Reporter

Stanford University researchers have solved the mystery behind the biological origin of blonde hair.

Researchers said that a single-letter switch in the genetic code is sufficient to generate blond hair in humans. The change from an "A" to a "G" in the four-letter DNA code is the factor that makes a person either blonde or brunette. In blondes, the switch reduces the activity of a gene called Kit ligand, which is associated with hair colour, by 20 percent as compared to brunettes.

Researchers also said that blonde hair is not related to any other characteristic or trait including eye color, skin color or intelligence - debunking the long-standing theory of blondes having lower aptitude.

"This particular genetic variation in humans is associated with blonde hair, but it isn't associated with eye color or other pigmentation traits," said David Kingsley, an HHMI investigator who led the study, in a press release. "The specificity of the switch shows exactly how independent color changes can be encoded to produce specific traits in humans."

Previous studies conducted on a fish, three-spined stickleback, discovered a gene that alters the organism's pigmentation. The scientists hence decided to see if a similar effect is found in other species like humans.

For the study, the researchers conducted experiments on mice as they have virtually same set of genes as humans. They found that the genetic switch caused the generation of light, golden-brown fur in rodents. For example, when the mice were given "A" at a point in DNA, they turned brown and when given "G", they turned light brown, Today Health reports.

 "This is a good example of how fine-tuned regulatory differences may be to produce different traits," Kingsley said. "The genetic mechanism that controls blond hair doesn't alter the biology of any other part of the body. It's a good example of a trait that's skin deep-and only skin deep."

The Kit ligand gene not only plays a vital role in the development of pigment-producing cells, but also influences the behaviour of blood stem cells, sperm or egg precursors, and gut neurons.

"We think the genome is littered with switches," said Kingsley. "And like the hair colour switch, many of the regulatory elements that control Kit ligand and other genes may subtly adjust activity."

The finding is published in the journal Nature Genetics.

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