London: Whether you are born blonde or brunette depends on a single letter of the genetic code, scientists have learned.

The DNA switch fine-tunes the activity of a gene known to be linked to hair colour. Changing just one of its chemical components - a single genetic code element - is enough to generate blonde hair.

Whether you are born blonde or brunette depends on a single letter of the genetic code, scientists have learned.

"This particular genetic variation in humans is associated with blonde hair, but it isn't associated with eye colour or other pigmentation traits," says lead researcher Professor David Kingsley, from Stanford University in the United States. "The specificity of the switch shows exactly how independent colour changes can be encoded to produce specific traits in humans."

Experiments with mice showed that the "blonde" switch caused animals to be born with light, golden-brown fur. The switch alters activity of a gene called Kit ligand that was already known to be associated with hair colour. Compared with its "brunette" version, which has a different DNA letter, it reduced Kit ligand activity by about 20 per cent.