Be blonde or brunette at the flick of a DNA switch

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.

“This particular genetic variation in humans is associated with blonde hair, but it isn’t associated with eye colour or other pigmentation traits,” said the lead researcher, David Kingsley, from Stanford University in the US.

“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.

Compared with its ‘brunette’ version, which has a different DNA letter, it reduced Kit ligand activity by about 20%. “This is a good example of how fine-tuned regulatory differences may be to produce different traits,” said Prof Kingsley, in the journal Nature Genetics.

“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.”

Many other regulatory elements may be scattered throughout the DNA that surrounds the Kit ligand gene, he added.

The gene not only aids the development of pigment-producing cells, but has a host of other it influences the behaviour of blood stem cells, sperm or egg precursors, and gut neurons.