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," says lead researcher Professor 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.
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.
"This is a good example of how fine-tuned regulatory differences may be to produce different traits," said Dr Kingsley, whose research appears in the journal Nature Genetics.
"The genetic mechanism that controls blonde 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 roles around the body. It influences the behaviour of blood stem cells, sperm or egg precursors, and gut neurons.
"We think the genome is littered with switches," said Dr Kingsley. "And like the hair colour switch, many of the regulatory elements that control Kit ligand and other genes may subtly adjust activity."
The switch was found by snipping out pieces of human DNA from the region of the genetic code around the Kit ligand gene.
Introduced into mice, one tiny piece of DNA was found to turn on gene activity in developing hair follicles.
"When we found the hair follicle switch, we could then ask what's different between blondes and brunettes in northern Europe," said Prof Kingsley.
Detailed examination of the DNA revealed a single letter of genetic code, or "single nucleotide polymorphism" (SNP), that separated people with different hair colour.
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