A single-letter change in the genetic code is enough to generate blond hair in humans, a new study has found.
Researchers from the Stanford University School of Medicine and colleagues found that a molecule critical to stem cell function plays a major role in determining human hair colour.
"We've been trying to track down the genetic and molecular basis of naturally occurring traits - such as hair and skin pigmentation - in fish and humans to get insight into the general principles by which traits evolve," said David Kingsley, professor of developmental biology.
"Now we find that one of the most crucial signalling molecules in mammalian development also affects hair colour," said Kingsley, who is also a Howard Hughes Medical Institute investigator, and senior author of the study.
The researchers found that the blond hair commonly seen in Northern Europeans is caused by a single change in the DNA that regulates the expression of a gene that encodes a protein called KITLG, also known as stem cell factor.
This change affects how much KITLG is expressed in the hair follicles without changing how it's expressed in the rest of the body.
Introducing the change into normally brown-haired laboratory mice yields an animal with a decidedly lighter coat.
The study found that even small, tissue-specific changes in the expression of genes can have noticeable morphological effects.
It also emphasised how difficult it can be to clearly connect specific DNA changes with particular clinical or phenotypic outcomes. In this case, the change is subtle: A single nucleotide called an adenine is replaced by another called a guanine on human chromosome 12.
The change occurs over 350,000 nucleotides away from the KITLG gene and only alters the amount of gene expression about 20 per cent - a relatively tiny blip on a biological scale more often assessed in terms of gene expression being 100 per cent "on" or "off", researchers said.
The study was published in the journal Nature Genetics.
