Scientists have been puzzling over the genetics of hair color for a long time, and now a team from Stanford University has figured out which variants lead to blond hair. The new study published in Nature Genetics finds that a switch of a single letter of the genetic code is responsible for lighter hair: An A (adenine) is changed to a G (guanine) on a region of human chromosome 12. The team says that because this specific genetic change only affects the hair follicle, other cell types — especially, say, brain cells — are not affected. This means that blond hair really is “only skin deep.”
“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 study author David Kingsley. “Now we find that one of the most crucial signaling molecules in mammalian development also affects hair color.”
The team first had a clue that a particular region of DNA might be responsible for hair color from earlier genetic studies of people in Iceland and the Netherlands. In the new study, the team pinpointed the exact genetic change and then used mice to test the theory: And those with the single nucleotide change did indeed have lighter coats.
What’s interesting is that the gene in which the change takes place encodes a protein, KITLG, which is also known as stem cell factor and is responsible for a host of different functions depending on where it’s expressed. When the single “switch” responsible for hair color occurs, it only affects hair follicles – not how the gene is expressed elsewhere in the body. Understanding how one tiny change in this remote “junk” region of DNA can affect various traits may give researchers a clue about the roles of these regions in disease states.
The results also underline that the genetic change behind blond hair cannot be linked to other attributes, like skin color or eye color. And most importantly for blondes who have been the punchlines of jokes over the years, it’s certainly not linked to intelligence.
“What we’re seeing is that this regulatory region exercises exquisite control over where, and how much, KITLG expression occurs,” said Kingsley. “In this case, it controls hair color. In another situation — perhaps under the influence of a different regulatory region — it probably controls stem cell division. Dialing up and down the expression of an essential growth factor in this manner could be a common mechanism that underlies many different traits.”
So if you’re having a “blond moment,” don’t let your friends to tease you about your hair color – you may just be having a bad day.
“It’s clear that this hair color change is occurring through a regulatory mechanism that operates only in the hair,” said Kingsley. “This isn’t something that also affects other traits, like intelligence or personality. The change that causes blond hair is, literally, only skin deep.”