Saturday, May 5, 2012

There's more than one way to make a blond

Image Credit: deanwissing.

Typically, if you want a look that combines dark skin with light hair there are two options. Depending on your starting point, you can either brighten your hair with chemicals like Beyoncé, or darken your skin with UV radiation à la New Jersey’s “tanning mom”. Yet on the South Pacific nation of the Solomon Islands, 5-10% of the population is just born that way. And now, a group of researchers believe they have traced the genetic cause of this unexpected blondness. And, well, big deal, because we’ve known for ages that hair color was genetically determined. Eye color too. It’s in our biology textbooks even. Nice going, science. But wait, it’s actually more interesting than you might think. It turns out that the Solomon Island blond results from a different, and simpler, genetic variation than the more familiar European brand of blond. This means that fair hair evolved separately at least two times in human history.

Prior to this recent study, which appeared in the latest issue of Science, the golden-haired inhabitants of the Pacific Islands had been the cause of much speculation. Perhaps their blondness resulted from some environment factor, such as diet or sun exposure. Or maybe fair hair was simply imported to the region by European visitors. To solve the mystery, scientists from several universities (including Stanford, located in blond-friendly California) scrutinized DNA samples from 43 blond and 42 dark haired Solomon Islanders. They found that the blonds did indeed have something different in their genes – a single nucleotide missense* mutation on an allele associated with pigmentation. Basically, there was a T (Thymine) where normally there would be C (Cytosine). Further genotyping of 918 Solomon Islanders and 941 individuals from elsewhere around the globe revealed that about 26% of the Solomon Islands population carried such an altered allele, but that it was essentially absent outside of the South Pacific, including European nations.

The findings suggest that South Pacific blondness is produced by a discrete recessive gene. It’s classic Mendelian genetics: individual carrying two mutated recessive alleles (TT) will be blond, whereas those with two standard issue alleles (CC) or a mixed set (CT) will be dark haired. European hair pigmentation, on the other hand, is determined by a bunch of different genes, yielding a variety of shades like platinum blond, golden blond and dirty blond. (Or “iced champagne”, “golden sunset” and the like, if you’re browsing the hair dye aisle.)

Globally, blond hair in adults is rare, and it tends to pair with fair skin. The Solomon Islands study indicates that human evolution has generated this hair pigmentation at least twice now, and seemingly under rather different environmental conditions. Whether the flaxen-haired phenotype confers any benefits on South Pacific individuals is unknown. It seems that light hair might help keep one’s head cool in hot, sunny regions. But then you also have to hear dumb blond jokes all day. Probably it just about evens out.

* DNA single nucleotide mutations (aka point mutations) come in a few flavors. Missense mutations result in a different amino acid being produced (think accidentally typing “tap” when you meant “cap”, it’s still a word), whereas nonsense mutations produce gibberish that shuts down the amino acid making process (more like “ctp” instead of “cap”, spell check does not approve). There’s also something called a silent mutation, which just results in the originally scheduled amino acid, but you don’t need to worry about those for today.

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