You don't see much of it outside during these cold winter months, but December seems a hot news month for skin. Surgeons recently completed the first successful face transplant, hundreds of biking New Yorkers who forgot to wear their Masques are probably getting frostbite this week, and scientists seem to be getting closer to solving the mystery of what causes variation in skin color.

Although skin color is perhaps one of the most visible characteristics of the human body--whether created so by social construct or not--it's been quite a tricky code for biologists to crack. It's well understood that melanocyte cells near the skin surface control skin tone level by variation in their content of the pigment melanin, what is not well understood is how this mechanism produces the differences between the skin color of a person with European versus one with African ancestry.

It's at this point in the game where scientists usually take the problem to a model organism, an animal that is well suited to the rigors of experimentation. Flies and Rats are the most popular, but in this case, the exotic sounding but diminutive zebrafish is the perfect candidate for skin pigment research. In reality, researchers at Penn State uncovered this fact by accident. While conducting cancer work using zebrafish, they became particular interested in those fish with mutations in the golden gene, a variation that produces fish with lighter stripes.

The team took the genetic sequence for golden and compared it against sequences in the human genome to see if they hit anything interesting. What they uncovered was the mouthful SLC24A5, far less romantically named than golden, but a gene with 69% similarity to the zebrafish sequence. SLC24A5 is involved in ion exchange across membranes, a key component of maintaining color in melanocytes. Such a high homology is often a strong indicator of similar function, but the researchers sealed the deal by injecting RNA of the human gene into zebrafish with light stripes. Sure enough, the fish's stripes darkened.

Moving the focus back to humans, the investigators examined a swath of SLC24A5 sequences across many racial subtypes to pinpoint common variations in the sequence. The search brought back two main variations, or alleles of the gene, each differing by only one amino acid within the protein sequence. After further analysis, 98% of Europeans were show to have one allele while 93% of Africans and Asians have the other.

If your eye caught the word "Asians" then you're on to something. Humans have two copies of each chromosome, so those two different alleles could produce a grand total of 4 different combinations of skin tone! Holy cow! So it seems this certainly isn't the only gene controlling skin color. Add that to the fact that most Asians clearly don't have as dark skin as most Africans, and its looking like there is more to this skin color story than just our man SLC.

Even so, the researchers elaborated on the gene finding by examining a racially mixed population. They compiled a sample cohort of 203 African Americans and 105 African Caribbeans, two groups with the most mixture of European and African ancestry, and correlated their skin tones to their genetics. Those with the lightest skin always had two copies of the "light" allele while the darkest always had two of the "dark" one. The gene is clearly a major player, but as most geneticists are staring to realize, its never as simple as one gene for one trait.

This new finding also supports the theory that as humans moved further north, lighter skin was selected for in order to allow the body to produce sufficient Vitamin D in climates with much less sun. The fact that there is only one amino acid difference between the "light" and "dark" versions of SLC is an elegant demonstration of natural selection at work. All that was necessary was a small mutation in this gene, altering this one amino acid and producing a change that was advantageous for these northern dwelling humans, and was thus selected for. You smell me Dover, PA? (We'll tackle the eye another week.) At this point in our world history, the concept of race is clearly more than skin deep, but it's interesting to ruminate on the fact that a portion of this skin variation is based off the difference in a single amino acid. Is there another single amino acid skin variation that makes some people able to dive into freezing ponds and oceans on the first of the year?

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