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As we celebrate black history month, we're reminded that skin color is only a small part of what defines race. But, as this ScienCentral News video reports, scientists have identified a gene that plays a big part in defining skin color.
Not Just Skin Deep
All together the lights and darks of people's skin make a gallery of color. Inside our cells, there are hundreds of genes at work mixing the pigments that shade our faces.
Geneticist Keith Cheng discovered that among these genes there is a master artist at work. It's a gene we all share and whose signature largely determines how light or dark our skin is.
"The variation in this particular skin color gene seems to account for more of the change in skin color than any other gene known to date," says Cheng, from Pennsylvania State University's Hershey College of Medicine. He adds that, "a variation in this gene seems to largely account for the lighter skin color in Europeans, and perhaps their hair and eye color variation as well."
When he compared traditional, dark zebra fish to one of it's blood orange-colored cousins, he saw that a genetic mutation in the gene changed their color completely. He found that the mutation caused the color change by cutting short a protein critical to the production of the pigment melanin.
Zebra fish
Skin color is due primarily to the presence of melanin, which also confers protection against the sun's damaging ultraviolet rays. Both light and dark complexioned people have this pigment, however, two forms are produced — pheomelanin, which is red to yellow in color, and eumelanin, which is dark brown to black. People with light complexioned skin mostly produce pheomelanin, while those with dark colored skin mostly produce eumelanin.
In addition, individuals differ in the number and size of melanin particles. The latter two variables are more important in determining skin color than the percentages of the different kinds of melanin. In lighter skin, color is also affected by red cells in blood flowing close to the skin. To a lesser extent, the color is also affected by the presence of fat under the skin and carotene, a reddish-orange pigment in the skin.
Using an electron microscope, Cheng compared normal zebra fish cells to those of its mutant cousin. He noticed that the little balls of pigment, which he says look like "canon balls" were smaller, lighter in color and fewer in number, than in the mutant fish.
"And what's remarkable about that is that those are the exactly the three changes you see when you compare lighter skinned humans with darker skinned humans," he explains
The gene mutation produces melanin particles that are smaller, fewer and lighter in color.
When Cheng and his colleagues compared the human and zebra fish genomes they found a lot in common. "We had found the great similarity in the sequence between the human and the zebra fish gene. They're both about 500 amino acids and they're identical in about 70 percent of their amino acids, their subunits, they're little bricks," says Cheng.
Cheng and his colleagues used online databases, including the global human gene database, so-called HAPMAP, to identify different variations of the gene in the human genome. They were surprised to discover that in the coding of human DNA there are only two versions of the gene: an "A" version that included the amino acid alanine, and a "T" version with the amino acid, threonine. Switching the "A" for the "T" in the amino acid coding of the SLC24A5 gene made the difference between dark and light skin. In a survey of 308 people, those who carried mostly the "A" version of the gene were darker than those who carried the "T" version — mostly people of European decent.
Cheng says his work suggests that this mutation has arisen due to selective pressure for lighter skin in some areas of the globe. "What other people have shown for sure is that we need to get vitamin D and that has to be generated by ultraviolet light from the sun," he explains. "And so if you live near the equator and there's a lot of sun and if you have dark skin, you can still get enough vitamin D. But if you live up north and you're darker skinned, you're not able to make enough vitamin D to prevent you from getting rickets, which can be fatal."
He points out that this gene does not explain all differences in skin color. "There are large number of genes that control skin color, the most significant of which is yet to be found, is that which makes East Asians, for example, Chinese and Japanese, to be lighter skinned," he says. "That gene, or genes, needs still to be found, but we know from our data that the gene that we found is not responsible for that variation." Nevertheless, Cheng points out that skin color alone cannot define race and that there are many other non-genetic factors, such as language, nationality and customs. "Race is a complicated term that includes both genetic and non-genetic components," he says.
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