Friday, November 8, 2013
Asian Indians share a gene with Europeans that plays a significant role in coding for lighter skin
The gene, which is responsible for 27% of skin color variation in Asian Indians, was positively selected for in North, but not South Indian populations. When something is "selected for," that means it provides some advantage and so gets passed down to offspring, becoming more prevalent in a population over time. The Indian subcontinent has an enormous variation in skin color. "We have dark brown [tones], yellow tones and whitish-pinkish tones," said Chandana Basu Mallick, a biologist at the University of Tartu in Estonia. "We have quite a range and diversity in the biological spectrum of skin color." But because South Asian gene studies are relatively rare, it wasn't clear which genes contributed to this variation. Past research has found at least 126 genes that code for pigmentation in general, Basu Mallick said. To find out, Basu Mallick and her colleagues took skin color measurements for about 1,228 individuals in Southern India. The researchers then conducted a genetic analysis and found that about 27% of the skin color variation was due to a variation in a skin pigmentation gene. Called SLC24A5, this gene codes for lighter skin and is present in almost 100% of Europeans. The team also examined the gene in 95 people around the subcontinent and found that both South Asian and European populations inherited this particular variant from a common ancestor who lived sometime between 22,000 and 28,000 years ago. The team then looked for the gene in more than 2,000 people from 54 ethnic groups around the subcontinent. Some groups, such as populations in Tibet and Burma, didn't have the gene variant at all, whereas the Northwestern tip of the subcontinent had a nearly 90% prevalence of the gene. Lighter skin has less dark melanin, a pigment that blocks the sun's UV rays; the body uses these rays to make vitamin D. The SLC24A5 gene is linked to less dark melanin production, so the gene may have become more common in Europe because it allowed people's skin to make more vitamin D in the continent's low-light conditions. But in India, the prevalence of the gene in different populations didn't correlate with latitude, but instead seemed strongly linked to language, geography and demographic history. The study also showed that the gene was positively selected for in North, but not South India (though both light- and dark-skinned people live in both regions). It's not clear exactly what caused the gene to be favored in certain regions, but it probably wasn't the production of vitamin D alone.