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October 5, 2009
Understanding Indian Ancestry
Researchers have analyzed genetic variation in people across India and discovered that most Indian people today descended from 2 ancestral populations. The findings set the stage for identifying genes that contribute to disease in people of Indian decent.
Most of our DNA, more than 99%, is the same as any unrelated person's. The small differences between people are called genetic variations. Genetic variations affect individual characteristics, such as eye color and blood group. They can also affect a person's risk for getting particular diseases.
Different populations can differ in the frequency of people at risk for particular diseases and conditions. But although India is the world's second most populous nation, Indian people have thus far been underrepresented in genome-wide surveys of human variation.
A group of scientists led by Dr. David Reich of Harvard and Dr. Lalji Singh of the Centre for Cellular and Molecular Biology in Hyderabad, India, undertook the largest survey ever done of genetic variation among people in India. Their effort was funded in part by NIH's National Human Genome Research Institute (NHGRI).
The team studied the DNA of 132 people from 25 diverse groups across India representing 13 states, all 6 language families, traditionally "upper" and "lower" castes and different tribal groups. The scientists analyzed over 560,000 single-letter DNA variations called single nucleotide polymorphisms (SNPs) across the genomes. Their results appeared in the September 24, 2009, issue of Nature.
The genetic differences between groups in India, the researchers found, are larger than those between European populations. Many groups in India seem to have been founded by a few individuals, with little subsequent genetic flow between those groups over thousands of years. Scientists call these group origins "founder events."
"The finding that a large proportion of modern Indians descend from founder events means that India is genetically not a single large population, but instead is best described as many smaller isolated populations," says Singh.
Most Indian populations, the researchers found, are primarily mixtures of 2 ancestral groups, which they call the Ancestral North Indians (ANI) and the Ancestral South Indians (ASI). Interestingly, a higher ANI proportion is associated with traditionally higher caste groups. ANI are genetically closer to Middle Easterners, Central Asians and Europeans. The proportion of ANI ancestry in most groups ranges from 39-71%. Indigenous Andaman Islanders have exclusively ASI ancestry, but such groups weren't found in mainland India.
These findings suggest that divisions in India by caste and tribe stem from ancient events, rather than being a more recent phenomenon. Medically, the results imply an increased risk of recessive genetic diseases in Indian populations.
“Further studies of these groups should lead to the rapid discovery of genes that cause devastating diseases, and will help in the clinical care of individuals and their families who are at risk,” Reich says.
—by Harrison Wein, Ph.D.