WASHINGTON (Reuters) - A genetic search of India's diverse populations shows most people have mixtures of European and ancient south Indian genes, and helps illustrate the deep roots of the country's caste system, researchers reported on Wednesday.
It also shows that Indians, much like the Finns and European-origin Jews, may be susceptible to recessive genetic diseases, they report in the journal Nature.
Dr. David Reich of Harvard Medical School in Boston and colleagues found "strong evidence for two ancient populations, genetically divergent, that are ancestral to most Indians today".
"One, the 'Ancestral North Indians', is genetically close to Middle Easterners, Central Asians, and Europeans, whereas the other, the 'Ancestral South Indians', is as distinct from ancestral north Indians and East Asians as they are from each other," they wrote.
"Nobody's even close to having all of one or the other," Reich said in a telephone interview. "People in India are almost all a mixture of these two ancestral populations."
And virtually all also carry a risk of genetic mutations that can confer disease, Reich said.
He said his group and others should look in to that, to see if Indians have higher rates of recessive diseases, which can include deadly and incurable cystic fibrosis, sickle cell disease, and Tay-Sachs disease.
Reich's team, including a group at the Broad Institute at Harvard and the Massachusetts Institute of Technology, looked at genetic mutations in people from 25 different Indian groups -- a small sampling of the 6,000 or so estimated groups that restrict intermarriage.
What they found, time and again, was the so-called founder effect -- large numbers of people descended from what was originally a small group of ancestors. People in Finland and Ashkenazi Jews are other groups marked by the founder effect.
The limits on marrying outside the group can create the risk of recessive diseases -- conditions that only occur if people have two mutated genes. Marriage within groups raises this risk.
"Many Indian groups have a pattern of having been founded by a small number of individuals. They have been isolated from other groups since that time by restricted marriage across groups," Reich said.
Some people get tested for recessive genes before having children, and some people also use assisted fertility techniques to test embryos for recessive diseases.
Reich said no one had documented a higher number of recessive diseases among Indians, but it also was not something anyone had looked for. "It probably affects hundreds of millions of people in India today," he said.
While the genes clearly show that the caste system has existed for hundreds of generations, the genes do not line up by caste.
"It is impossible to distinguish castes from tribes using the data," Kumarasamy Thangaraj of the Center for Cellular and Molecular Biology in Hyderabad, India, who worked on the study, said in a statement.
"This supports the view that castes grew directly out of tribal-like organizations during the formation of Indian society."
Editing by Julie Steenhuysen and David Storey