DNA trawl shows long history of India's castes

FOUNDER EFFECT

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)