Most people carry some disease-associated mutations that have no effect because they’re present only in one parent’s genes. In an endogamous group, however, it’s more likely that two individuals carry the same mutation from a common founder. If they reproduce, their offspring have a higher risk of inheriting that disease.

Rare conditions are therefore disproportionately common in populations with strong founder events. Among Finns, for instance, congenital nephrotic syndrome, a relatively rare kidney disease, is uniquely prevalent. Similarly, Ashkenazi Jews are often screened for diseases like cystic fibrosis or Gaucher disease.

To measure the strength of different founder events, Dr. Reich and Dr. Thangaraj’s team looked for long stretches of DNA shared between individuals from the same subgroups. More shared sequences indicated a stronger founder event.

The strongest of these founder groups most likely started with major genetic contributions from just 100 people or fewer. Today, 14 groups with these genetic profiles in South Asia have estimated census sizes of over one million. These include the Gujjar, from Jammu and Kashmir; the Baniyas, from Uttar Pradesh; and the Pattapu Kapu, from Andhra Pradesh. All of these groups have estimated founder effects about 10 times as strong as those of Finns and Ashkenazi Jews, which suggests the South Asian groups have “just as many, or more, recessive diseases,” said Dr. Reich, who is of Ashkenazi Jewish heritage himself.

The next step, the authors say, is to map out and study the genetic origins of diseases prevalent in different groups. As proof of concept, they screened 12 patients from southern India for a gene mutation known to cause a joint disease called progressive pseudorheumatoid dysplasia. Of the six people that had the mutation, five instances could be traced to founder effects, and one case could be traced to a marriage between close relatives.

This distinction is important because it’s well documented that marriage between close relatives can increase the possibilities of recessive disease. But many South Asians are not yet aware that they should also look out for genetic risks among broader populations, said Svati Shah, an associate professor of medicine at Duke University who was not involved in the research.

“There’s a tendency to think, ‘This will never happen to me because I will never marry my first cousin,’” Dr. Shah said. “But that’s not what’s happening here, according to the data.”