Along with David Reich, a geneticist at Harvard Medical School, Dr. Thangaraj led an effort to analyze data from more than 2,800 individuals belonging to more than 260 distinct South Asian groups organized around caste, geography, family ties, language, religion and other factors. Of these, 81 groups had losses of genetic variation more extreme than those found in Ashkenazi Jews and Finns, groups with high rates of recessive disease because of genetic isolation.
In previous studies, Dr. Reich, Dr. Thangaraj and colleagues found that social groups in South Asia mixed between around 4,000 and 2,000 years ago. After that, the solidification of India’s caste system resulted in a shift toward endogamy. “You can see writ in the genome the effects of this intense endogamy,” Dr. Reich said.
Today, South Asia consists of around 5,000 anthropologically well-defined groups. Over 15 years, the researchers collected DNA from people belonging to a broad swath of these groups, resulting in a rich set of genetic data that pushes beyond the field’s focus on individuals of European ancestry, Dr. Reich said.
The scientists then looked at something called the founder effect. When a population originates from a small group of founders that bred only with each other, certain genetic variants can become amplified, more so than in a larger starting population with more gene exchange.
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…