From the NYT:
In South Asian Social Castes, a Living Lab for Genetic Disease
By STEPH YIN JULY 17, 2017
In certain states in southern India, anesthesiologists know to ask anyone undergoing surgery whether they belong to the Vysya, a regional group traditionally associated with traders and businesspeople.
Anecdotally, medical workers know that some people with Vysya ancestry — who live primarily in Andhra Pradesh and Telangana — have had fatal responses to common muscle relaxants, so doctors will use a different combination of drugs.
The Vysya may have other medical predispositions that have yet to be characterized — as may hundreds of other subpopulations across South Asia, according to a study published in Nature Genetics on Monday. The researchers suspect that many such medical conditions are related to how these groups have stayed genetically separate while living side by side for thousands of years.
South Asians should be viewed not as a single population but as thousands of distinct groups reinforced by cultural practices that promote marrying within one’s community….
Marriage within a limited group, or endogamy, has created millions of people who are susceptible to recessive diseases, which develop only when a child inherits a disease-carrying gene from both parents, said Kumarasamy Thangaraj, an author of the study and a senior scientist at the Center for Cellular and Molecular Biology in Hyderabad.
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.
A complicated place …
