Happily never after

Caste bar on marriages fuelling genetic disorders in India

By Jyotsna Singh
Published: Sunday 15 September 2013

Happily never afterIT IS well-known that endogamy, or marriages within the same gene pool, accentuate the risk of genetic disorders in a population. Despite this most Indian families forbid marriage outside of their caste citing “traditional values”.

It would come as a shock to such people that such caste-based discrimination is a fairly recent phenomenon and our ancestors freely married across communities. A study, based on a genetic analysis of Indian population, provides evidence that modern day India is the result of such population mixing among people of different social groups.

The study, by scientists from Centre for Cellular and Molecular Biology (CCMB) in Hyderabad and Harvard Medical School in Boston, US, shows a lot of admixture was happening between different communities in India 1,900- 4,200 years ago. It shows that our society was more open in earlier times than today and certain tribes, such as Palliyar (from Tamil Nadu) and Bhil (from Central India), that remain isolated now were also part of such intermingling.

The study was published online in American Journal of Human Genetics on August 8.

In 2009, the same team had published a paper in Nature based on an analysis of 25 different Indian population groups. They had demonstrated that in pre-historic India, there were only two ancestral populations: Ancestral North Indians (ANI), related to Central Asians, Middle Easterners, Caucasians and Europeans, and Ancestral South Indians (ASI), who were primarily from the subcontinent. The analysis showed that all contemporary populations in India are a genetic admixture of these two groups. However, at that time the researchers could not establish the precise date of admixture.

“We were sure of the admixture, but the dates could not be estimated. In the new study, we have analysed 73 different Indian population groups,” says lead author of the study, Kumarasamy Thangaraj, who is a senior principal scientist at CCMB. “Nearly 4,000 years ago, ANI and ASI were very distinct. We came to the conclusion that it was 1,900-4,200 years ago that the two must have mixed,” adds Thangaraj.

The paper shows that the first wave of mixing happened nearly 4000 years ago but ANI admixed again nearly 2000 years ago. This would have been possible only if marriages were happening across communities and castes. As this admixing reduced after that period, the scientists conclude that endogamy came to be practised more rigidly and samecaste marriages became a norm. “Only a few thousand years ago, the Indian population structure was vastly different from today,” says co-author David Reich, a professor of genetics at Harvard Medical School. “The caste system has been around for long, but not forever. Prior to about 4,000 years ago there was no mixture. After that, widespread mixture affected almost every group in India, even the most isolated tribal groups. And finally, endogamy set in and froze everything in place,” he adds.

The team studied the chromosomal data of 73 population groups. “We analysed chromosomes from the subject population till we reached original ANI and ANI structure. That indicated the dates of admixture,” says Thangaraj. The paper goes on to demonstrate that endogamy can lead to community specific diseases and hamper cure for them. “There is 25 per cent chance of genetic disorders being transferred if parents are from same gene pool,” says Ratna Dua Puri, senior consultant at the Center for Medical Gene tics of Sir Ganga Ram Hospital in New Delhi.

Thangaraj says, “An important conclusion of the study is that the high incidence of genetic and population-specific diseases, characteristic of present-day India, is likely to have increased only in the last few thousand years when groups in India started following strict endogamous marriage.”

He cites the example of the Vysya community of Andhra Pradesh to prove his point. The community is allergic to the drug used in anaesthesia. Genetic evidence shows this trait has been passed down from generations. “Genetic analysis of Vysyas shows they have experienced negligible gene flow from outside the community for an estimated 3,000 years. At some point mutation in their genes must have happened giving rise to this condition. If they mixed with other groups, this could have been cured,” says Thangaraj.

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