Study throws light on genetic diversity of Indian population

A paper published online today (September 24) in the journal Nature, shows that all diverse groups seen spread out in India today come from two major ancient populations that are genetically divergent.

The two ancient populations are the Ancestral North Indians (ANI) and the Ancestral South Indians (ASI). The study was based on genetic analysis.

While the Ancestral North Indians (ANI) group is genetically close to Middle Easterners, Central Asians, and Europeans, the Ancestral South Indians (ASI) are not related to any group outside India, notes the paper.

The study was undertaken by the Hyderabad-based Centre for Cellular and Molecular Biology (CCMB) and three other institutions in the U.S.

Samples from 132 people representing 25 groups from 15 States and speaking six language families (including two language families from the Andaman Islands) were studied.

The study also looked for genetic variations based on caste — upper and lower caste — from two the States of Uttar Pradesh and Andhra Pradesh.

The study found the groups (seen today) that emerged from the two ancient populations have distinct genetic affinity. “The populations (groups) that emerged from ANI show 40-80 per cent genetic affinity to European population.

“But the populations that emerged from ASI don’t show any affinity to any population outside India,” said Dr. Kumarasamy Thangaraj, Senior Scientist at CCMB, and one of the authors of the paper. “The Hyshi and Ao Nage population from north-east India show genetic affinity to the Chinese.”

The indigenous population seen in the Andamans have more affinity to the ASI. “Otherwise, they have no relationship with any other population anywhere in the world,” said Dr. Thangaraj.

Unlike the European and Chinese population, the Indian population are more scattered in a genetic sense.

Medical implications

While consanguinity is often implicated for many recessive gene disorders in a population, this study found that the “shared descent from a common ancestral population plays a bigger role. This is called as the ‘founder effect.’

Founder effect is nothing but the fact that many groups seen today have descended from a small group of founding individuals, and these founding individuals in turn have been isolated from other groups, genetically speaking.

It is based on this fact that the authors state that the founder effect plays a bigger role. “We propose that the founder effect is responsible for an even higher burden of recessive diseases in India than consanguinity,” the paper states.

Dr. Thangaraj explained the significance of this. A disease can occur due to the presence of a recessive gene due to mutation. In a small population with high endogamy [where people marry within the population], the mutation persists and spreads to more number of people.

After a point of time, a large number of people have the recessive gene and this increases the chance of a child receiving a recessive gene from both the parents and thus becoming diseased.

Existence of caste

Contrary to popular perception by historians that the caste system seen today is an invention of colonialism, the study found scientific evidence to show that “many current distinctions among groups are ancient.”

“The caste system is not recent,” said Dr. Thangaraj. “The social stratification existed right from early human divergence, some 50,000-60,000 years ago when initial settlement happened in India.”

The paper adds a word of caution: “Models in population genetics should be treated with caution. Although they provide an important framework for testing historical hypotheses, they are oversimplifications.”

Published in The Hindu on September 24, 2009