PARIS, September 23, 2009 (AFP) - Nearly all Indians can claim descent from two ancestral groups, a study released on Wednesday said, adding that millennia of inter-marriage may have left the country's population more at risk to some inherited diseases.

US and Indian scientists took blood samples from 132 individuals from 25 diverse groups in India, representing 13 states, all six language families as well as tribal groups and "upper" and "lower" castes.

They then unravelled the volunteers' genetic code, sifting through key areas of DNA for similarities and differences.

Two ancestral populations emerge, and their genes dominate the Indian genome today, the researchers said.

"Different Indian groups have inherited 40 to 80 percent of their ancestry from a population that we call the Ancestral North Indians, who are related to western Eurasians, and the rest from the Ancestral South Indians, who are not related to any group outside India," said Harvard Medical School geneticist David Reich.

The north-and-south findings chime with a scenario whereby a small number of venturers, the so-called Austro-Asiatic people, first moved into the sub-continent about 60,000 years ago.

Around 5,000 years ago, the arrival of a Dravidian-speaking tribe caused the community to disperse. Its members went on to form enclaves of small, tightly-knit groups.

The Dravidians themselves were then driven to the south when Indo-European tribes arrived around 500 years years later.

These early events helped form a patchwork of groups that is visible today, according to this theory.

Adding DNA spice have been India's successive waves of conquerors, from the Persians, the Macedonians, the Portuguese, the Mughals and the British.

Trade and regional contacts have added to the mix.

The Siddi people in southwestern India have a signature of African genes, consistent with their origin, which involved the Arab slave trade. The Nyshi and Ao

Naga groups in the far northeast, cluster with Chinese genotypes, which correlates with their use of Tibeto-Burman languages.

The investigators, whose work is published by the British journal Nature, point to a range of intriguing discoveries.

One is the strong genetic similarities within groups that can be traced to a "founder event" -- the establishment of a community by a tiny number of people who migrated after the ancestral population was dispersed.

Six Indo-European- and Dravidian-speaking groups have evidence of "founder events" that happened 30 generations ago, while the Vysya group had a "founder event" that occurred at least 100 generations in the past.

"Founder events" mean a community starts with a relatively small gene pool whose confines may then be maintained or reinforced by geographical isolation or by endogamy, the term for marriage within a tribe or caste.

The paper says endogamy's role in shaping India's DNA has deeper roots than many might think.

"Some historians have argued that 'caste' in modern India is an 'invention' of colonialism in the sense that it became more rigid under colonial rule," it says.

"However, our results indicate that many current distinctions among groups are ancient, and that strong endogamy must have shaped marriage patterns in India for thousands of years."

The downside of "founder events" and enduring endogamy is that genetic flaws which boost the chance of inherited disease get transmitted through the generations, rather than get erased by mixing genes with other communities.

Screening and mapping India's diversity could have benefits in health, enabling doctors to help people at risk from their genetic inheritance, says the study.

It gives the example of a tiny deletion on a gene called MYBPC3, which increases the risk of heart failure by about sevenfold. Around four percent of the Indian population have this genetic variant, yet it is nearly absent elsewhere.

Little is known about the genomics of India. The country has the world's second-largest population but has been under-represented in surveys of the human genome.