SALT LAKE CITY--Although outlawed in the 1960s, the Hindu caste system constrained the marriage choices of Indians for 3000 years. This rigid social system has left a clear mark on the genes of modern Hindus, a group of Indian and American researchers reported here last week at the annual meeting of the American Association of Physical Anthropology. The genes confirm that women could marry up and ascend into higher castes, but men stayed in the castes in which they were born.
To examine the impact of social rules on the human genome, University of Utah, Salt Lake City, pediatric geneticist Michael Bamshad and research specialist Scott Watkins worked with anthropologists Bhaskara Rao and J. M. Naidu of Andhra University in Vishakhapatnam, India. The team collected blood samples from 300 unrelated men from all ranks of the Hindu caste hierarchy. The researchers compared DNA on the Y chromosome, which traces paternal ancestry, with DNA from a cellular organelle, the mitochondria, which is inherited only through the maternal line.
The mitochrondrial DNA revealed a slight blurring of caste lines for men in closely ranked castes. This means that the men's maternal ancestors had mixed the genes of closely related castes by changing caste by a rank or two. And historical and social records show that women moved up caste, not down. But the paternal genes showed that male ancestors rarely crossed caste lines: The collection of genetic markers on the Y chromosome remained distinct for each caste.
The study confirms a pattern found in cultures worldwide--that women can move up in social rank, because higher ranking males will marry lower ranking females, but that low-ranking males have the least choice in mates, because they have no access to high-ranking females, notes molecular anthropologist Mark Stoneking of Pennsylvania State University in University Park.
What's more, the work shows an Asian origin for people in most castes, but the DNA of upper castes resembles that of Caucasians. This fits historical records that say the caste system was imposed by Caucasians sweeping in from the northwest. Says Jorde: "It should make us optimistic about the power of genetic studies to reveal history."