Aug. 18, 1999 Can the rules of society, like those of nature, affect the genetic makeup of human populations? In this month's issue of Genome Research, Nitai Pada Bhattacharyya (Saha Institute of Nuclear Physics), Partha Majumder (Indian Statistical Institute) and colleagues take a look at this issue by asking whether social customs in India have restricted the flow of male genes between ethnic populations.
About 80% of the people in India belong to Hindu caste groups of varying social rank, and marriage between individuals of equivalent rank is customary. If, however, a man marries a woman of lower rank, he may retain his own caste and rank; the converse situation, a woman marrying a man of lower rank, is very rare. Bhattacharyya and colleagues examined the effect of this social system on male gene flow by comparing Y chromosome DNA from males of 10 different castes or tribes in the northern and eastern regions of India.
In particular, the researchers examined six Y chromosome markers, unique DNA sequences that can differ slightly between individuals; when any two males had identical sequence at all six markers, they were classified in the same "haplotype." Bhattacharyya and colleagues discovered that few haplotypes (15%) were shared between different castes or tribes. When they divided the castes according to rank (upper, middle, and lower), they also found little haplotype sharing between ranks. These results suggest limited flow of male genes between subpopulations in India, consistent with its historically prevalent marriage system.
Intriguingly, what little sharing the researchers did observe between populations displayed certain trends. Upper castes in fact occasionally shared haplotypes with lower castes, but only with those from a different geographical region. Bhattacharrya and colleagues speculate that this skewed sharing reflects a typical fate of high-disparity unions between upper caste men and lower caste women. In such unions, they propose, the couple may frequently leave their home, settle in a new region, and merge into a lower, rather than upper, caste. Such movements could provide what this study suggests is a rare conduit for male genes across social ranks in India.
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