Editorial: Indo-African gene admixture

Published in The Hindu on July 28, 2011

India is a classic example of genetic potpourri. Though the Ancestral South Indians (ASIs) found in the four southern States are not related to any group outside India, populations in the north Indian States known as Ancestral North Indians (ANIs) exhibit a close genetic affinity to middle easterners, central Asians, and west Eurasians. The Nyshi and Ao Naga populations from north-east India show genetic affinity to the Chinese. The small Siddi population found in Karnataka, Gujarat and some parts of Andhra Pradesh shows an African affinity. Though the genetically admixtured tribal Siddi group exhibiting Indian and typical African features like dark skin, curly hair, and a broad nose has been around for a few hundred years, no comprehensive study was done to establish its African genetic kinship. This lacuna has been finally addressed by a study published in The American Journal of Human Genetics. The study is robust as the authors have analysed genetic markers from non-sex chromosomes (autosomes), and paternally derived Y-chromosomes and maternally derived mitochondrial DNA (mtDNA). An analysis of both Y-chromosomes and mtDNA, and autosomes in Siddis and the adjoining Indian tribal populations has helped in tracing the direction of gene flow between the African descendent and Indians. Sixty Siddis and 90 people from the adjoining tribal populations in Junagadh district of Gujarat, and 94 Siddis and 178 people from adjoining tribal populations in Uttara Kannada district of Karnataka were studied.

There is overwhelming evidence of gene flow from Indians to the Siddi population. Nearly 65 per cent of maternally derived mtDNA found in the Siddis represents Indian ancestry (with minor contribution from Europeans). Even in the case of the Y-chromosome, which is transmitted by fathers only to sons, nearly 30 per cent has come from Indians. The evidence is further strengthened by the non-sex chromosomes. Nearly 30 per cent of Indian gene signature is seen in the case of the autosomes, which are equally contributed by mother and father to the offspring. Incidentally, no genetic signature of African ancestry is found in the Indian populations adjoining the Siddis. All these together establish an unequivocal unidirectional gene flow from the local Indian groups to the Siddis. The study has for the first time traced the Siddi’s African ancestry to the Bantu-speaking population in the sub-Saharan region, and established that the gene admixture happened only over the past eight generations — about 200 years ago.