Mixed genes

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When individuals from different groups interbreed, their offspring's DNA becomes a mixture of the DNA from each admixing group. Pieces of this DNA are then passed along through subsequent generations, carrying on all the way to the present day. Researchers from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, Oxford University and University College London (UCL) have now produced a global map detailing the genetic histories of 95 different populations across the world, spanning the last four millennia.

The interactive world map, that is accessible via the internet, details the histories of genetic mixing between each of the 95 populations across Europe, Africa, Asia and South America. It shows likely genetic impacts of historical events including European colonialism, the Mongol Empire, the Arab slave trade and European traders near the Silk Road mixing with people in China.

The study, published this week in Science, is the first to simultaneously identify, date and characterise genetic mixing between populations. To do this, the researchers developed sophisticated statistical methods to analyse the DNA of 1490 individuals in 95 populations around the world. "DNA really has the power to tell stories and uncover details of humanity's past", said Simon Myers of Oxford University's Department of Statistics and Wellcome Trust Centre for Human Genetics, co-senior author of the study. "Because our approach uses only genetic data, it provides information independent from other sources. Many of our genetic observations match historical events, and we also see evidence of previously unrecorded genetic mixing. For example, the DNA of the Tu people in modern China suggests that in around 1200CE, Europeans similar to modern Greeks mixed with an otherwise Chinese-like population. Plausibly, the source of this European-like DNA might be merchants travelling the nearby Silk Road."

The powerful technique, christened 'Globetrotter', provides insight into past events such as the genetic legacy of the Mongol Empire. Historical records suggest that the Hazara people of Pakistan are partially descended from Mongol warriors, and this study found clear evidence of Mongol DNA entering the population during the period of the Mongol Empire. Six other populations, from as far west as Turkey, showed similar evidence of genetic mixing with Mongols around the same time.

"What amazes me most is simply how well our technique works", said Garrett Hellenthal of the UCL Genetics Institute, lead author of the study. "Although individual mutations carry only weak signals about where a person is from, by adding information across the whole genome we can reconstruct these mixing events. Sometimes individuals sampled from nearby regions can have surprisingly different sources of mixing. For example, we identify distinct events happening at different times among groups other than the Hazara sampled within Pakistan, with some inheriting DNA from sub-Saharan Africa, perhaps related to the Arab Slave Trade, another from East Asia, and yet another from ancient Europe. Nearly all our populations show mixing events, so they are very common throughout recent history and often involve people migrating over large distances."

The team used genome data for all 1490 individuals to identify 'chunks' of DNA that were shared between individuals from different populations. Populations sharing more ancestry share more chunks, and individual chunks give clues about the underlying ancestry along chromosomes.

"Each population has a particular genetic 'palette'", said Daniel Falush of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, co-senior author of the study. "If you were to paint the genomes of people in modern-day Maya, for example, you would use a mixed palette with colours from Spanish-like, West African and Native American DNA. This mix dates back to around 1670AD, consistent with historical accounts describing Spanish and West African people entering the Americas around that time. Though we can't directly sample DNA from the groups that mixed in the past, we can capture much of the DNA of these original groups as persisting, within a mixed palette of modern-day groups."

As well as providing fresh insights into historical events, the new research might have implications for how DNA impacts health and disease in different populations.

INFORMATION:

Original publication: Garrett Hellenthal, George B.J. Busby, Gavin Band, James F. Wilson, Cristian Capelli, Daniel Falush, Simon Myers
A Genetic Atlas of Human Admixture History
Science, 14 February 2014