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The first in-depth genetic scan of the British Isles shows their violent history of invasion after invasion lives on in the people.

The blood of Norwegian Vikings still flows in the residents of the Orkney Islands. The Welsh-speaking folk of northern Wales are distinct from their English-dominant neighbors. And, surprisingly, there’s no one group of Celts. The ancient Cornish are genetically distinct from the Scots and the original Celtic inhabitants of Northern Ireland.

The hope is to shed light on the genetics of disease, but it’s also filling in a lot of gaps for archeologists and historians.

“It is really the first time that scientists have looked in great detail within a country at the genetic level,” said Peter Donnelly of the Wellcome Trust Centre for Human Genetics in Oxford.

“What was striking was the pattern of variation that we saw and the way it aligned with geography,” added Donnelly, who led the study published in the journal Nature.

The team got samples of DNA from more than 2,000 people living in rural parts of England, Wales, Scotland and Northern Ireland whose four grandparents had stayed put in their home towns or villages.

“What was striking was the pattern of variation that we saw and the way it aligned with geography."

“Because we inherit DNA from our parents…the DNA of any person is roughly one-fourth, one-fourth, one-quarter, one-fourth from your grandparents,” Donnelly told reporters. “In effect, it looks at genetic variation in the UK at the end of the 19th century.”

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They wanted to look back because there’s been so much population movement in the past 100 years, with much more mixing than in the previous millennia.

For a comparison, the researchers used DNA samples from 6,000 people in continental Europe who had taken part in a medical study.

Map of the UK showing clustering of individuals based on genetics, and its striking relationship with geography. Each of the genetic clusters is represented by a different symbol (combining shape and colour, with legend at the sides). There is one symbol plotted on the map for each of the individuals in the study. The ellipses give a sense of the geographical range of each genetic cluster. Stephen Leslie | EuroGeographics

The findings were startling, even to Britons who know how strong regional loyalties can be on the small cluster of islands.

“The genetic groups we find in Cornwall and Devon almost exactly match the modern county boundaries,” Donnelly said.

“Even as statisticians, we were struck by the concordance with history,” said Stephen Leslie of Royal Children’s Hospital in Victoria, Australia, who worked on the study.

One thing historians had wondered about. Had the Anglo-Saxon invaders who moved in after the collapse of the Roman Empire perhaps committed genocide? Historical, linguistic and archeological records all show a mass shift of culture.

But the DNA shows something different.

“Even as statisticians, we were struck by the concordance with history."

Anglo-Saxon DNA makes up less than half of the genetic mix in southeast England. This suggests a more subtle takeover, in which the invaders intermarried with and perhaps imposed their culture on the locals.

As historians would predict, eastern, central and southern England is made up of a single, relatively homogeneous, genetic group. This reflects the influence of the Romans, who built roads in these regions, broke down borders and whose economy encouraged trade.

Older groups were pushed to the edges — to cold Scotland and the craggy coasts of Cornwall, Devon and Wales.

The most ancient group? The Welsh. Their DNA predates even a mysterious genetic infusion from Europe long before the Romans arrived.

“By using genetics and powerful statistical methods, we have been able to tell the story of the masses,” Leslie said.