Rare DNA previously found only in people from West Africa has turned up in white males from northern England, a new study reports.
The surprising discovery was made during a survey of genetic diversity in the United Kingdom based on the male Y chromosome.
This sex-determining chromosome is copied from father to son, providing a record of male ancestry.
The uncommon DNA, a chromosome called hgA1, had previously been detected only in a region of West Africa that includes Mali, Senegal, and Guinea-Bissau, the team says (Africa map).
"It's a really special chromosome, one that's only been reported before in a handful of men in Africa," said Mark Jobling, a genetics professor from the University of Leicester who led the research team.
The hgA1 chromosome lies near the root of the family tree of Y chromosomes in Africa, Jobling added.
"It's an ancient type that's African specific."
But the team found hgA1 in one white British male who took part in the survey, despite the man having no known African family connection.
According to the research, published online this week in the European Journal of Human Genetics, the unusual DNA has been present in Britain for at least 250 years.
Distinctive Surname
After making the surprising find, Jobling's team tested other British men who shared the same east Yorkshire surname as the original man found with the African chromosome.
(The researchers haven't revealed the surname, which is derived from a Yorkshire place name, to preserve the anonymity of the study participants.)
Seven out of 18 of those tested also had the rare chromosome, even though the men weren't known to be related.
Genealogical research and further genetic testing were used to date the arrival of the African DNA in northern England.
Records such as birth and marriage certificates traced the men's surname to two individuals who were born in Yorkshire in the 1780s.
This closely matched the date reached from analyzing mutations in the studied Y chromosome (get an overview of human genetics.)
Such mutations build up through generations at a predictable rate, allowing the study team to work back to the time when the men likely shared a common ancestor.
"Both those lines of evidence say that this chromosome has been around since at least the mid-18th century," Jobling said.
The finding suggests that black people have contributed to the "indigenous" British gene pool despite previous evidence to the contrary.
Africans were first recorded in northern England some 1,800 years ago, part of a Roman garrison brought in to defend Hadrian's Wall against raids by tribes in what is now Scotland, the study team said.
But slaves from West Africa, Jobling said, were the most likely source of the African DNA revealed in the study.
"The first boatful of slaves showed up in 1555 in England, and so from that time on their numbers increased," Jobling said.
In 1601 Queen Elizabeth I issued an edict "that black people should be expelled from Britain because there were too many of them around, which everybody ignored," he added.
Historian Ron Ramdin, author of Reimaging Britain: 500 Years of Black and Asian History, said that by the end of the 18th century an estimated 10,000 black people were living in Britain, mostly concentrated in cities.
Despite this long history of contact, previous studies of the genetic makeup of Britons haven't detected evidence of African Y-chromosome lineages, the study team noted.
In a book released in 2005, David Miles, research fellow at the Institute of Archaeology in Oxford, England, said that evidence suggests that about 80 percent of the genes of most white Britons have been passed down from a few thousand Ice Age hunters.
The University of Leicester's Jobling concedes that African DNA probably exists "at a very low level" in the native British gene pool. But, he said, the latest findings show that "what it means to be British is complicated and always has been."
Mark Thomas, from the Centre for Genetic Anthropology, University College London, said other Y-chromosome lineages in Britain from the last 1,000 to 2,000 years probably also have an African origin.
"For example, there's a lineage that's very common in North Wales that's usually found in places like North Africa and Ethiopia," he said.
The new study, Thomas added, "makes the point that we do all have very mixed ancestry."
http://news.nationalgeographic.com/news/2007/01/070124-british-genes.html
