Kayser's and Novembre's teams uncovered the gene-geography pattern only by analysing hundreds of thousands of common gene variants called single nucleotide polymorphisms (SNPs) across the genomes of people from about two dozen countries. SNPs are places in the genome where one person's DNA might read A, while another's T.

Though the teams worked independently, they used some of the same DNA samples, which were gathered by the pharmaceutical company GlaxoSmithKline to help hunt for genes linked to drug side effects. The researchers recorded the results alongside the country of origin for each subject as well as that of their parents and grandparents when possible.

For each subject, the researchers decoded half a million SNPs. However, to get an overall assessment of the difference between any two genomes, the researchers used a mathematical trick that scrunched the hundreds of thousands of SNPs into two coordinates, with each person's genome represented by a point. The greater the distance between two points, the greater the difference in their genomes.

When both teams plotted thousands of genomes on a single graph along with their country of origin, a striking map of Europe emerged. Spanish and Portuguese genomes clustered "south-west" of French genomes, while Italian genomes jutted "south-east" of Swiss.

These cardinal directions are artificial, but the spatial relationships between genomes are not. In general, the closer together two people live, the more similar their DNA. The same is known to be true of animals .

The map was so accurate that when Novembre's team placed a geopolitical map over their genetic "map", half of the genomes landed within 310 kilometres of their country of origin, while 90% fell within 700 km.

Both teams found that southern Europeans boast more overall genetic diversity than Scandinavians, British and Irish.

"That makes perfect sense with the major migration waves that went into Europe," says Kayser, noting Homo sapien's European debut 35,000 years ago, post-ice age expansions 20,000 years ago, and movements propelled by the advent of farming 10,000 years ago. In each case, members of established southern populations struck north.

"A pattern in which genes mirror geography is essentially what you would expect from a history in which people moved slowly and mated mainly with their close neighbours," says Noah Rosenberg, a geneticist at the University of Michigan in Ann Arbor.

Source: NewScientist