Frequency of the I^B allele of the ABO blood group system

The I^B allele is present at highest frequency in Central Asian populations (>30%), and declines westward in a cline to a low point in Basque populations in Northern Spain and Southwestern France (~0%). The cline apparently reflects Mongol migration into the European population following the collapse of the Roman Empire (400AD). [Figure from Klug & Cummings 1997]