About 90% of the human genome variance is common across the world. The remaining 10% differentiate individuals along continental and on a finer scaler population lines. Ancestry informative markers is a subset of polymorphisms, which under selection or genetic drift, have substantial difference in frequency between populations. Thus this smaller subset can be used as a surrogate of the whole genome in population prediction of an individual.
P. Paschou, J. Lewis, A. Javed, P. Drineas, Ancestry informative markers for fine-scale individual assignment to worldwide populations, J Med Genet. Dec 2010 [pubmed]