An ancestry-informative marker (AIM) is a set of polymorphisms for a locus, generally from humans, which exhibits substantially different frequencies between populations from different geographical regions.
By using a number of AIMs one can estimate the geographical origins of the ancestors of an individual and ascertain what proportion of ancestry is derived from each geographical region. By using a suite of these markers more or less evenly spaced across the genome, they can be used in a cost-effective way to discover novel genes underlying complex diseases in a technique called admixture mapping or mapping by admixture linkage disequilibrium.
As one example, the Duffy Null allele (FY*0) has a frequency of almost 100% of Sub-Saharan Africans, but occurs very infrequently in populations outside of this region. A person having this gene is thus more likely to have Sub-Saharan African ancestors.
Collections of AIMs have been developed that can estimate the geographical origins of ancestors from within Europe.
- Elizabeth Pennisi, Human Genetic Variation, Science 21 December 2007: Vol. 318. no. 5858, pp. 1842 - 1843 doi:10.1126/science.318.5858.1842 
- Bauchet M, McEvoy B, Pearson L, Quillen E, Sarkisian T, Hovhannesyan K, Deka R, Bradley D, Shriver M. 2007. Measuring European population stratification with microarray genotype data. American Journal of Human Genetics 80(5): 948-956. doi:10.1086/513477