Genetic variation is generated, maintained, and lost in a population by the interplay of various evolutionary forces, including mutation, migration, selection, and random genetic drift. A locus with two or more alleles (e.g., a site represented by two or more different nucleic acid sequences) in a population is considered polymorphic. Polymorphisms can be assayed across a range of molecular levels from changes in single nucleotides to differences in protein structure (e.g., blood groups) to gross chromosomal differences (e.g., inversions).