Linkage disequilibriumrefers to the occurrence of specific alleles at 2 loci with a frequency greater than expected by chance. If the alleles at locus A are a1 and a2 with frequencies of 0.7 and 0.3, and alleles at locus B are b1 and b2 with frequencies of 0.6 and 0.4, the expected frequencies of haplotypes would be a1b1, 0.42; a1b2, 0.28; a2b1, 0.18; and a2b2, 0.12. Even if the 2 loci are closely linked, unrestricted recombination should result in allelic combinations in the general population that are close to the frequencies given above. When a particular combination occurs at a higher frequency, for example a2b2 at a frequency of 0.45, this is called linkage disequilibrium. Linkage disequilibrium may result from natural selection or by chance. When a disease mutation arises on a founder chromosome and not much time has elapsed since the mutational event, the disease mutation will be in linkage disequilibrium with alleles from loci close to the gene. Thus, linkage disequilibrium can be a powerful tool for genetic mapping.