The ability to combine information from multiple loci strengthens the statistics of a match and lends real power to STR typing. For random match probabilities to be combined from multiple loci, it is required that those loci are independent of one another (i.e., that recombination occurs between them). When STR loci or any DNA sequence is transferred independently of another DNA segment during meiosis, the two DNA regions are said to be in linkage equilibrium.
Independence of frequencies for all genotypes can be tested with exact tests. These tests use the probability of the observed genotypic array, conditional on the observed allelic array. Exact tests are calculated for the database being considered as well as for a series of new databases obtained by permuting the alle-les among individuals. Typically 2000 shuffles or permutations are performed with computer programs. Shuffling through permutation testing is performed to enable a look at a virtual population database that is much larger than the one actually measured (Guo and Thomson 1992).
Multiplication of allele frequencies in the form of the product rule benefits demonstration that a DNA profile is extremely rare. If the product rule cannot be used, then the power of a genetic test is vastly reduced. Therefore, the assumption that loci and alleles are inherited independently is an important one so that match probabilities can be multiplied.
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