The triallelic patterns were classified into two types based on the relative intensities of their component alleles. In the Type 1 pattern, the alleles had uneven intensity; whereas, in the Type 2 pattern, all three alleles had even intensity. Evidence was found that the more frequent Type 1 pattern results from somatic mutation at a heterozygous locus, and the Type 2 pattern results from a localized chromosomal rearrangement at a heterozygous locus. The size difference between the progenitor and mutated allele were determined from the Type 1 pattern, and all mutational changes were found to be multiples of four nucleotides, suggesting the loss or addition of one or more tetrameric repeat units. Aberrant diallelic patterns were identified by analysts due to an unexpectedly large difference in intensity between alleles at a heterozygous locus. Although some of these diallelic patterns are likely caused by the same genetic phenomena previously described at a homozygous locus, others are caused by a mutation in the primer binding sequence, leading to a reduction in amplification efficiency of one allele. Based on a visual inspection of a profile, this study concluded that it is possible to infer a likely genetic basis directly from the triallelic pattern; however, the aberrant diallelic patterns could be due to any one of a number of genetic effects. 1 table, 5 figures, and 19 references