Chromosomal abnormalities do exist and can give rise to extra allele peaks at a particular STR locus. Chromosomal translocations, somatic mutations and trisomies may occur in the cells of the donor of a forensic stain. However, the STR profile from the individual with the chromosomal abnormality would most likely show only a single extra peak and the same pattern would be present in both the forensic stain and the reference sample from the matching suspect (Clayton et al. 1998). The rare cases where a chromosomal abnormality is observed can even help strengthen the final conclusions.
An excellent example of a chromosomal abnormality is found in the standard cell line K562. Three peaks are obtained at the D21S11 locus and at least five other STR loci have heterozygous peak patterns that are not balanced (D.N.A. Box 7.1). At first glance, one might suspect this sample to have arisen from more than one source rather than a sample with an abnormal number of chromosomes.
As described in Chapter 6, tri-allelic patterns have been reported for a number of STR loci (see Figure 6.7). In fact, more than 56 different tri-allelic patterns have been reported spanning all 13 CODIS core loci (see http:// www.cstl.nist.gov/biotech/strbase/tri_tab.htm).
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