Typing With Degraded Dna Samples

The primary selling point of SNPs in the forensic arena is the ability to make small PCR products that can overcome challenges of strong PCR inhibitors or samples possessing highly degraded material. Smaller PCR products should result in greater recovery of information from badly damaged samples. PCR products for SNP markers can be small because the target region is only a single nucleotide rather than an expandable array of 20-60 nucleotides as is present in tetra-nucleotide STRs with 5-15 repeats. While the promise of SNPs in this area has been promoted for many years, as of early 2004 no clear advantage of SNPs has been demonstrated scientifically in a quantitative fashion. Studies directly comparing SNP assays to STR assays on the same degraded DNA samples are needed.

SNPs have successfully been used on highly degraded samples from victims of the World Trade Center disaster of 11 September 2001 (see Chapter 24). Also valuable in those identification efforts was the use of reduced size STR markers (miniSTRs) (Butler et al. 2003). The PCR products from some STR markers can be reduced from over 300 bp to less than 100 bp in length by developing primers that anneal immediately adjacent to the repeat region (see Chapter 7). These reduced size STR assays have an advantage over SNPs in that more alleles exist to produce a higher power of discrimination. More importantly miniSTR assays offer compatibility with current DNA databases housing millions of convicted offender DNA profiles (see Chapter 18).

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Stammering Its Cause and Its Cure

Stammering Its Cause and Its Cure

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