DNA profiling using singlelocus probes SLPs

To circumvent the limitations of MLPs, specific cloned minisatellites were used as SLPs to produce simpler 'DNA profiles' and were applied in criminal casework even before MLPs were commercially established as the standard method for paternity testing (Figure 5.2). Since each SLP detects only a single minisatel-

Figure 5.2 DNA profile using single-locus probes: results from a paternity test exploring five loci. Samples were obtained from the mother (lane 1), child (lane 2) and alleged father (lane 3). The numbers at the top of each autorad represent the name of the loci examined. Size marker is indicated by 'm'. Paternity was successfully established

Figure 5.2 DNA profile using single-locus probes: results from a paternity test exploring five loci. Samples were obtained from the mother (lane 1), child (lane 2) and alleged father (lane 3). The numbers at the top of each autorad represent the name of the loci examined. Size marker is indicated by 'm'. Paternity was successfully established lite, it produces two band (two allele) patterns, but is still highly polymorphic due to the use of hypervariable minisatellites. Single-locus probes have considerable advantages over MLPs in analysing forensic specimens. The method is far more sensitive, with the limit of band detection at around 10 ng of genomic DNA. Mixed DNA samples such as semen in vaginal swabs can be analysed with relative ease because the original DNA (from the victim) has only two bands and subsequent autoradiography will reveal whether the profile has more than two bands. Secondly, because allele sizes can be estimated and included in databases, the comparison of samples does not require side-by-side electro-phoresis, which overcomes the inter-blot comparison problem associated with DNA fingerprinting. However, owing to continuous allele size distribution and the resolution limits of agarose gel electrophoresis, SLP allele sizing cannot be done with absolute precision. Two procedures for determining allele frequencies and match probabilities in the face of measurement errors are the floating-bin and the fixed-bin methods (Herrin, 1992). The true discriminating power of SLPs was compromised for no appropriate reason by genetically inappropriate calculations based on the 'ceiling principle' or 'interim ceiling principle' invented by the National Research Council of the United States (National Research Council, 1992); these calculations were abandoned in 1996 (National Research Council, 1996).

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