A new method of STR interpretation using inferential logic -development of a criminal intelligence database

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Abstract
A short tandem repeat (STR) system consisting of seven multiplexed loci has recently been introduced in the UK to support a National strategy to create large DNA databases for criminal intelligence purposes. The process uses automated sequencers, employing dye-labelled primers. Identification of tetrameric loci such as HUMTH01 are straightforward. Sizing windows are estimated by running a series of control allelic ladders on several gels and ‘unknown’ samples are designated if they fall within a defined window. However, utilisation of complex STRs (eg. D21 S 11) characteristically have common variants which differ by just 2 bp. In addition, rare alleles are encountered which may differ by just 1 by from a common variant. To assist with the identification of alleles, we have introduced a series of allelic ladders, so that direct comparisons with ‘unknown’ samples can be made on the same gel. To designate an allele, it should be within 0.5 by of an allelic ladder marker. Not all alleles (in particular rare alleles) can be included within an allelic ladder, however their expected positions can be easily calculated by reference to existing alleles in the ladder. Measurement of band shift is also a useful diagnostic tool. A series of guidelines are described to enable reliable allelic identification. These guidelines can be converted into computer programmes, which form the basis of an expert system.