Fluorescence-based DNA segment analysis in forensic science

Beyond STRs: The Role of Diallelic Markers in Forensic Genetics

Short tandem repeat (STR) polymorphisms have been firmly established as standard DNA marker systems since more than 15 years both in forensic stain typing as well as in paternity and kinship testing. However, when analyzing genetic relationships in deficiency cases, STRs have a couple of disadvantages due to the sometimes poor biostatistical efficiency as well as the possibility to observe one or more genetic inconsistencies that could also be explained by mutational events. In such situations, additional robust markers with negligible mutations rates such as single nucleotide polymorphisms (SNPs) and insertion/deletion markers (indels) can be used as adjuncts to provide decisive genetic information in favor for or against the assumed relationship. Both SNPs and indels can now be typed more easily using multiplexes of up to 50 loci based on fragment length analysis on instruments available in all routine forensic and paternity testing laboratories, thus making it possible to extend the range of markers beyond the currently used STRs.

Fluorescence-based classification of microsatellites using a single-wavelength semiautomatic sequencer: genotype assignment and identity tests by analysis of comigrating peak profiles

Microsatellite analysis, based on fluorescein labeling and reading through a semiautomatic single wavelength sequencer, is described. Pairs of labeled polymerase chain reaction (PCR) samples, mixed in equimolar proportion, were electrophoresed and the specific peaks read in a single gel lane. Identity was asserted when peaks overlapped in a unique fluorescent signal which, compared with individual sample profiles, had a twofold intensity. Classification was achieved by blending individual PCR products to 'locus specific allelic ladders' (composite samples containing a repertory of fragments allelic to a given locus) and by noticing the specific peak enhancement. The resulting protocol of analysis assigned no size and classified allelic forms by tandem repeat number. Applied to a large repertory of PCR products and compared with manual electrophoresis, this protocol proved to be reliable and reduced times and costs of genotype analysis. Analysis of comigrating peak profiles is highly objective and provides convincing evidence for diagnostics and identity tests.

Analysis of 6 VNTR loci by 'multiplex' PCR and automated fluorescent detection

The polymerase chain reaction was used to amplify six small variable number of tandem repeat loci in two reactions (D19S20 co-amplifying with D17S5 and D1S80; D17S766 co-amplifying with D16S83 and D17S24). When coupled with fluorescent detection of the products, this provides a rapid, highly discriminating automated test. Preferential amplification of small alleles, leading to 'allelic dropout' was found to occur in D19S20 and D16S83. Population databases are presented for Caucasians and Afro-Caribbeans at loci D19S20, D16S83 and D17S24, and for Asians at D19S20.

DNA profiling

Although some concerns still remain in standard DNA profiling technology over the assumptions from population genetics used to calculate expected match frequencies, forensic scientists are preparing for the introduction of the next generation of DNA profiling techniques based on the polymerase chain reaction. These new techniques offer the prospect of dramatically increasing the speed and sensitivity of DNA profiling and have already been applied in some casework studies.