PCR is used in forensic science. (A) The DNA sequences that create the variability used in this analysis contain runs of short, repeated sequences, such as GTGTGT..., which are found in various positions (loci) in the human genome. The number of repeats in each run is highly variable in the population, ranging from 4 to 40 in different individuals. A run of repeated nucleotides of this type is commonly referred to as a hypervariable microsatellite sequence, also known as a VNTR (variable number of tandem repeats) sequence. Because of the variability in these sequences, individuals will usually inherit a different variant of each VNTR locus from their mother and from their father; two unrelated individuals therefore do not usually contain the same pair of sequences. A PCR reaction using primers that bracket the locus produces a pair of bands of amplified DNA from each individual, one band representing the maternal variant and the other representing the paternal variant. The length of the amplified DNA, and thus its position after electrophoresis, will depend on the exact number of repeats at the locus. (B) In the schematic example shown here, the same three VNTR loci are analyzed from three suspects (individuals A, B, and C), producing six bands for each person after polyacrylamide gel electrophoresis. Although some individuals have several bands in common, the overall pattern is quite distinctive for each. The band pattern can therfore serve as a ³fingerprint² to identify an individual nearly uniquely. The fourth lane (F) contains the products of the same PCR reactions carried out on a forensic sample. The starting material for such a PCR reaction can be a single hair or a tiny sample of blood that was left at the crime scene. The more loci that are examined, the more confident we can be about the results. While examining the variability at 5-10 different VNTR loci, the odds that two random individuals would share the same fingerprint by chance are approximately one in 10 billion. In the case shown here, individuals A and C can be eliminated from inquiries, while B remains a clear suspect. A similar approach is now used routinely for paternity testing.  Fair Use and Copyright info