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Use of PCR in Forensic Science
Medical Image Databases on the Internet, 2003(A) The DNA sequences that create the variability used in this analysis contain runs of short, repeated sequences, such as GTGTGT .
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 VNTR (variable number of tandem repeat) sequence.
Because of the variability in these sequences, each individual will usually inherit a different variant of each VNTR locus from their mother and from their father; two unrelated individuals will therefore 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) The DNA bands obtained from a set of four different PCR reactions, each of which amplifies the DNA from a different VNTR locus.
In the schematic example shown here, the same three VNTR loci are analyzed from three suspects (individuals A, B, and C), giving six bands for each person.
You can see that although some individuals have several bands in common, the overall pattern is quite distinctive for each.
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 that was left at the scene of the crime.