Techniques for DNA Testing

DNA typing is performed by demonstrating differences in length of specific DNA sequences. This can be done by digestion of DNA with restriction enzyme(s), followed by Southern blot hybridization using a probe specific for the polymorphic site. Polymerase chain reaction (PCR) techniques are becoming widely applied to the same task, and have several advantages over Southern blotting - for example, much less DNA is required and in many cases, typing can be done using partially degraded DNA. For PCR analysis, the primers are designed to flank the VNTR locus and the size of the PCR product is dependent on the number of repeats. The general term "DNA fingerprinting" is used to describe all these procedures for characterizing VNTRs, RFLPs and other sequence polymorphisms.
Two conceptually different types of fingerprinting are commonly performed for either VNTR or RFLP analyses:
Single locus DNA fingerprinting: Polymorphism at a single locus is characterized, usually through use of a specific probe or specific PCR primers. Because the single loci detected by this method are characterized, one obtains a DNA genotype from single locus methods.
Multilocus DNA fingerprinting: Polymorphism at multiple loci is simultaneously identified. This can be performed by application of a mixture of single locus probes or application of a single probe that identifies multiple similar sequence polymorphisms. In the latter case, one is detecting unidentified fragments of DNA and the result is therefore a DNA phenotype rather than a genotype.
Each of these methods has advantages over the other in specific situations. For example, single locus but not multilocus methods are useful when the DNA is degraded and for mixed (i.e. victim and pertetrator) samples. On the other hand, multilocus fingerprinting typically provides more information per sample than single locus fingerprints. Examples of both types of fingerprinting follow.