Techniques for DNA Testing

Medical Genetics Index Glossary

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.

Example 1: Single Locus Fingerprinting

Minisatellite fingerprinting to demonstrate kinship using mixtures of two or three single locus probes (probe sets 1 and 2). The loci detected in the child (C) are clearly a composite of those present in the mother (M) and father (F).

Example 2: Multilocus Fingerprinting

Microsatellite fingerprinting to establish parentage. The probe, (CAG)₅, recognizes a large number of loci. Examine the bands detected in DNA from the child that are not detected with DNA from the mother. Which male is the biologic father of the child?

Example 3: Multilocus Fingerprinting

Multilocus fingerprinting to match trace evidence from a crime with suspects. Which suspect matches the specimen?

Next Topic for Forensic DNA Testing: Applications of DNA Testing

References and Reviews

Decorte R, Cassiman J: Forensic medicine and the polymerase chain reaction. J Med Genetics 30:625, 1993. [technical discussion of PCR-based DNA testing]
Krawczak M, Schmidtke J. DNA Fingerprinting. Bios Scientific Publishers, Ltd., Oxford, 1994. [Good elementary review of DNA fingerprinting and its genetic basis]

Last updated on March 24, 1996

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Example 1: Single Locus Fingerprinting Minisatellite fingerprinting to demonstrate kinship using mixtures of two or three single locus probes (probe sets 1 and 2). The loci detected in the child (C) are clearly a composite of those present in the mother (M) and father (F).
Example 2: Multilocus Fingerprinting Microsatellite fingerprinting to establish parentage. The probe, (CAG)₅, recognizes a large number of loci. Examine the bands detected in DNA from the child that are not detected with DNA from the mother. Which male is the biologic father of the child?
Example 3: Multilocus Fingerprinting Multilocus fingerprinting to match trace evidence from a crime with suspects. Which suspect matches the specimen?


	*Next Topic for Forensic DNA Testing:* Applications of DNA Testing
	References and Reviews Decorte R, Cassiman J: Forensic medicine and the polymerase chain reaction. J Med Genetics 30:625, 1993. [technical discussion of PCR-based DNA testing] Krawczak M, Schmidtke J. DNA Fingerprinting. Bios Scientific Publishers, Ltd., Oxford, 1994. [Good elementary review of DNA fingerprinting and its genetic basis]