Techniques used by forensic DNA laboratories for human identity testing purposes are based on the same fundamental principles and methods used for medical diagnostics and gene mapping. A person's genetic makeup can be directly determined from very small amounts of DNA present in blood stains, saliva, bone, hair, semen, or other biological material. Because all the cells in the human body descend by successive divisions from a single fertilized egg, the DNA material is (barring mutations) identical between any cells collected from that individual and therefore provides the same forensic information.
Primary approaches for performing DNA typing can be classified into restriction fragment length polymorphism (RFLP) methods and polymerase chain reaction (PCR)-based methods. Some of the characteristics of these techniques are compared in Table 2.1. PCR-based methods have rapidly overtaken RFLP
Comparison of RFLP and PCR-based DNA typing methods.
Time required to obtain results
Amount of DNA needed Condition of DNA needed Capable of handling sample mixtures Allele identification
Form used in analysis
Power of discrimination
Automatable and capable of high-volume sample processing
Commonly used DNA markers
6-8 weeks with radioactive probes; ~1 week with chemiluminescent probes
High molecular weight, intact DNA
Yes (single-locus probes)
Binning required since a distribution of sizes are observed
DNA must be double-stranded for restriction enzymes to work
~1 in 1 billion with 6 loci No
D1S7, D2S44, D4S139, D5S110, D7S467, D10S28, D17S79
May be highly degraded Yes
Discrete alleles obtained
DNA can be either single-stranded or double-stranded
~1 in 1 billion with 8-13 loci (requires more loci)
methods due to the ability of PCR to handle forensic samples that are of low quantity and of poor quality. The desire for a rapid turnaround time and the capabilities for high volume sample processing have also driven the acceptance of PCR-based methods and markers. The most recent and probably most rapidly accepted forensic DNA markers are short tandem repeats (STRs) due to a number of advantages.
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