ABSTRACT
If the evidentiary sample contains an insufficient quantity of DNA, or if the DNA is degraded, a polymerase chain reaction (PCR)-based test may be used to obtain a DNA profile. The PCR-based tests generally provide rapid results that can serve as an alternative or as a complement to other DNA testing. The process involves the isolation of DNA from a biological specimen (e.g., blood, semen, saliva, fingernail clippings), followed by an assessment of DNA quality and quantity. Next, the PCR amplification technique is used to produce millions of copies of a specific portion of a targeted DNA segment that contains polymorphic DNA selected for forensic and parentage evaluations (Figure 13.1). The PCR amplification procedure is similar to a molecular photocopying machine. The amplified PCR products are then separated and identified by either gel-based or capillary electrophoresis where chemical staining (e.g., ethidium bromide or Coomassie blue) or chemiluminescent labels are used, respectively. The resulting DNA profiles are routinely interpreted by direct comparison to allele standards (i.e., allelic ladders for each locus) and known reference samples. Probability calculations are determined based upon classical population genetic principles.
