Forensic DNA Typing or Profiling
Each person has a unique set of fingerprints. As with a person’s fingerprint no two individuals share the same genetic makeup. This genetic makeup, which is the hereditary blueprint imparted to us by our parents, is stored in the chemical deoxyribonucleic acid (DNA), the basic molecule of life. Examination of DNA from individuals, other than identical twins, has shown that variations exist and that a specific DNA pattern or profile could be associated with an individual. These DNA profiles have revolutionized criminal investigations and have become powerful tools in the identification of individuals in criminal and paternity cases.
Polymerase Chain Reaction-Based Tests. If the evidentiary sample contains an insufficient quantity of DNA or if the DNA is degraded, a PCR (polymerase chain reaction)-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 testing. The process involves the isolation of DNA from a biological specimen (e.g., blood, semen, saliva, fingernail clippings, etc.). Next, the PCR amplification technique is used to produce millions of copies of a specific portion of a targeted DNA segment. The PCR amplification procedure is similar to a "molecular photocopying machine." The amplified PCR products (e.g., short tandem repeat [STR]) are then separated and identified by gel or capillary electrophoresis. Such detection procedures eliminate the need for DNA probes thus reducing the analysis time from several weeks to 24 to 48 hours. The resulting DNA profiles are routinely interpreted by direct comparison to DNA standards. Probability calculations are determined based upon classical population genetic principles.
Mitochondrial DNA Analysis. Mitochondrial DNA (mtDNA) typing is increasingly used in human identity testing when biological evidence may be degraded, when quantities of the samples in question are limited, or when nuclear DNA typing is not an option. Biological sources of mtDNA include hairs, bones, and teeth. In humans, mtDNA is inherited strictly from the mother. Consequently, mtDNA analysis cannot discriminate between maternally related individuals (e.g., mother and daughter, brother and sister). However, this unique characteristic of mtDNA is beneficial for missing person cases when mtDNA samples can be compared to samples provided by the maternal relative of the missing person.
For humans, the mtDNA genome is approximately 16,000 bases (A, T, G, and C) in length containing a "control region" with two highly polymorphic regions. These two regions, termed Hypervariable Region I (HV1) and Hypervariable Region II (HV2), are 342 and 268 base pairs (bp) in length, respectively, and are highly variable within the human population. This sequence (the specific order of bases along a DNA strand) variability in either region provides an attractive target for forensic identification studies. Moreover, since human cells contain several hundred copies of mtDNA, substantially more template DNA is available for amplification than nuclear DNA.
Mitochondrial DNA typing begins with the extraction of mtDNA followed by PCR amplification of the hypervariable regions. The amplified mtDNA is purified, subjected to sequencing (Sanger et al., 1977. PNAS 74: 5463-5467), with the final products containing a fluorescently labeled base at the end position. The products from the sequencing reaction are separated, based on their length, by gel or capillary electrophoresis. The resulting sequences or profiles are then compared to sequences of a known reference sample to determine differences and similarities between samples. Samples are not excluded as originating from the same source if each base (A, T, G, or C) at every position along the hypervariable regions are similar. However, due to the size of the mtDNA database and to the unknown number of mtDNA sequences in the human population a reliable frequency estimate is not provided. Consequently, mtDNA sequencing is becoming known as an exclusionary tool as well as a technique to complement other human identification procedures.
Restriction Fragment Length Polymorphism. The first widespread use of DNA tests involved RFLP (restriction fragment length polymorphism) analysis, a test designed to detect variations in the DNA from different individuals. In the RFLP method, DNA is isolated from a biological specimen (e.g., blood, semen, vaginal swabs) and cut by an enzyme into restriction fragments. The DNA fragments are separated by size into discrete bands in a gel (gel electrophoresis), transferred onto a membrane, and identified using probes (known DNA sequences that are "tagged" with a chemical tracer). The resulting DNA profile is visualized by exposing the membrane to a piece of x-ray film which allows the scientist to determine which specific fragments the probe identified among the thousands in a sample of human DNA. A "match" is made when similar DNA profiles are observed between an evidentiary sample and those from a suspect’s DNA. A determination is then made as to the probability that a person selected at random from a given population would match the evidence sample as well as the suspect. The entire analysis may require from 6 to 10 weeks for completion. However, this techniques has essentially be replaced with STR analysis.