Forensic Methods of DNA Testing
Multiple Techniques Used to Identify DNA Evidence in Cold Cases
Nov 1, 2009
Four-year-old Gregory Villemin was found dead in 1984. His killer was never found, though his parents were once suspected. The ensuing case became known in the French media as The Gregory Affair. In 2009, at the parents' request, old evidence was re-examined with new forensic technology and found to contain DNA belonging to an unknown man and woman. A variety of DNA testing methods may be used to answer questions in this case and other unsolved mysteries.
DNA Testing for Identification of Human Samples
According to The Human Genome Project, forensic DNA testing is used to:
- compare suspects' DNA to samples left at crime scenes
- clear people who may be innocent of crimes of which they're accused or have been convicted
- identify victims of mass casualties
- DNA paternity testing.
Human DNA is very similar from person to person. Even though the difference is 0.1%, a specific DNA sample can be identified by chemical base pairs that occur together. These base pairs create genetic markers that can be tested by DNA probes. The genetic markers that match a suspect's DNA specimen to that left at a crime scene, the more likely the person is to have left the DNA. There are several different technologies in forensic DNA testing used for this type of investigation.
Restriction Fragment Length Polymorphism (RFLP) DNA Analysis
RFLP is one of the oldest form of DNA testing and takes several weeks to complete. DNA is split into fragments with specific enzymes. Several million copies of these DNA fragments are made using polymerase chain reaction (PCR) technology. Different enzymes break the DNA into strands at different places. A DNA sample from a crime scene investigation can be compared to a suspect DNA sample by checking to see if the samples are broken by the enzymes at the same places. The similarities they have, the more likely the samples match. Because of the length of time required for RFLP analysis, it's been mostly replaced by faster techniques.
STR (short tandem repeats) DNA Analysis
STRs have somewhat replaced RFLP since less DNA material is required. STR technology is one type of analysis used in the FBI's CODIS database, which is a DNA repository used to compare and store unknown DNA samples for identification. STRs are short DNA fragments that include only two to five base pairs. CODIS uses 13 of these STR regions for comparison. The odds of two people having the same pattern of these 13 SRT regions is approximately one in a billion according to Human Genome Project Information.
Mitochondrial DNA Testing Technique
Both STR and RFLP analysis require DNA found in the nucleus of cells extracted from the sample. For some types of evidence like hair, teeth and bone, specimens do not contain cells that have a nucleus. In these cases, mitochondrial DNA can be used to identify the sample. DNA is removed from cellular organelles known as mitochondria. These are passed down from mother to child making this technique very helpful in missing persons identification.
Forensic Uses of DNA Testing
DNA forensic testing is useful in a variety of applications for identification of suspects and victims, endangered species and even wine heritage. Victims of the September 11 disaster were identified using DNA testing. DNA samples from Holocaust survivors, victims and their descendants are being collected in a database to help reconnect families. Applications for DNA testing are numerous.
 |
 |
