Human DNA Testing to Identify a Single Individual at a Crime Scene is Not As Easy As It Seems
According to Daniel Drell of the U.S. DOE Human Genome Program, we need several factors to key into the personal DNA of any individual. Drell uses the example of identifying a suspect at a crime scene to illustrate his point.
Suppose a blood sample was found at the crime scene and that sample was type O. It is a start, but not the end of the process, type O is found among 45% of the human population. Supposing a blond hair was found at the scene of the crime as well, this additional information is necessary information however, there are still many people with blond hair who have the type O blood type.
Supposing you find a footprint that is identified as a size 11 and the print matches Nike Air Jordan running shoes ( that has a distinctive pattern on the sole), you know you have a person who is blood and has a type O blood type who also has a size 11 foot size and wears Air Jordan running shoes. As you see each piece of evidence narrows the DNA puzzle and brings the identification of the individual a little closer.
Determining a suspect at crime scene or determining paternity in the family courts
One or two DNA matches, will not connect two people together even for paternity testing when the individual is present and ready for testing. It is even more difficult when there is no suspect for a crime scene and the individual must be picked out of the general public. The full loci of 13 sites must be used to try and match a suspect with the crime scene. The court system will also need proof that the 13 loci points have been tested and there is sufficient evidence to convict a criminal, or even prove a male is the father of a child in the non criminal health related DNA testing scenario.
How testing for DNA matches are performed
Scientists will test bone, body tissue, body products such as saliva, blood, and hair to do identify the DNA present in any individual. It is quite interesting to note that only one-tenth of a percent of DNA will differ among individuals. When we go back to our crime scene evidence, scientists must match the DNA evidence found at the scene with the suspect.
Forensic scientists use specific genetic markers.
Forensic scientists take small pieces of DNA information they have gathered called probes to see if this DNA sampling matches (binds) to the complementary DNA sampling that they have. A series of these probes that match forms a distinctive pattern and then that pattern is compared to the DNA sample of the suspect or the
individual in question. Again, one marker is not enough, there would have to be about 5 markers that match to identify the suspect. Of course more markers would be even better, but it is very time consuming and very expensive and is not demanded in court cases that much because of these factors.
At this point the chances are very good that the suspect was present at the scene of the crime. Note that DNA testing is not 100 percent accurate and there is a very slim chance, but a chance nevertheless, that someone else could have the same DNA profile for the probe or genetic marker sets. The courts then decide how accurate the DNA evidence is. It may at that point ask for the full 13 loci to be conducted. Even if only 5 loci are considered because that is all the evidence available (crime scene investigation is a piecemeal procedure and does not afford the DNA testing available for paternity testing, crime scene DNA is far superior to any evidence we have at present, including eyewitness accounts which are only 50 percent accurate.
DNA chip technology may soon be on the horizon to make stronger more accurate matches.