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Strengthening Forensic Science in the United States: A Path Forward
contain amylase, including the particular type (AMY 1) that is associated with saliva.
Although the forensic use of nuclear DNA is barely 20 years old, DNA typing is now universally recognized as the standard against which many other forensic individualization techniques are judged. DNA enjoys this preeminent position because of its reliability and the fact that, absent fraud or an error in labeling or handling, the probabilities of a false positive are quantifiable and often miniscule. However, even a very small (but nonzero) probability of false positive can affect the odds that a suspect is the source of a sample with a matching DNA profile.6 The scientific bases and reliability of other types of biological analysis are also well established, but absent nuclear DNA, they can only narrow the field of suspects, not suggest any particular individual.
Testing biological evidence in the laboratory involves the use of a logical sequence of analyses designed to identify what a substance is and then from whom it came. The sequence begins with a forensic biologist locating the substance on the evidence. This is followed by a presumptive test that would give more information about the substance, typically using the same tests employed by scene investigators: the ALS, enzymatic, chemical, or immunological tests. Once the material (e.g., blood, semen, or saliva) is known, an immunological test or a human DNA test is run to determine whether the sample comes from a human or an animal.
The final step in the analytical sequence procedure is to identify the source of the biological material. If a sufficient sample is present and is probative, the forensic biologist prepares the material for DNA testing. The analyst who conducts the DNA test may or may not be the same person who examines the original physical evidence, depending on laboratory policies.
A decision might be required regarding the type of DNA testing to employ. Two primary types of DNA tests are conducted in U.S. forensic laboratories: nuclear testing and mitochondrial DNA (mtDNA) testing, with several variations of the former. For most biological evidence having evidentiary significance, forensic DNA laboratories employ nuclear testing routinely,7 and testing for the 13 core Short Tandem Repeat (STR)
W.C. Thompson, F. Taroni, and C.G.G. Aitken. 2003. How the probability of a false positive affects the value of DNA evidence. Journal of Forensic Sciences 48(1):47-54.
T.R. Moretti, A.L. Baumstark, D.A. Defenbaugh, K.M. Keys, J.B. Smerick, and B. Budowle B. 2001. Validation of short tandem repeats (STRs) for forensic usage: Performance testing of fluorescent multiplex STR systems and analysis of authentic and simulated forensic samples. Journal of Forensic Sciences 46(3):647-660.