|MadSci Network: Genetics|
Biochemical and molecular analysis of evidence in criminal cases has been around for a long time, and new techniques are being developed at a fast rate. Some of the earliest tests used biochemical methods to determine whether the suspect’s blood type (A, B, AB, or O) matched samples from the crime scene. Recently, even these tests have been converted into a “genetic” system where the blood type can be determined by analyzing DNA from evidence such as bloodstains, vaginal swabs, cigarette butts, hair roots, and even 10-year old stamps. DNA typing, as this is known, can generate results within hours and can provide information from very small specimens and even from samples with partially degraded DNA. Because there are only a limited number of blood types, and therefore many indivividuals who will have the same blood type as an evidentiary sample, this analysis is only useful as a first-step screening or for cases where there are a limited number of suspects (such as paternity testing). Other molecular techniques are much more useful and take advantage of the DNA variation which occurs at thousands of sites in the human genome and which defines an absolute uniqueness for each individual. In most cases, this variation is detected in VNTR (variable number of tandem repeat) regions in the human genome. In each of these regions a particular short (2 to 30 base) DNA sequence is repeated many times. In the past, these regions were characterized, or typed, using RFLP (restriction fragment length polymorphism) analysis. In this method, DNA from evidence is isolated and treated with enzymes which cut the DNA at a few specific sites. The resulting DNA fragments are separated by electrophoresis, which allows them to be visualized. Differences in the length of a VNTR located between two enzyme sites results in longer or shorter fragments, and this DNA “profile” or “fingerprint” can then be compared to the suspect’s DNA fingerprint. This technique, known as Southern blotting, is pretty labor- intensive and time-consuming, and therefore not of much use if there are many suspects. It also requires fairly large amounts of DNA, which are often not available. Most of the recently developed techniques utilize the polymerase chain reaction, or PCR. In this method an enzyme is used to “amplify” regions of DNA by making many copies, which can then be detected by electrophoresis. DNA fingerprinting using PCR is more sensitive and can generate data from tiny, degraded samples (such as in the ABO genotyping described in the first paragraph); it can resolve samples which are mixtures of evidence from several individuals; and it is very specific, i.e. it has a high power to discriminate among individuals. This method also utilizes the variation present in regions of repeated DNA. PCR is used to amplify regions containing VNTR loci, and the length of the resulting “PCR products”, which vary depending on the number of repeats, are compared. By looking at a large number of VNTRs, the likelihood that an individual will match a crime sample at all the loci can be a millionfold higher than other techniques. This technique is very sensitive, and recently forensic scientists were able to generate genetic profiles from DNA isolated from actual fingerprints (fingerprint fingerprinting!) as well as items that had been handled briefly (such as telephones) and hands swabbed after a handshake. There are some drawbacks to such a sensitive test, related to possible technical errors. Because the sample is amplified by PCR, it is possible that it is not the sample that gets amplified, but contamination. Thus the technician may accidentally contaminate the evidence with DNA from the suspect, or both the evidence and the suspect samples might be contaminated with the technician’s DNA. Extensive precautions are always taken to avoid contamination, and experimental controls are included to identify false results. Ideally, different samples are handled by different technicians in different labs, providing independent confirmation of the results. Some references that might prove useful are: http://bio.taiu.edu/class/ksmp003/dnafinger.html http://www.geom.umn.edu/docs/education/chance/course/topics/DNA.html van Oorschot, R.A.H., and Jones, M.K. (1997) DNA fingerprints from fingerprints. Nature 387:767 Hochmeister, M.N. (1995) DNA technology in forensic applications. Molecular Aspects of Medicine 16:315 Pena, S.D., Prado, V.F., and Epplen, J.T. (1995) DNA diagnosis of human genetic individuality. Journal of Molecular Medicine 73:555.
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