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DNA Fingerprinting

DNA fingerprinting, or DNA typing, is the use of DNA analysis to identify an individual. A common technique used to fingerprint DNA is based upon the variability between individuals of several noncoding regions of DNA in the genome, called VNTRs (variable number of tandem repeats). These regions are composed of a fixed nucleotide sequence that is repeated 2 to 10,000 times. The exact number of repeats is highly variable among individuals. Thus, most individuals are heterozygous because they have inherited a different number of repeats from each parent. Also, it is unlikely that any two unrelated individuals will have the same two VNTR alleles. In fact, if enough VNTR loci are used, even related individuals can be distinguished.

Sir Alec Jeffreys, Swee Lay Thein and Vicky Wilson invented DNA fingerprinting in 1985 as a way to identify genetic diseases. However, it soon became useful for other types of identification - such as proving biological relations and resolving immigration cases. Recently it has become prevalent in determining the guilt or innocence of crime suspects as well. Prior to DNA typing, the fingerprint (the type that actually comes from your fingers) was the only type of evidence that could positively link a suspect to a crime. However, in many crimes, fingerprints weren't left behind, but samples of blood and hair could be recovered. Without DNA typing there was no unique way to connect these kinds of samples to a suspect. This animation will take us through the steps needed to fingerprint such DNA evidence.

DNA fingerprinting for a trial begins with samples of DNA from both the crime scene and the suspect. DNA is found in most body tissues and fluids, including blood, hair and saliva. The DNA from each sample is cut into manageable pieces using restriction enzymes, which cut the DNA at specific sequences. Care is taken at this point to Ensure that the enzyme does not cut within the VNTR that is being analyzed. Often, there is not enough DNA in the crime scene sample and so the polymerase chain reaction (PCR) is used to make copies of it. PCR is described in detail in an earlier section of this web site. After the DNA has been cut, the segments are separated by size using a process called gel electrophoresis. The segments are then marked with radioactive DNA probes and exposed to X-ray film so that they form a pattern of black bands. This pattern is the DNA fingerprint. As you can see in the animation, our suspect's DNA clearly does not match that seen in the crime scene, so the suspect is innocent.

Though DNA typing has been effective, there have been some problems that have led to its inability to hold up in legal battles, such as the O.J. Simpson trial. DNA typing is based on the variation of tandem repeats between individuals. When creating a DNA fingerprint, scientists use specific DNA probes to determine if two samples are from the same source. If the samples are found to result in the same fingerprint, then the odds that they came from the same individual are calculated using the frequency that that particular pattern should occur within a population. The problem that arises has to do with the fact that the frequency of different patterns varies across the population. Therefore, it is difficult to give a set number of individuals within the population that should have the same DNA pattern correlating to the probes that were used. With such uncertainty, juries have found it difficult to place suspects at the crime scenes. There are other problems, too, with the standards that different laboratories use when carrying out the tests. Since DNA fingerprinting is a relatively new technology, there are no set guidelines in place for how the fingerprints should be prepared. This further weakens the argument that it can positively identify suspects.