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DNA Evidence In Criminal Trials
DNA evidence in criminal trials is common in our San Diego courts. Here’s how it works.
Until the 1990s, the only sure-fire way to establish the identity of an individual was to examine his or her fingerprints. Because each individual’s fingerprints have a unique pattern, fingerprint evidence is readily admitted into court. Now, DNA is rapidly becoming the method of choice when it comes to linking individuals with crime scenes and criminal assaults.
The universally accepted theory underlying DNA analysis is that every person (except an identical twin) has certain elements of his or her DNA (deoxyribonucleic acid, which makes up chromosomes) that are unique.
Different methodologies allow experts to identify these distinguishing elements. The most common of these methodologies is “RFLP,” or the “Restriction Fragment Length Polymorphism Technique.” By comparing an individual’s known DNA with a sample of DNA from a crime scene (for example, in a droplet of blood or a strand of hair), an expert can give an opinion concerning the likelihood that both samples came from the same individual.
This sort of technology is extremely complex; few people are able to understand it. And yet the major crime labs and, increasingly, local police agencies, are becoming adept at comparing DNA samples left at the scene of a crime with DNA taken from a suspect — and concluding on that basis that the suspect is the culprit.
The conclusion that a DNA match proves the defendant’s guilt is based primarily on the assumption that the probability against one individual’s DNA matching another’s is in the hundreds of millions, or even billions, depending on who is crunching the numbers. However, as overwhelming as these figures may seem, it’s still possible to whittle them down to far less overwhelming odds if it can be shown that the methods used by the laboratories doing the testing were flawed in some manner. It is this approach, among others, that the defense team in the O.J. Simpson case used to mount its defense against what appeared to be overwhelming DNA evidence implicating O.J. Simpson as the guilty party.
DNA evidence is also proving to be a powerful tool in determining the innocence of prisoners who were tried before DNA testing in its current form was an option. If bodily samples, such as blood or semen, were collected from the crime scene or victim and that evidence is still available for DNA testing, a showing that the prisoner’s DNA doesn’t match the crime-scene DNA can be a powerful reason to conclude that the prisoner is innocent and should be released. A number of prisoners who were sentenced to death have been cleared through this technology.
DNA: Greater Accuracy
The use of DNA (deoxyribonucleic acid) as a method of identification is relatively new, but it has proven an effective means of identifying criminals and perhaps more important, eliminating people as crime suspects. A fingerprint is the only unique identification source (identical twins have the same DNA). But if a criminal leaves no prints behind, law enforcement officials must rely on minute DNA samples from blood, saliva and other bodily fluids, hair, or skin. DNA testing is also used in paternity disputes to determine the identity of the actual father in custody, inheritance, or child support suits.
DNA testing can be done by standard techniques such as restrictive fragment length polymorphisms (RFLP), polymerase chain reaction (PCR), short tandem repeat (STR), and mitochondrial analysis. In RFLP testing, a DNA sample is mixed with a chemical substance that helps examiners isolate and identify specific key fragments of the sample that can be used in comparison analysis. A drawback of RFLP is that it requires a fairly large DNA sample. With PCR, a series of chemical reactions helps generate copies of a minute DNA sample, thus amplifying a small or degraded piece of information. In STR, various DNA regions in a sample are compared with other samples for similarities. The FBI uses STR using special software that can identity thirteen of these regions in a DNA sample. Mitochondrial DNA analysis is often used for extracting samples from bones and teeth, for which the other methods are not effective.
The FBI keeps a computerized databank of DNA samples called CODIS (Combined DNA Index System), which contained about 1.7 million DNA profiles as of 2003. The profiles stored in CODIS can be used to convict criminals, and also to exonerate innocent people. There are numerous examples of criminals whose DNA matched a profile from an earlier crime and who were then charged with the crime; likewise, there are examples of individuals whose innocence was confirmed when DNA found at a crime scene turned out to belong to another person identified through the profiles.
DNA Evidence In Criminal Trials as an Exoneration Tool
Not only can DNA be used to convict criminals, it has successfully been used to exonerate individuals, some of whom were wrongly imprisoned for more than two decades.
Often, the person who is wrongly convicted of a serious crime such as murder or rape has a criminal record for petty crimes, which means a record already exists. These individuals are frequently convicted on eyewitness testimony, but without any physical evidence tying them to the crime.
If you have any questions about DNA Evidence In Criminal Trials contact San Diego Criminal Defense Attorney Vik Monder 619.405.0063 or visit San Diego Criminal Defense