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The use of deoxyribonucleic acid profiling, commonly referred to as DNA profiling, has become commonplace in the legal system of many countries, including the United States, since its discovery in the 1980s. Along with its widespread use to both convict suspects and to exonerate previously convicted offenders, have come questions regarding the reliability of DNA profiling. The answer to the question, "How reliable is DNA profiling?" depends on who you ask. Figures typically used in courtrooms put the chance of a DNA profile match being a random or accidental match at somewhere in the astronomical range of one in 15 quadrillion. Scientists, mathematicians, and statisticians, however, question the accuracy of that figure — or a figure anywhere close to that.
DNA is a nucleic acid which basically contains all the ingredients, or building blocks, for life. Humans share over 99.9% of the same DNA. The last 0.01%, however, is enough to make each of us unique — except in the case of monozygotic twins. As a result, the use of DNA to identify a suspect, or rule out a suspect, is an incredibly powerful tool for law enforcement and prosecutors alike.
The science of DNA profiling has come a long way since its birth in the 1980s. Although a very complicated process, DNA profiling basically looks at a few selected areas, called "loci" and compares them to each other to see if they match. Different jurisdictions use different methods and/or different numbers of loci. In the United States, the Federal Bureau of Investigation (FBI) uses 13 core loci when creating or comparing a DNA profile.
The commonly accepted method for determining the reliability of DNA profiling is known as the "product rule." When the product rule is used to determine the likelihood that a match of all 13 core loci is a random chance, the probability comes out to one in 15 quadrillion, give or take a few billion. Mathematicians, scientists, and statisticians, however, question whether or not the product rule should be used at all to determine the accuracy of DNA profiling. When other methods of determining its reliability are used, the accuracy rate of DNA profiling can drop substantially.
Of greater concern, however, than the science or math used to determine reliability when working with DNA evidence is the chance of human error or contamination. In many cases, a DNA sample passes through a number of hands from the site of retrieval to the courtroom, and the chance of error or contamination during the process may be significantly higher than the chance of a random, inaccurate match. While safeguards are usually in place to try and prevent the possibility of human error or contamination, the possibility exists nonetheless.
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