DNA profiling is widely used in legal systems, but its reliability is questioned by scientists and statisticians. DNA contains unique information, making it a powerful tool for identifying suspects. The process compares selected areas of DNA to determine a match, with the FBI using 13 core loci. The commonly accepted method for determining reliability is the ‘product rule’, which estimates a one in 15 quadrillion chance of a random match. However, other methods can significantly reduce accuracy, and human error or contamination is also a concern.
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 exonerate previously convicted offenders, questions have arisen about the reliability of DNA profiling. The answer to the question “How reliable is DNA profiling?” Depends Who You Ask The figures typically used in courtrooms indicate the chance of a DNA profile match being a random or accidental match somewhere in the astronomical range of one in 15 quadrillion. Scientists, mathematicians and statisticians, however, question the accuracy of that figure – or anything close to it.
DNA is a nucleic acid that contains basically all the ingredients, or building blocks, for life. Humans share over 99.9% of the same DNA. That last 0.01%, however, is enough to make each of us unique, except in the case of monozygotic twins. As a result, using DNA to identify a suspect, or exclude a suspect, is an incredibly powerful tool for both law enforcement agencies and prosecutors.
The science of DNA profiling has come a long way since its inception in the 1980s. While 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 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 the DNA profile is known as the ‘product rule’. When the product rule is used to determine the probability that a match of all 13 core loci is a random possibility, the probability rises to one in 15 quadrillion, plus or minus a few billion. Mathematicians, scientists, and statisticians, however, question whether or not the product rule should be used to determine the accuracy of DNA profiling. When other methods are used to determine its reliability, the accuracy rate of the DNA profile can drop significantly.
Of greater concern, however, than the science or mathematics used to determine reliability when working with DNA evidence, is the possibility of human error or contamination. In many cases, a DNA sample goes through a number of hands from the retrieval site to the courtroom, and the chance of error or contamination during the process can be significantly greater than the chance of a random and inaccurate match. While there are usually safety measures in place to try and prevent the possibility of human error or contamination, the possibility still exists.
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