DNA Technology in Forensics & Ethics
DNA technology is invaluable in forensic science, where investigators try to find clues to identify the perpetrator of a crime. DNA fingerprinting offers detectives the ability to compare DNA from samples at a murder scene to samples from suspects; in theory it's more accurate and reliable than many other methods. DNA technology has also caused controversy as well, especially where law enforcement agencies keep DNA samples on file from people who have neither been charged nor convicted.
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Older Techniques
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The first kind of DNA fingerprinting was RFLP or restriction fragment length polymorphism analysis. Investigators would slice a DNA sample up into smaller pieces using restriction enzymes, proteins that cut DNA, then separate the fragments by size. A radioactive DNA probe is added and will bind to a matching sequence in the sample. RFLP is a slow laborious process and requires large amounts of sample DNA; moreover, it's less accurate than more modern techniques, so it's largely been supplanted by STR analysis.
Modern Techniques
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DNA fingerprinting is typically done through a procedure called STR or short tandem repeats. There are parts of the genome where short sequences of 2-5 bases are repeated many times. The number of times these sequences are repeated varies widely between unrelated individuals; one person might have a sequence repeated 10 times, for example, while another has 50 repeats. Using a technique called polymerase chain reaction(PCR), investigators isolate specific STRs from a DNA sample then separate them by size and compare them to the size of STRs in another sample to see whether the two samples are from the same person. Thanks to PCR, even very small samples provide enough DNA for STR testing.
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Reliability
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The greater the number of STR regions analyzed, the more accurate the DNA fingerprinting test. For a typical test using 13 markers, the theoretical odds that two people will have identical DNA profiles are thought to be between 1 in 10 billion and 1 in several trillion. The frequency of different STRs can vary from one population or ethnic group to another, however, and if the procedure is improperly performed the reliability will naturally be far less. A contaminated sample can also yield false results; this is most often a problem in murder cases where the samples may include DNA from both victim and perpetrator.
Controversy
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DNA fingerprinting is often used to determine whether a sample from a crime scene matches a sample from a suspect. Some critics have expressed concern that jurors may not understand the science involved or may overestimate the reliability of forensic techniques. TV series have popularized some aspects of DNA fingerprinting in forensics and may leave jurors with the belief that these techniques are infallible. According to a 2008 survey cited by the Economist, 62 percent of defense lawyers and 69 percent of judges polled thought jurors had unrealistic expectations of forensic science.
Overturned Convictions
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DNA fingerprinting has been used to exonerate a number of people believed to have been wrongfully convicted. In one case, a man named Earl Washington was sentenced to death in 1984 for the rape and murder of Rebecca Williams. After 17 years in jail, DNA fingerprinting showed Washington was innocent. He was released in 2001.
DNA databanks
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Some critics contend that law enforcement agencies are storing DNA samples from people who have neither been charged nor convicted of a crime; they believe this practice constitutes a potential violation of civil liberties. In 2007, for example, an American Constitution Society briefing cited several cases of so-called DNA dragnets, where police agencies in the US or the UK had collected DNA samples from hundreds or even thousands of people in the course of investigating a crime. The same briefing noted that current law empowers enforcement agencies to keep DNA samples on file from people who are arrested even if they are never charged with a crime.
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References
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