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How DNA Profiling Works

Controversy in DNA Profiling

When DNA profiling was first used in criminal cases, it was often difficult for prosecutors and defense attorneys, as well as the experts they hired to testify, to explain the significance of their DNA match to the jury. Fingerprints are still considered by most people to be an ironclad way to identify someone, but an expert testifying about fingerprints discusses them in terms of "points of similarity." DNA matches are discussed in terms of statistical probability using what is currently known about DNA similarity within the general population. This often confused the jury or was interpreted incorrectly.

dna sequence exam
Rob Melnychuk /PhotoDisc/Getty Images
A scientist views an autoradiograph, which is one of the first methods of DNA profiling.

For example, an expert testifying about DNA profiling for the prosecution might say that the DNA profile created from the crime-scene evidence has a 4-to-5 probability (or 80 percent chance) of matching the DNA profile created from the defendant's sample. Saying that the probability of the match is 80 percent, however, is not the same thing as saying that the probability of the accused person's guilt is 80 percent.

On the other hand, an expert testifying about DNA profiling for the defense could say something like, "The likelihood that this person's DNA was found at the crime scene, but he did not commit the crime, is 1 in 10 (or 10 percent)." That isn't a very high probability, but it doesn't take into account the fact that the accused isn't just some random person plucked off the street. It's not likely that the DNA profile is the only reason why he or she was arrested for the crime. DNA is just one piece in a very large puzzle.

The DNA profiling and its interpretation have come under fire. RFLP analysis was in part discontinued because of the possibility for error. The risk of a coincidental match using RFLP is 1 in 100 billion. However, in laboratory settings, this risk is probably higher because technicians may misread similar patterns as identical or otherwise perform the analysis incorrectly. A 2002 study of accuracy of DNA laboratories in the United States conducted by the University of Texas showed that 1 in 100 profiles may give a false result.

STR analysis is not as subjective, but any DNA profile can give a false result if it is contaminated. Although there have been no documented cases of a laboratory worker intentionally contaminating a DNA sample, DNA samples have been contaminated or even faked by criminals in order to avoid prosecution. In 1992, Dr. John Schneeberger was accused of raping one of his patients while she was sedated. A DNA profile was created using the sample that he left on the victim. A profile from a sample of his blood did not match the crime-scene sample, and the case was closed. The victim persisted, and eventually Dr. Schneeberger was convicted after additional DNA samples showed a match. He was able to avoid the initial match by implanting a drain in his arm filled with another man's blood and an anticoagulant, and skillfully getting the technician who drew his blood to do so from that spot.

Ultimately, DNA profiling has proved to be an amazing tool. However, it's just one of the many tools used to find the truth in criminal investigations, genealogy searches and testing for disease. There is rarely a 100-percent certainty of anything.

For lots more information on DNA profiling and related topics, explore the links on the next page.