DNA profiling is a method used by forensic scientists to help identify a criminal suspect based upon his unique genetic code. If you watch some of the crime-scene shows that appear frequently on television, you would think that DNA profiling is infallible. (In fact, one prosecutor in a case that I tried referred to DNA as the "gold standard" of evidence.)
But is DNA profiling really infallible? The answer depends upon whether the individual doing the testing is qualified to do so and whether the testing methods that she uses are sound.
In 2005, the United States Congress passed a law authorizing the National Academy of Sciences to conduct a study on such areas of forensic science as fingerprint evidence, firearms examinations, and DNA evidence.
As regards DNA evidence, that study, while conceding the fact that the "probative power of DNA is high," also observed that "[a]lthough DNA laboratories are expected to conduct their examinations under stringent quality controlled environments, errors do occasionally occur. They usually involve situations in which interpretational ambiguities occur or in which samples were inappropriately processed and/or contaminated in the laboratory. Errors also can occur when there are limited amounts of DNA, which limits the amount of test information and increases the chance of misinterpretation. Casework reviews of [mitochondrial DNA] analysis suggest a wide range in the quality of testing results that include contamination, inexperience in interpreting mixtures, and differences in how a test is conducted" (italics added).
In some cases, DNA evidence is not all that helpful in solving a crime. For example, in a case in which a girlfriend accuses her boyfriend of having raped her, it would not be unusual to find the boyfriend's DNA inside the girlfriend's vagina if they had been routinely engaging in sexual intercourse prior to the alleged rape. However, if that same woman were raped by a stranger, then DNA evidence might well identify her attacker and thus help solve the crime.
When DNA evidence does make up an important part of the prosecution's case against one of my clients (such as in the case of a woman raped by a stranger mentioned above), I make it a point to have that DNA examined by my own expert in order to make sure that it does not, in fact, point to someone other than my client.