What the Genes Say
DNA (deoxyribonucleic acid), the basic chemical in human genes, tells the cell what it should become. Its use as evidence in court, introduced in 1986, is controversial—DNA has been ruled inadmissible in criminal cases in six stales and the District of Columbia—because the distinctions it shows are based on probability, not certainty.
"We may see a battle royal over DNA evidence in this case, "says William Thompson, a psychologist and lawyer, who is one of the nation's leading authorities on the use—and abuse—of DNA evidence in the courtroom. Thompson spoke with Chicago bureau chief Giovanna Breu from his office at the University of California, Irvine, where he is an associate professor of law and society.
Is every person's DNA unique?
By some estimates, 99.9 percent of human DNA molecules are the same. When we use DNA for identification purposes, the tests have to focus on the few areas where there are differences among people rather than the majority of areas where people are largely very similar.
Scientists identify those differentiating areas and measure their lengths. A set of these measurements is called a DNA profile or fingerprint. Conceptually, it is similar to measuring one's shoe size, belt size, coat size. In theory, if you measured enough parts of the body, no two individuals would be exactly alike. If you took only a few measurements, the probability that more people would match is a good bit higher.
Where do you find DNA in a crime case?
Blood, semen, hair—any cell with a nucleus will have DNA in it. Even saliva. DNA from saliva from an envelope was evidence in New York City's World Trade Center bombing case. As little as a single drop of blood will do.
How about a hair follicle such as the one found in the cap at the murder scene in the Simpson case?
It'll be an iffy proposition whether they can get enough DNA out of that hair. Apparently there's some variation among hair follicles as to how much DNA they contain. When we are talking about a single hair, we are clearly working at the very limits of this technology. It's much easier to get DNA out of blood.
How is it done?
The procedure has become pretty routine, although dealing with crime scene samples can be tricky. It's one thing to use fresh, clean laboratory samples. It's another to test something you scraped up off the sidewalk. Contamination by salt, chemicals or even denim can affect the test and make it more difficult to interpret. The human cells are exposed to chemicals that cause the DNA to spill out. Enzymes are used to divide the DNA into fragments, which are then sorted by length by an electric current. The result, a series of dark bands, which indicate the lengths of the fragments, take six to eight weeks to achieve.
Is this what the jurors will see?
Yes, they will see dark bands on X-ray film. If the bands, each of which represents a fragment of DNA, align closely enough, it is a match.
Can DNA fingerprinting indicate decisively guilt or innocence?
Only innocence. Establishing guilt is tougher. A mismatch between two samples can be conclusive evidence that they are from different people. We've had a dozen or more cases in which prisoners have won their release when DNA tests showed they were innocent. If, however, a match is found between two samples, experts can only cite statistics on how rarely these characteristics are found to match.
So why is the use of DNA as criminal evidence controversial?
The science behind the DNA testing is sound. However, there have been serious disagreements among scientists about just how rare those matching characteristics are. There is also debate over whether particular laboratory procedures are reliable or not. Human error in handling samples may produce false matches in two cases out of 100.
What do you think will happen in the O.J. Simpson case?
It is going to be quite a spectacle. It could be a pivotal case for DNA testing. California has been one of the stricter stales, saying that scientific evidence is admissible only if it is produced by a technique that is generally accepted as reliable by the scientific community. The Simpson case will really focus people's attention on what the strengths and limitations of DNA technology are. If we can draw national attention to these issues, it may be very positive for forensic science and science in general.
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