Dna 'Fingerprinting'

August 23, 1994|By Kim Clark

What is DNA "fingerprinting"?

Testers dissolve blood stains found at the scene of a crime in a test tube with a chemical that extracts DNA, which carries each individual's unique genetic code. They fix the DNA to a membrane, then soak it in a radioactive solution that sticks only where it finds certain genetic sequences.

Then, the technicians place a piece of X-ray film on the membrane, and stick the two in a deep freezer, which speeds up the radiation's exposure of the sequence's pattern on the film.

At Germantown-based Cellmark Diagnostics Inc., which performed the O. J. Simpson tests, the technicians then reapply four other radioactive solutions, which lock on to four other genetic sequences, to take a total of five snapshots of the sample's genes. Each solution has to be applied and frozen for a week.

At the end of five weeks, the technicians compare the X-ray exposure, which is called a DNA "fingerprint," of the sample's genetic sequence to a similar photograph of a genetic sequence taken from a suspect's blood or hair sample.

If they don't match, the two samples did not come from the same person.

If they do match, the scientists look up how often each of those patterns occur in other people to figure out the odds of any two people having the same five patterns.

How definitive is DNA "fingerprinting"?

The accuracy of DNA fingerprinting is under debate.

Opponents, including several scientists and attorneys who have been hired by the Simpson defense, point out that laboratories ++ have mistakenly matched samples from different people in the past. And the critics argue that the statistics the labs use to estimate the chances of a random match are optimistic. The odds of a random match could be as low as one in several thousand, the critics say.

But proponents, including Mark D. Stolorow, director of operations at Cellmark Diagnostics, say the tests are nearly infallible. He says that while the odds of each individual match are different, it usually boils down to one chance in several million that two samples from different people would have the same five sequences.

Are DNA "fingerprints" admissible in court?

In 28 states, including Maryland, legislatures have passed laws requiring courts to accept DNA testing, or courts have made definitive rulings accepting the tests.

The tests have already been used in the convictions of several hundred criminals across the nation. And the tests have been used to clear people improperly convicted.

But the courts in a handful of states -- most notably California -- are still skeptical about the reliability of the tests. Some California judges have allowed the tests as evidence.

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