DNA fingerprints to aid sleuths.

It's elementary, my dear Watson. The DNA minisatellite probes show that only suspect X could be the rapist. All 15 bands in the DNA fingerprint of the recovered semen match those of his blood sample. The chance of error, Watson, is only 1 in 30 billion.

While DNA patterns have not yet condemned any criminals, forensic applications of molecular biology appear both imminent and powerful. "It is envisaged that DNA fingerprinting will revolutionize forensic biology particularly with regard to the identification of rape suspects," says geneticist Alec J. Jeffreys of the University of Leicester, U.K., and Peter Gill and David J. Werrett of the Home Office Forensic Science Service in Reading, U.K. In the Dec. 12 NATURE, they described new analyses of blood and semen samples.

Their technique is one of several that are being developed to identify individuals and their relationships by analyzing genes (SN: 8/31/85, p. 140). These methods have promise for more definite determinations of paternity and maternity, as well as forensic applications.

The new techniques take advantage of segments of human DNA that vary among individuals. Jeffreys and his colleagues Victoria Wilson and Swee Lay Thein have described a class of short segments that they call hypervariable minisatellite regions. Each segment contains a core sequence of 10 to 15 DNA subunits, known as base pairs, which is repeated many times. The number of these repeats varies from persons to person.

To analyze the hypervariable regions, the scientists enzymatically cut a sample of DNA into pieces and radioactively tag those containing minisatellite regions. This procedure creates a characteristic pattern of bands. Except for identical twins, even close relatives can be distinguished by these DNA "fingerprints." The patterns are inherited: Each parent contributes about half his or her bands to each offspring.

To demonstrate that these patterns can be valuable in forensic work. Gill, Jeffreys and Werrett produced DNA fingerprints from samples that might be available to a detective: 4-year-old bloodstains and semen stains on cloth. They also developed a method that might identify rapists. The investigators separated the nuclei of sperm from other material that is found in vaginal swabs taken more than 6 hours after intercourse. They then produced DNA fingerprints of the semen donor that match the DNA fingerprints produced from a blood sample.

"These preliminary results demonstrate that DNA fingerprints are capable of changing completely the emphasis of blood-grouping [biochemical identifications] in forensic science," say Gill, Jeffreys and Werrett. Currently, a series of tests can rule out suspects, but it cannot produce a positive identification. With the DNA testM using two types of minisatellite DNA, the chance of a mistaken identification is less than 5 X 10.sup.-.sup.19.

Although the potential s great, DNA fingerprints are not expected replace immediately the current battery of tests. The new methods still must be subjected to extensive family and population testing. Barbara E. Dodd of London Hospital Medical College says that the DNA fingerprint test, as JEffreys currently performs it, is very time-consuming and needs meticulous expertise.

In at least one case already DNA fingerprinting has been used to settle a dispute. Jeffreys helped a Ghanian woman living in the U.K. to convince immigration officials that a boy returning to the U.K. was her son and not the son of one of her sisters. The problem was especially difficult because the boy's father and the woman's sisters were not available to test. Jeffreys calculated that the chance that the woman is the body's aunt, rather than his mother, is less than 1 in 100,000. Jeffreys and his colleagues conclude, This difficult case demonstrates how DNA fingerprints can give unequivocal positive evidence of relationship, even in some cases where critical family members are missing."