Technique opens avenues for police: Terry Kirby looks at murder investigations in which genetic fingerprinting has been used successfully

THE KILLER of Johanna Young, the 14-year-old girl murdered last month, is most likely to be one of the 3,500 males living around the small town of Watton in Norfolk, detectives believe.

At the moment, they are not saying - and probably do not know - whether scientists have created a genetic fingerprint from samples taken from her body. But if a print from a blood or semen sample is obtained, it would create the scenario for a mass genetic fingerprinting of the males in the area. If testing went ahead, it would have to be voluntary, but those who declined would merit special attention.

It was the attempt in 1987 by Colin Pitchfork to persuade a friend to impersonate him in the mass screening of males around the Leicestershire village of Narborough that led to his conviction for the murders of two teenage schoolgirls. The case was the first where genetic fingerprinting was used.

Mass screening can be carried out where the 'catchment area' of the suspect is clearly defined. It is unlikely to be of use in the investigation of the murder on Wimbledon Common, south-west London, last year of Rachel Nickell. But in that case and Johanna Young's, a genetic print can be used to confirm a suspect found by other means. Suspects can refuse to provide a sample, but where there is other circumstantial evidence this can be used against them as indication of guilt.

Attempts to solve murders and rapes more than a decade old can now be made as a result of DNA profiling. In 1978, when Candice Williams, a 13-year-old Birmingham schoolgirl was raped and strangled, Patrick Hasset, who lived close by, became a suspect because of earlier convictions for attacking girls. But his girlfriend provided an alibi and as police had no other evidence, Hasset was not charged.

In 1988, tests were conducted on swabs of semen taken from the body of the schoolgirl. The samples had been kept in frozen storage at the West Midlands Forensic Science Laboratory.

Detectives had to wait another three years because Hassett, in prison for other sexual offences, refused permission to take samples for matching; on release in 1991, he was re-arrested and consented to a hair root sample. The match was made and Hassett was jailed for life last year, 14 years after the murder.

Hair root samples are not as good as blood or semen for obtaining a DNA fingerprint, but Hassett's was one of a number of cases where convictions have been obtained. The first occured in 1988 when a man suspected of two rapes was convicted almost solely on the basis of a hair root sample match, said to be of a 1 in 150 million chance of being repeated.

Genetic fingerprinting can now be conducted 'by proxy'. In 1989, Ian Simms, 33, the landlord of a public house in the Lancashire village of Billinge, was convicted of the murder of Helen McCourt, 22, a computer operator, attacked on her way home from work.

Blood stains found on Simms's clothes were matched to genetic prints taken from Ms McCourt's parents; scientists estimated that the blood was 126,000 times more likely to come from a child of the McCourts than from anyone else. The case was only the third murder prosecution where the body was never found.