Page last updated at 12:41 GMT, Friday, 19 March 2010

When criminals are caught by relatives' DNA

By Michael Stoddard
BBC News

A child will share exactly half of its parent's DNA profile

For 20 years Keith Davison thought he had got away with raping a 24-year-old waitress on the Isle of Wight.

Police had exhausted their investigations into the 1990 case. But a DNA sample taken from Davison's daughter when she was cautioned for a minor assault led to his eventual conviction.

A technique known as "familial searching" linked DNA from the crime scene with the DNA his daughter gave.

Davison was finally sentenced to eight years in jail at Portsmouth Crown Court on Friday.

He was also put on the sex offenders register for life.

The same technique last year helped police in Southampton find a DNA match from a 1979 crime scene.

In that case, David Lace, who killed himself in 1988, was named by police as the man responsible for the murder and rape of Teresa De Simone.

'Not foolproof'

Sean Hodgson had spent 27 years in jail for the crime. He was released after it was discovered DNA found at the scene did not match his, but was linked to Lace through a family member.

But how does familial searching work?

It was first used by the Forensic Science Service (FSS) in 2001 to solve a 29-year-old murder of three teenage girls in south Wales.

Keith Davison
DNA from Keith Davison's daughter linked him to the rape

DNA from the crime scene was linked to the family of Joe Kappen. His body was exhumed and his DNA proved a match to a profile from the crime scene.

A DNA profile is made up of 10 pairs of numbers, with parents and children sharing exactly half of the numbers.

However, DNA profiles of numerous unrelated people could also share exactly half of the numbers, major crime specialist adviser at the FSS Dr Colin Dark said.

"We have inherited DNA from our parents, a child will have half from each one.

"But the technique is an intelligence tool; you cannot use the term foolproof.

"Police will set parameters, like location, age ranges and we will present a list which can contain hundreds of names with a parent-child match.

"Then maybe one of those names is known to police already.

"It is one piece of a giant intelligence jigsaw."

The technique has since been used in more than 200 cases, including the Davison conviction.

In his case, his daughter's DNA sample matched 50% of the DNA recovered from the rape scene.

Davison was then identified as living in Ryde, where the rape took place, leading to his arrest.

Police on the Isle of Wight admitted their inquiry was at a dead-end until the DNA breakthrough.

David Lace
David Lace's body was exhumed and his DNA proved a match

Det Ch Insp Bob Maker, head of CID, said: "At the time there were no leads which led to the suspect.

"We managed to gather a full DNA profile from a sample at the scene of the crime in 2004, but there wasn't any positive DNA hits on the database at the time."

It was two years later when a sample from Davison's daughter showed a link and he was arrested.

"This has been one of the most high profile cases, in DNA terms, in Hampshire but we must remember the DNA database is not only used to convict people," Det Ch Insp Maker added.

"It also goes some way to disprove someone's involvement in a crime.

"It is a tool we use, but a DNA hit does not mean a conviction. There is an awful amount of work which still goes into a case."

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