Under the Microscope: How does familial DNA searching help solve crimes?

Answered by: Dr Colin Dark, Major Crime Consultant, Forensic Science Services

Asked by Kate Jenkins, Edinburgh

Analysing DNA

DNA is individual, person-specific; a blueprint for life, carrying all the genes we are made up of. What forensic scientists have done is to identify parts that we can analyse and use in comparisons for identifying criminals. We can't analyse it all – too time-consuming – but we look at 10 different areas of DNA that are known to be highly variable.

These 10 areas aren't genetic. They don't control anything like having brown eyes. They're the bits in-between. Think of DNA as being like a string with knots in it. The knots are the genes, and the string in-between simply connects them. But those bits are still DNA, so they follow the rules of inheritance: half come from your mum and half from your dad. Each of the 10 areas has two numbers. One number represents DNA from mum, one from dad. So a DNA profile is a set of 20 numbers, in 10 pairs, which can be added to a database, which can then be searched.

Complete matches and familial searching

So you can load up a crime-scene profile and the computer compares it with all the profiles on the database. If the profile matches an existing sample, you can give that information to the police. But if you've got a large DNA database and you're not getting a complete match, you can widen out the search for familial matches – so you can find the offender through following up parents or siblings with similar DNA who are on the database. This has a sociological basis: the assumption is that criminality will run in families. Also, families likely to commit crime are also likely to stay in the same geographical area.

If your crime sample is an exact half-match to another sample, that's a parent-child match. Then there's sibling comparison – a close match of 18 or 19 out of 20 numbers. We then pass maybe 100 of these profiles to the police, who can research and see if any is a good suspect.

Real-life uses

In the recent 'Grim Sleeper' case in Los Angeles, where a serial killer's DNA had no complete match on the DNA database, they used familial searching to find his son. The police observed that the father [Lonnie David Franklin, charged this month on 10 counts of murder] was a good suspect, and they took a sample from a cup he had used to get a DNA match that proved the case.

DNA was first used in crime-solving back in the late 1980s. But familial searching really came into use in 2000. Mine was the first case to be solved using this technique, in 2001. It was the murder of three girls in 1973 in South Wales.

I got involved in the 1990s, but it wasn't until we had the database search that we really made progress. We were getting no direct matches, so we hit on the idea of trying to find a relative. It was all done by hand as we didn't have a computer programme then. But we whittled down a list of about 100 close matches and realised there was a strong suspect. He was deceased but relatives volunteered their DNA, so we were able to do a reverse paternity test, which made it look very likely it was him. We eventually exhumed the suspect's body, and found a complete match.

As long as you have a DNA sample from the crime scene, there's no time limit. I'm working on a case from 1946 now, the murder of a 12-year-old girl. We've got a DNA sample, and we're trying to find a match that will lead us to the offender's grandson. We've analysed the Y-chromo-some from the semen, which will be identical to his son and his grandson. As long as there's a male line, you can just keep going.

Benefits and limitations

Familial searching is an intelligence tool – not proof. It's extremely valuable in cases involving serial offenders, murders or sexual cases. But there's a certain amount of police work needed into the family background. Familial searching is used in countries that have decent-sized databases; in Britain, there are 5.6 million people on the National DNA database.

The decision to use it is case-dependent. There are no guidelines for what it can be used for, but it is a time-consuming exercise. Also, because it's an intelligence service, its use has to be approved at the highest police level. There's only so much you can do with familial searching – it's limited to parents and siblings. You couldn't search for someone's aunt, say, as in that case only a quarter of the DNA is shared.

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