DNA evidence frees man after 38 years in prison

A British man who spent nearly four decades in prison for a murder he always said he didn’t commit has had his conviction overturned by the Court of Appeal. Peter Sullivan was jailed in 1987 for the killing of 21-year-old Diane Sindall in Merseyside. But now, new DNA evidence has cleared him - revealing someone else’s genetic material at the scene. We thought this was an ideal opportunity to learn a bit more about the science of DNA forensics, and Turi King - a world-renowned geneticist and expert on forensic DNA at the University of Bath - has been examining the case for us, beginning with her reaction to the story…

Turi - Oh gosh, I mean what a tremendous miscarriage of justice really and as a geneticist a bit alarming as to how long they've taken to bring DNA evidence to bear on the case really.

Chris - When we approach a case forensically and use DNA, how does it work?

Turi - What we're doing is that we're looking at little sections of our DNA that we know vary between individuals and they're quite variable. So they're very, very good for distinguishing people from one another and that technology has been available. So the very early days of it were sort of 1984 when DNA fingerprinting was first invented, but it required large amounts of DNA. But since then, you know, technology has been moving on and it's now possible to use quite small amounts of DNA to be able to determine whether or not a DNA sample comes from a particular individual or not.

Chris - Given the ubiquity of DNA and the sensitivity, as you're saying, of the techniques now, how do we make sure it's safe as a technology though?

Turi - That is a really important question. So it starts right from the crime scene in terms of collecting evidence and this happened in 1986. So very, very early days of being able to kind of sample for this sort of thing. You're worried about contamination with the DNA from another source such as the person who might have been collecting the DNA and I understand that they've ruled that out. And I think the other thing that's really important to say about genetic evidence is it's never taken in isolation. You have to take it alongside good old-fashioned police work. It's a strand of evidence that you bring to bear on the case.

Chris - How far back can we go with it? So in other words, if you've got a sample, a potentially a forensic sample, how long can that lay in an evidence locker and you could get that out and say, I can get a DNA signature from that and I can therefore prove that this person did or didn't have anything to do with a particular crime?

Turi - Ah, so it all depends on how it's stored. So DNA degrades and heat and water, moisture, those are the sorts of things that degrade it. But if something's frozen. So there's a really lovely case. It was around the Golden State killer case, a chap in California who'd been committing rapes and murders. And one of the samples that helped break that case was one that had been stored in a freezer for nearly 40 years. And they were able to go back and get a really good DNA sample from it and they could use that. I have to confess, I don't know what the time limits are, but I can tell you it will absolutely depend on how it's been stored.

Chris - So why do you think, going back to what we've been discussing, why has it taken so long to look again at the present case, given how long this poor guy, Peter Sullivan, has been locked up?

Turi - I am really surprised. Why has it taken this long? Because we've had really good technology around this in terms of doing DNA testing. So the National DNA Database was set up in 1995. So we've had various kind of kits that can be used for this for decades. I mean, the one that's used now, called DNA 17, has been available since 2014. I don't know why they've taken so long, because the technology has been around to do this for quite a number of years now.

Chris - And given that it has exonerated one person, that means that this case doesn't have a suspect now. So can DNA actually help there? Because you mentioned there is a DNA database. People's DNA when they've committed a range of crimes are just added to that, aren't they? So is now the approach that you go to that database and start just trawling to look for any matches or hits? Is that how it tends to work?

Turi - Well, so yes. I mean, my understanding is they've done that already with the DNA profile. They've had a look on the National DNA Database and they haven't got a hit, so they haven't got a match. But there's other methods that can be used. So there's something known as familial DNA searching. This is because you inherit half of your DNA from each of your parents. The little genetic variants that you've got, you'll share some of those with your siblings or aunts or uncles or this kind of thing. It's usually parents, children, or siblings that they look for. But they could do a search and go, is there anybody matching on this database that looks like it could be a close relative of the perpetrator? And my understanding is that they've done a sweep. So they have screened over 250 men who were suspects originally to see whether or not they're getting any DNA matches there. And of course, for me, given the work that I do on this television program called DNA Family Secrets, there's the possibility of forensic investigative genetic genealogy, but it's not something that's allowed in this country, but it is allowed in other countries.

Chris - And you sort of predicted or presaged my question, which was, it's been very en vogue in the last 10 years or so to find your family tree and send off samples of DNA to commercial entities who will then read bits of it for you. So there are now huge databases internationally of people and their families out there. Could some of this actually be used? Could those companies, if they've got the right legal rights, let's say, could they just offer them to the police and say, would you like to borrow our database to trawl through to look for crimes? Or can that not be done with the samples and the data that they have?

Turi - So there's a few things here. So the first of them is how good is the DNA sample? Because the genetic variation that you look at with DNA fingerprinting is different than what is used for these companies. Obviously, there's ethical issues around this because people who take these DNA tests, you know, for companies such as Ancestry and 23andMe, haven't consented to being part of a forensic investigation. So these companies don't allow the police to use their databases. But there are databases that are available that can be used by the police. So there's a couple of issues there. Is the DNA of sufficient quality to be able to do the DNA typing that you would use for investigative genetic genealogy? And then what databases can you use to do that? It's something that's not allowed in this country at the moment.