Forensics, DNA Fingerprinting and Human Origins

04 December 2005
Presented by Chris Smith, Kat Arney


This week we take a foray into forensics, as DI Alan Cook talks about how DNA is used to solve crime, Alec Jeffreys helps us brush up on how DNA fingerprinting works, Tamsin O'Connell describes how DNA can help track down human origins, and in Kitchen Science we have a live DNA fingerprinting race in which schools battle it out to discover which of the Naked Scientists is the foul footed felon with the criminally smelly feet.

In this episode

Aging Secrets Are in The Cards...

A new set of trading cards from SAGE Crossroads offers a fun look at the brains behind the latest theories on aging and immortality. Each card includes information about an anti-aging research scientist, and feature Alzheimer's researchers such as Rudy Tanzi, and people who believe aging is curable, such as Aubrey de Grey. The idea behind the cards is to raise awareness of the goals of aging science and the projects taking place across the globe. The cards are available online at, and are suitable for all ages...

- Reservoir of Ebola Discovered

Scientists track down the source of Ebola virus infections...

Reservoir of Ebola Discovered

Writing in this week's edition of Nature, Eric Leroy and his colleagues have solved a thirty year puzzle about the lethal Ebola virus - where it comes from, and where it goes when it's not causing outbreaks amongst humans and primates.

To track down Ebola's 'natural' host, the researchers set traps to catch all of the small animals that they could close to sites of Ebola outbreaks in Gabon.

In all they collected over 1000 specimens which they then analysed for signs of the virus.

The tests showed that three species of fruit bats were positive.

The researchers suspect that the virus could be finding its way into humans when hungry locals capture and eat infected bats, and are urging people to avoid this practice - so we'd better keep an eye on Ozzy Osbourne if he pays a visit to the Congo at some time in the future.

- How long does DNA evidence last?

The Naked Scientists spoke to Tamsin O'Connell, Alan Cook

How long does DNA evidence last?
with Tamsin O'Connell, Alan Cook

Tamsin O'Connell and Alan Cook discuss how long DNA evidence remains recoverable...

After how many years can you still find DNA evidence? Does it break down quite quickly?

Alan - Yes. When we're actually recovering things from crime scenes care and prevention of cross contamination is a big issue, but so is care of the exhibits once we've actually got them. We need to get them into fridges and freezers as quickly as possible so that scientists can go on and look at them.

Chris - In what ways does DNA get damaged or deteriorate with time?

Tamsin - For archaeologists, water is a real problem because it helps to break down the bonds within the DNA and eventually washes it away. Preservation of protein and the whole sample is also a problem, so we have to look for well preserved samples. These include things like mammoths found in the natural freezer, or permafrost, in Russia. Things in the desert are an absolute disaster because of seasonal rain and the fact that it's very dry. This helps the DNA break down easily as well.

Chris - How far back would we need to go to find the first humans?

Tamsin - To find modern Homo sapiens, you'd probably have to go back about 100 000 years to find the first Homo sapiens, which is not very far back in terms of human evolution. We split apart from the great apes about 6 million years ago, and there are various other hominin species in between them and modern Homo sapiens sapiens.

Chris - In the past 100 000 years or so we've had us, plus those hobbit people in Flores, and there are deposits of Neanderthal man until as recently as 20 000 years ago.

Kat - I think some of them are still hanging around in London actually!

DNA helix

- DNA Fingerprinting

The Naked Scientists spoke to Professor Sir Alec Jeffreys from the University of Leicester

DNA Fingerprinting
with Professor Sir Alec Jeffreys, University of Leicester

Professor Sir Alec Jeffreys, from the University of Leicester, discovered the process that we no call "DNA fingerprinting". Chris Smith heard how it works...

Chris - Thank you for joining us. What's involved in the process of genetic fingerprinting?

Alec - The technology has evolved quite a lot over 21 years since the first genetic fingerprint. What we do nowadays is we start with a genetic sample. This could be a little bit of blood, saliva or whatever. Firstly, we have to extract the DNA from that and then amplify up the DNA using a wonderful technology called the Polymerase Chain Reaction (PCR). We amplify lots of short pieces of DNA which we know vary a lot from one person to another. That variation is in terms of the length of the DNA. So the next stage is to separate those fragments of amplified DNA by size, measure the size of the fragments and then store the information on a database. Nowadays, most of this can be done by an automated process.

Chris - How much do we vary genetically from one person to the next?

Alec - We're all absolutely unique genetically unless you have an identical twin. In this case, you will be the same genetically. In terms of differences between any other two people, we're talking well into the many many millions of differences. In DNA fingerprinting, we don't look at the whole lot. All you need to do is to pick out ten (for the UK) highly variable sections of DNA.

Chris - How much DNA do you need to actually do a DNA fingerprint? Presumably if someone were to go through a dustbin and find a letter with a stamp on it that I have licked, they'll find DNA. Would it be sufficient in order to retrieve my genetic profile?

Alec - Yes, most certainly. We quite routinely carry out analysis on DNA from a single human cell or a single DNA molecule. We showed many years ago that you can extend that to DNA fingerprinting. It's very very sensitive.

Chris - So what are the chances of getting an incorrect result and potentially arresting the wrong person?

Alec - It's not zero. The so-called random match probability, which is the chance that your DNA profile is the same as mine, is about one part in ten trillion. Which is pretty low.

Chris - Now in addition to fingerprints, are we in the position to infer what people might look like on the basis of their DNA sequence?

Alec - There's a lot of interest in this. You can determine gender from DNA, and you can determine with reasonable accuracy whether the person is a red head or not. There is a test for eye colour, but it's pretty iffy actually. There are markers out there that can give you clues on ethnic origin, but not very strongly. That's really about it. All the kind of fanciful notions you can plough into DNA from a crime scene is, at the moment, science fiction. It's science fiction, for example in the case of facial features, because we really don't understand the genetic basis of how genes and their variation control the length of the nose or how much your ears stick out. I think there's also another issue. The current DNA systems use DNA variation which is actually pretty irrelevant to you as an individual. I think this stops there being any problems with genetic privacy. If the police start looking at very important genetic characters, such as ethnic origin or facial appearance, then I think that raises very important issues about genetic privacy, and the issue of whether the police should have access to that type of genetic information. I don't think they should.

- Would an object kept in a box for 70 years still carry a DNA fingerprint?

We have a picture of my wife's father in the police force in 1947. He's wearing a hat and holding a truncheon, which we now have in our p...

Would an object kept in a box for 70 years still carry a DNA fingerprint?

I think in this case detective work rather than forensic work is going to be more useful in finding out whether the items belonged to him. After all this time, I think it would be difficult to prove this. However, if these had come from a modern crime scene, there is actually something we can do with them. Hats are especially good because we can usually extract DNA from headbands. However, in this person's case, they'd be better off popping it along to the Essex Police museum and seeing if they can match up the item from the period with the photograph. Cross contamination is also a major problem for us when using DNA. DNA is great and helps us out a lot, but it also presents its own problems.

- How can someone extract and sequence DNA from something that's been buried for centuries?

How can someone extract and sequence DNA from something that's been buried for centuries?

How can someone extract and sequence DNA from something that's been buried for centuries?

Well it's actually extremely difficult. Cross contamination is a big problem, and so first of all you have to make sure that your sample is clean. If the person excavating it touched it, you don't want to end up extracting their DNA by accident. We bleach it to get rid of all the DNA on the outside and then have to try and find some cells inside the sample that are still whole. We then break those open and extract the DNA.

- How are DNA fingerprints made from samples left on clothing?

When you have DNA left on a piece of clothing, how do modern techniques match the DNA with the guilty suspect?

How are DNA fingerprints made from samples left on clothing?

There is a national DNA database. When offenders are brought in, we take a sample of their DNA and it goes onto this database. Later, we try and match stains from a crime scene to people on that database. Obviously this assumes that the DNA is already on the database. This means that unless we have a suspect, there'll be no match.

- Are solar panels on spacecraft mistaken for UFOs?

Are solar panels on spacecraft mistaken for UFOs sometimes? - Matthew has emailed us with an addition to an answer last week.

Are solar panels on spacecraft mistaken for UFOs?

On the 27th edition of the Naked Scientists, John from Clacton said he saw a bright flash in the sky, and wondered if it was a gamma ray burst. Our guest on the show, Mike Hobson, said it definitely couldn't be a gamma ray burst because our eyes aren't sensitive to gamma rays. However, without having seen it, he wasn't sure what it could have been. Matthew says that there are a network of satellites used for a satellite telephone system called Iridium. They're in a low Earth orbit and have large solar panels that can catch sun light and reflect it down to the Earth. They appear as a bright flash that lasts several seconds. This is a potential explanation for what John saw that night.

- Is there a single origin for Homo sapiens?

I've heard that there's a single origin for the emergence of Homo sapiens. Is there any scientific background to this claim?

Is there a single origin for Homo sapiens?

It's pretty difficult to say exactly where we came from, but the evidence is stacking up that we came from Africa, probably about 100 000 years ago. This is based on a combination of looking at the Y-chromosome in DNA, which is the male line, and also mitochondrial DNA, which you have to inherit from your mother. There's a lot more diversity in Africa than anywhere else, and it looks like we can track the migration of humans around the world. East Africa is looking pretty good as our origin.

- What are the chances of DNA fingerprinting being wrong?

What is the possibility of erroneous matching of DNA? There was a case I saw about 8 years ago in which a man was convicted. When the evi...

What are the chances of DNA fingerprinting being wrong?

With the number of strands we actually look at, it means that the chances of it being someone else are about one in a billion. The biggest problem that you have with DNA as opposed to something like a fingerprint, is that if your fingerprint is at a crime scene, there's no doubt that you've actually been there. With DNA, you have to eliminate the possibility that it was brought in by someone else and caused cross contamination of the evidence. It is a very real issue for us in investigations, but it something that we are always aware of and try to rule out.

- Can a bone marrow transplant affect a DNA test?

I've heard that bone marrow transplants can cause havoc with DNA testing. Is this the case, and if so, how can the problem be resolved?

Can a bone marrow transplant affect a DNA test?

You're right. A bone marrow transplant can cause a problem because what you essentially become is a mixture of two people. Someone takes away your normal bone marrow and replaces it with the bone marrow of somebody else in order to make up for the damage caused by your own bone marrow. This means that you potentially leave behind two DNA fingerprints at a crime scene and could even implicate your donor.

- Is there any such thing that could erase or change DNA?

Is there any such thing that could erase or change DNA?

Is there any such thing that could erase or change DNA?

You can definitely get rid of DNA because there are enzymes which the body makes and are found in the environment called DNAase enzymes. This means that they literally eat DNA and cut it up. So you can definitely get rid of DNA. Leaving DNA in the sun can also break it down. Changing DNA is actually very different. Changing happens in all of us due to a process called mutation, in which your DNA accidentally swaps a few of the DNA letters around. These letter swaps usually get fixed by DNA repair machinery within the cell. If it's not, it can sometimes lead to cancer. However, there's not actually a technique in which you can physically change DNA, say, at a crime scene, to implicate somebody. The best way to do that is to literally sprinkle some of their DNA at the scene.


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