The Shortcomings Of Dna Evidence

[LazyGun]

There have been several posts, over the years, on the value of DNA testing for the determination of the guilt of a potential suspect.

TV series and various stories in papers have tended to reinforce the notion that DNA testing is virtually infallible.

Thought you might be interested in a rather disturbing story from France, which highlights both the benefits and the flaws of DNA evidence.

Although the DNA evidence was sufficient to narrow the suspects down, it is currently almost impossible to distinguish between 2 identical twins - and unless they can find some means to positively distinguish between them, the serial rapist is likely to be acquitted.......

http://www.popsci.com/science/article/2013-02/dna-twins-rape

[mrs_overall]

Perhaps they should both be in court - one of them is a rapist and the other will probably be committing perjury. If they were given identical sentences, that would be a just outcome IMO

[jake-the-peg]

How is one comitting perjury if they both accuse the other ?

I don't know the quality of jurors these days ;c)

[rojash]

"one of them is a rapist and the other will probably be committing perjury"

If one of them is a rapist, then the other is telling the truth.

[Barmaid]

I've never subscribed to the idea that DNA evidence is conclusive. This is a perfect example. If we take it one stage further, what if these identical twins had been separated at birth and brought up by different adoptive families, neither being aware of the other's existence?

In some cases I accept it can provide compelling evidence when taken with the other evidence. In others, I fear over reliance could lead to miscarriages of justice.

I've seen no end of posts on here where people say things such as "Now we have DNA evidence we can be 100% sure of guilt so bring back hanging". I couldn't disagree more.

[LazyGun]

@Barmaid - Yep, I agree.

[DangerUXD]

DNA is often portrayed as some sort on infallible inidicator means it is not and we should not be blinded by what is after all just some evidence. The presence of a persons DNA proves nothing beyond the fact that the DNA is present. DNA can get to a crime scene in a variety of ways. It does not even prove the person was at the scene. The twins issue is inherent in the problems of course but no more an issue to DNA testing than it is to video and photographic evidence. In these cases the investigators will have to use other means to establish the likely guilty or the likely innocent.

[Old_Geezer]

Without other evidence DNA can't even reduce the person's DNA to just two individuals. At the level it is checked (is checkable ?) even high certainty in a large population leaves a lot of possible DNA donors. It can only help to build on a case that is already a good one.

[Peter Pedant]

.

yeah OK Barmaid, not hanging

but what about bringing back flogging ?

[emmie]

it would worry me that people are convicted solely on DNA evidence, seeing as how one can leave DNA evidence any place. say you are in a cafe, there is a robbery, you are there, but nothing to do with the robbery, if someone had a mind for not nice reasons to fit you up, then your DNA is at a crime scene, they could swear blind you were involved, and no other witnesses are on hand, does this mean you could be implicated in a robbery that you had no part in, just because the DNA evidence says you were at the scene of a crime...

[pdq1]

I get a bit worried when they claim it will be accurate to one in a million. Based on the UK alone there could be 34 guilty people at large.

[DJHawkes]

in an episode of CSI they didn't rely on the DNA evidence when twins were suspects, they analysed the bloods and identified certain medication one had been taken that confirmed they were the killer as trace was recovered at the crime scene

[LazyGun]

~DJ - now, if only life imitated art :) It seems it is not going to be as simple as that, in this case at least...

[I_Hate_Infinity]

A similar set of circumstances involving twins occurred early last century in USA, though for different reasons. Two Siamese twin brothers, joined at the waist, eared their living as a 'freak show act' and one of the brothers took to drinking, becoming an alcoholic. The same brother then started a brawl in a bar injuring a man who then pressed charges of common assault. When it came to court, the judge ruled that the drunken twin was indeed guilty of the crime but ruled that it would amount for false imprisonment of the other brother if they were both sentenced to prison, so he let them both go.

I think DNA testing of genetic evidence found at the crime scene should only be part of a more rounded case for the prosecution, but it's undeniably strong evidence that the defendant was at the scene of the crime. In the case of identical twins you mention, DNA evidence should be ruled out to prevent a miscarriage of justice occurring, in my opinion. But interesting scenario :)

And I don't imagine a identity parade would help either!!

IHI

[jim360]

Cases like these are pretty unusual, so shouldn't really be used to shoot down DNA evidence. Of course it's important to recognise that the Science has limits, and while in the future DNA matching will only become more accurate, I really wanted to agree with Barmaid's post. No one piece of scientific evidence is ever 100% certain.

[Mojo-Jo-Jo]

I think it is patently ridiculous to dismiss DNA evidence as a valid tool when the only case in 30 years to throw a curve ball is in the case of identical twins.
I remember the very first use of DNA genetic fingerprinting because I lived there and my mum worked with the culprit.
It was right then and it's right now.
In fact, it's right in this case. They are genetic twins. Unless you've suddenly discovered a totally new gene strand I can't see any fault with the technique.

[Barmaid]

No one is dismissing it as a valid tool Mojo. Indeed, everyone seems to accept its value. I think what the OP was referring to is the numerous threads that have been on here saying "Now we have DNA, we can be 100% sure so we can bring back hanging". But DNA can never be 100% proof of guilt. It might put someone at the scene, it might put them in contact with the victim, it might place them in any number of circumstances, but that is as far as it goes.

[Mojo-Jo-Jo]

Having re-read the OP Barmaid I can't agree.
The OP questions whether DNA evidence is infallible yes, but this is a terrible example to use.
Of course the DNA is going to be identical with identical twins.
As for whether we should hang someone based on DNA?
I see no reason to hang anybody. Nothing is a given. Nothing.

[Barmaid]

But he also agrees with my post later on - that is, taken as a whole with the rest of the evidence, yes it may prove compelling. But over reliance could lead to miscarriages. Putting the emotive subject of corporal punishment aside, you can still never get 100%.

[Mojo-Jo-Jo]

I actually think we agree for the most part Barmaid.
Nothing in science is 100% but DNA is a damn good marker.
I still wouldn't want to see anyone executed based on it though.

[LazyGun]

@MoJoJo

Can you point to anyone here who is advocating dismissing DNA evidence entirely? Sorry, but that is not the point of the OP or the article I linked to at all.

It is important however that people appreciate the limitations of the current form of forensic DNA testing, and more widely the limitations of DNA evidence in securing a conviction. Most posters here have rightly said we should be wary of convicting based upon DNA evidence alone.

And while a case such as this is at the far end of what might be conidered unusual, the idea that we can have falsely matched DNA is not as unusual as watching programmes like CSI would have you believe....