BS: How can DNA convict a person?

29 May 11 - 06:06 PM (#3162300)
Subject: BS: How can DNA convict a person?
From: gnu

There is talk of the French minister recently accused of sexual assult being convicted on DNA evidence.

I would like to know how that is possible. OJ could not be convicted based on DNA evidence. The OJ prosecution presented the DNA evidence which indicated he and approximately 85,000 other people in the US could have done it. WTF is that?

DNA evidence can exonerate but how can it convict?

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29 May 11 - 06:12 PM (#3162302)
Subject: RE: BS: How can DNA convict a person?
From: Greg F.

DNA evidence can exonerate but how can it convict?

Some levels of abysmal ignorance simply cannot be dealt with effectively on a web forum.

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29 May 11 - 06:18 PM (#3162305)
Subject: RE: BS: How can DNA convict a person?
From: CET

OJ was proved guilty 9 ways from Sunday. The reason he was acquitted was partly because the prosecution was not handled well, but mostly because the jury refused to do their duty. There was overwhelming evidence of guilt quite apart from the DNA evidence. You should read Vincent Bugliosi's book on the trial.

It is very early days to be talking about DSK being 'convicted' of anything. We have only press reports about samples, and nothing has been presented in court. However, with a suitable sample and modern techniques identity can be proved to a much higher degree of certainty than 1 in 85,000. I can't explain the science behind it unfortunately.

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29 May 11 - 06:48 PM (#3162313)
Subject: RE: BS: How can DNA convict a person?
From: GUEST,Jon

Greg, please enlighten gnu and I.

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29 May 11 - 06:54 PM (#3162315)
Subject: RE: BS: How can DNA convict a person?
From: gnu

"Some levels of abysmal ignorance simply cannot be dealt with effectively on a web forum."

That sounds like you are calling me ignorant. Prove it or... you know.

As far as the jury not convicting OJ... I wouldn't have voted guilty either, and not just because he was not proven guilty.

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29 May 11 - 07:53 PM (#3162341)
Subject: RE: BS: How can DNA convict a person?
From: GUEST,Eliza

I believe that DNA profiling can determine identity, as one's DNA is unique, at least, they say, 33million-to-one certainty. The problems arise when the sample hasn't been properly collected, is contaminated or insufficient in quantity to analyse with any degree of success. Also, the presence of someone's DNA doesn't exclude he possibility of other individuals having being present, thus one cannot necessarily say the DNA's owner was the actual perpetrator of the crime, merely that at some point, they were there.

"abysmal ignorance..." Greg?? Don't understand why a straightforward question should provoke that response! Rather arrogant of you, IMO.

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29 May 11 - 08:14 PM (#3162355)
Subject: RE: BS: How can DNA convict a person?
From: CET

Gnu:

I can only refer you to Bugliosi's book. I don't have the time or inclination to restate the evidence from OJ's trial.

However, your statement that you would not have voted guilty 'and not just because he was not proven guilty' is astounding and I hope you didn't really intend the literal meaning of those words. What you said, in effect, that even if he had been proven guilty, you would still have voted to acquit. If guilt had been established beyond a reasonable doubt, it would have been your duty to convict.

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29 May 11 - 08:14 PM (#3162356)
Subject: RE: BS: How can DNA convict a person?
From: DrugCrazed

The misconception here is that the DNA evidence has a 1 in 85000 chance of getting the right man. Going from a vague recollection of Ben Goldacre's book "Bad Science", if the evidence is wrong then there is are 84999 other people the evidence points to.

I can't fully remember though.

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29 May 11 - 08:16 PM (#3162357)
Subject: RE: BS: How can DNA convict a person?
From: DrugCrazed

Also, OJ's trial was a fair few years ago. We'll have got better at it since then.

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29 May 11 - 08:50 PM (#3162363)
Subject: RE: BS: How can DNA convict a person?
From: Bill D

Two members of OJs jury admitted later that they INTENDED to acquit him from the start...no matter what the evidence. You can present DNA evidence to the Nth degree, and if the jury simply closes their ears, there's not much else you can do.

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29 May 11 - 09:56 PM (#3162390)
Subject: RE: BS: How can DNA convict a person?
From: Little Robyn

But if the poor girl has someone's sperm inside her, DNA testing can certainly nail (or free) the person she accuses.
Slightly different from OJ.
These days the scientists have started checking on old cases because the testing is so much better. And I believe they're picking up people who have escaped justice for many years, putting rapists behind bars where they can't hurt innocent people any more.
Robyn

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29 May 11 - 10:17 PM (#3162394)
Subject: RE: BS: How can DNA convict a person?
From: Joe Offer

DNA evidence can prove that a person was at a given location. Whether DNA can prove guilt, depends on the nature of the crime. Rape with transfer of sperm can provide damning DNA evidence.
This page gives pretty good information. This Wikipedia article is pretty good, too. Oh, and one more (click).

-Joe, retired investigator-

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29 May 11 - 11:08 PM (#3162398)
Subject: RE: BS: How can DNA convict a person?
From: wysiwyg

DNA does not convict. People do. People are fallible. See?

~S~

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29 May 11 - 11:32 PM (#3162403)
Subject: RE: BS: How can DNA convict a person?
From: Kent Davis

Gnu,

Assume that you have a DNA sample from an unknown assailant and further assume that DNA testing shows that "approximately 85,000 other people in the US could have" left the DNA in question. Given the population, that is the same as saying that there is roughly one chance in 4000 chance that an INNOCENT person picked randomly would match that DNA. Which is to say that, about 3,999 times out of 4000, the DNA of an innocent person picked randomly would NOT match.

If there were no other evidence EXCEPT the DNA, such evidence would not be sufficient to convict. Suppose, however, that based on other evidence, you already suspected X was the assailant. Based on this suspicion, you test X's DNA. If you find a match, the odds are increased that X is really the assailant. Exactly what those odds are depends on the other evidence.

Hope this helps.


Kent

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29 May 11 - 11:50 PM (#3162404)
Subject: RE: BS: How can DNA convict a person?
From: Kent Davis

Gnu,

I hasten to add that the figure of "approximately 85,000 people in the U.S." was a hypothetical chosen only because you used that figure. The real figure would be better than that for a good sample (because of better technology now than during Mr. Simpson's trial), but could be worse than that for a degraded or contaminated sample.

Kent

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30 May 11 - 01:23 AM (#3162423)
Subject: RE: BS: How can DNA convict a person?
From: GUEST,Jon

I don't see that the technology itself mattered in OJ's trial. If there is doubt about the handling of the sample (in this case, known to have been carried around in someone's lab coat pocket for a day) and the case involves the testimony of a lying and racist policeman (copper later done for perjury even), I think it unlikely that I'd be finding anyone guilty.

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30 May 11 - 02:29 AM (#3162432)
Subject: RE: BS: How can DNA convict a person?
From: GUEST,Alan Whittle

I was in the boot Inn in Weymouth and I was earwigging this conversation on the next table.

This guy had been arrested , given a blood test - and he was feeling quite chipper - he knew the DNA test would rule him out as a suspect for this crime the cops were investigating.

However when he was a young tearaway - twenty years before - he had broken into somewhere and cos he didn't like the people he was burglarising, he had shit on the bed. he was amazed to find the cops had got him bang to rights for this earlier indiscretion. The DNA of the phantom shitter had been put on a database.

he got 12 months.

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30 May 11 - 03:10 AM (#3162448)
Subject: RE: BS: How can DNA convict a person?
From: Musket

As an aside, I used to drink in The Boot years ago, when I had family living in Weymouth.

Anyway, onto the thread..

My understanding would be that a footprint would indicate some shoes you own, add the red threads from a cardigan that you own and the footprints being heavier on the left side than the right, indicating a back problem.

Any one of these could indicate lots of people. Add them together and Sherlock Holmes would call it elementary. DNA is a bit like that but with many more variables to cut down the likelihood till you are left beyond reasonable doubt that it narrows to one person.

Or I've been reading too many books again.

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30 May 11 - 03:50 AM (#3162464)
Subject: RE: BS: How can DNA convict a person?
From: GUEST,Paul Burke

First, as others have pointed out, Simpson was acquitted because of a racially biased jury. Second, that was in 1995, and DNA tests have improved over the last 16 years of research. In particular, tests are much faster and more sensitive, and test more of the DNA, thus decreasing the probability of a false match. Thirdly, if you have a DNA match that could apply to, say, 10000 other people, but you have evidence that one of those people was at the scene, you can simply calculate the odds that another matching person was present at the same time.

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30 May 11 - 03:53 AM (#3162467)
Subject: RE: BS: How can DNA convict a person?
From: DMcG

DNA evidence can prove that a person was at a given location.

Absolutely crucially, that is not the case, Joe. It proves their DNA was at the crime, not that they were. For example, here's a bit about the current state of DNA testing:

=======
One area of confusion is the difference between touch DNA and Low Copy Number (LCN) DNA. With LCN DNA, forensic scientists are able to analyse a very tiny sample of DNA, with anywhere from five to twenty cells.

However, a touch DNA sample will be analysed in much the same manner as a sample from blood or another common type of sample. It is also generally well recognised in a criminal investigation that goes to court proceedings. Commonly, PCR analysis is used to process a touch DNA sample.


Pinpointing a Suspect
So long as a form of contact occurs, there is virtually always an exchange of some kind. Assuming that during a crime, the person committing the crime transfers a reasonable amount of skin cells on some object at the scene of the crime – and if the item is identified and collected rapidly after the crime – then DNA analysis can yield information about the criminal.

=======

So if you have touched the victim, there's some chance your DNA will be found on them. Perhaps you both needed to get to bar or through a turnstile and jostled against each other. Maybe the victim left a party and gave several people a farewell kiss; their DNA will be present. I'm sure you can think of others. These are the sort of things that the victim may well do with dozens of strangers or near-strangers every day, and they illustrate that having someone's DNA at a crime scene is not quite as strong evidence as it appears at first sight.

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30 May 11 - 03:58 AM (#3162468)
Subject: RE: BS: How can DNA convict a person?
From: GUEST,Peter Laban

That very much depends on the form in which the DNA is found on the scene doesn't it?

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30 May 11 - 04:05 AM (#3162470)
Subject: RE: BS: How can DNA convict a person?
From: DMcG

Well, yes, some forms are more relevant than others, sperm being the most obvious example (but even there, it proves the couple had intercourse recently but not necessarily at the crime scene, and it would need other evidence to sort out the timing).

I'm not dismissing DNA evidence by any means - it is incredibly powerful. But it still needs to be backed up by lots of other evidence.

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30 May 11 - 04:17 AM (#3162473)
Subject: RE: BS: How can DNA convict a person?
From: MGM·Lion

Of course it does, DMcG; but I take it that the Crown Prosecution Service, or its equivalents elsewhere, are perfectly well aware of all the constraints and exceptions rubricated here, and will be sure to take all these possible objections into account before authorising any prosecution based solely on DNA evidence; if indeed any such prosecution might be contemplated. They are not idiots, you know, and really do know as much about the matter as some of the smartarses so triumphantly raising "well, what about?"s above.

~Michael~

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30 May 11 - 04:21 AM (#3162475)
Subject: RE: BS: How can DNA convict a person?
From: DMcG

I agree as far as the CPS and their equivalents are concerned. I don't think it follows that the popular media are so careful and so it behoves us lesser mortals to always bear that famous motto in mind "It ain't necessarily so" when they trumpet their cause of the moment, pro or anti some outcome.

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30 May 11 - 06:24 AM (#3162516)
Subject: RE: BS: How can DNA convict a person?
From: MGM·Lion

Indeed so again ~~ but, happily, theirs is not the say-so, for all their huffings'n'puffings!...

~M~

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30 May 11 - 06:53 AM (#3162530)
Subject: RE: BS: How can DNA convict a person?
From: GUEST,Jon

A couple of quotes from Wikipedia on the OJ Trial:

"What should have been the prosecution's strong point became their weak link amid accusations that bungling police technicians handled the blood samples with such a degree of incompetence as to render the delivery of accurate and reliable DNA results almost impossible."

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"In March,[LAPD detective] Fuhrman testified to finding blood marks on the driveway of Simpson's home, as well as a black leather glove on the premises, which had blood of both murder victims on it as well as Simpson's. Despite an aggressive cross-examination by F. Lee Bailey, Fuhrman denied on the stand that he was racist or had used the word "nigger" to describe black people in the 10 years prior to his testimony. But a few months later, the defense played audio tapes of Fuhrman repeatedly using the word – 41 times, in total. The tape had been made in 1986 by a young North Carolina screenwriter named Laura McKinny. She had interviewed Fuhrman for a screenplay she was developing on police officers. The Fuhrman tapes became one of the cornerstones of the defense's case that Fuhrman's testimony lacked credibility.

In September, Fuhrman was called back to the witness stand by the defense to answer more questions about the discovery of the blood marks and leather glove that he supposedly found on Simpson's property hours after the murders took place. When questioned by Simpson attorney Gerald Uelmen, Fuhrman, with his lawyer standing by his side, pleaded the Fifth Amendment against self-incrimination to avoid further questioning after his integrity was challenged at this point."

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30 May 11 - 07:04 AM (#3162538)
Subject: RE: BS: How can DNA convict a person?
From: GUEST,Alan Whittle

too bad OJ didn't shit on the bed as well, I guess that would have clinched it.

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30 May 11 - 07:13 AM (#3162542)
Subject: RE: BS: How can DNA convict a person?
From: MGM·Lion

Why am I reminded, no doubt entirely irrelevantly, of my interest in observing that the neglectful security man whose carelessness is the initial cause of the eponymous conflagration in The Towering Inferno was played by none other than ~~ guess who? Why, OJS himself.

~M~

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30 May 11 - 08:35 AM (#3162580)
Subject: RE: BS: How can DNA convict a person?
From: McGrath of Harlow

But of course only "a racially biased jury" could possibly have believed that Fuhrman was a racist, and therefore not to be trusted...

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30 May 11 - 08:50 AM (#3162584)
Subject: RE: BS: How can DNA convict a person?
From: GUEST,Jon

Exactly, McGrath....

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30 May 11 - 10:38 AM (#3162626)
Subject: RE: BS: How can DNA convict a person?
From: pdq

"...audio tapes of Fuhrman repeatedly using the word – 41 times, in total. The tape had been made in 1986 by a young North Carolina screenwriter named Laura McKinny. She had interviewed Fuhrman for a screenplay she was developing on police officers."

Mark Fuhrman was asked to help develope the character of a racist cop from North Carolina. The tape never had anything to do with the real personality of Mark Fuhrman, the LA cop. Demonizing him for that tape is a bit like sending an actor to prison for playing the part of Al Capone.

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30 May 11 - 12:41 PM (#3162674)
Subject: RE: BS: How can DNA convict a person?
From: Bill D

"...the testimony of a lying and racist policeman."

well, Mark Fuhrman was a flawed character...but to give him credit for FINDING that glove at the crime scene, not mentioning it to anyone else, wrapping it securely to keep from 'getting any blood on himself', then carrying it, hidden, TO Simpson's house when Simpson was not yet even a suspect and planting it and disposing of whatever he had wrapped it in....well, that gives even an ass of a cop too much credit.
MUCH easier to reason that Simpson dropped it while sneaking in to avoid being seen by the limo driver waiting in his driveway. And Kato HEARD a thump against the house where Fuhrman said he found the glove. Even a naughty cop can occasionally do something right.

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30 May 11 - 12:46 PM (#3162676)
Subject: RE: BS: How can DNA convict a person?
From: Bill D

Remember...all they had to do was plant 'doubt' in the minds of jurors with 'maybes' about the blood/DNA samples. The odds against any minor errors in handling of evidence actually causing contamination that falsely pointed to OJ are astronomical.... but if the jury accepts the possibility...and didn't intend to convict him anyway... there you are!

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30 May 11 - 01:33 PM (#3162694)
Subject: RE: BS: How can DNA convict a person?
From: McGrath of Harlow

The thing is, a reasonable doubt requires a jury member to vote to acquit, even if they think the accused is probably guilty.

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30 May 11 - 02:07 PM (#3162709)
Subject: RE: BS: How can DNA convict a person?
From: Bill D

Indeed... and they leave the lawyers to set the definition of 'reasonable' in jurors' heads.

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30 May 11 - 02:13 PM (#3162712)
Subject: RE: BS: How can DNA convict a person?
From: Bill D

Oh...and people sometimes say "it's only 'circumstantial' evidence."...but that's what MOST evidence is in these sort of trials. That's why they have trials. It is seldom that they have videos of the crime being committed. All the evidence, taken as a whole, even with 'some' doubt about a few bits, pointed directly at OJ Simpson. His lawyers succeeded in focusing on just a few items.

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30 May 11 - 02:20 PM (#3162720)
Subject: RE: BS: How can DNA convict a person?
From: McGrath of Harlow

It's the job of the judge to guide jurors on what "reasonable" means - and to slap down the lawyers on either side if they try to mislead the jury about that.

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30 May 11 - 03:29 PM (#3162759)
Subject: RE: BS: How can DNA convict a person?
From: Bill D

Yep...and then the judge, and the defendant are, finally, at the tender mercy of whatever the jurors choose to believe....or ignore.

I'm sure you know the old saying.."A trial is the way the criminal justice system goes about methodically finding out who has the better lawyers."

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30 May 11 - 03:30 PM (#3162760)
Subject: RE: BS: How can DNA convict a person?
From: MGM·Lion

Yes ~~ but the jury is perfectly entitled to ignore a judge's directions, wherever they may seem to lead: and some summings-up are a disgrace ~~ read Peter Wildeblood's Against the Law, a bit dated now esp since repeal of long overdue anti-homosexual legislation, but still worth a look as a warning to potential jurors that the judge himself is not always to be trusted either ~~ & NOBODY can contradict or even query the verdict they see fit to deliver, except by going thru formal appeals processes.

~M~

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31 May 11 - 11:05 AM (#3163116)
Subject: RE: BS: How can DNA convict a person?
From: Bee-dubya-ell

In all likelihood, DNA evidence won't have much bearing in the Strauss-Kahn case. DNA evidence can prove that physical contact of one sort or another took place, but it can't prove whether or not that contact constitutes assault. Strauss-Kahn's defense will be that whatever took place was consensual, not assault. If his lawyers can convince the jury that no crime took place, it doesn't matter how much DNA he left behind.

The only way I can see that DNA evidence would carry much incriminating value would be if she had his skin cells under her nails from attempting to fend him off and he had corresponding scratches on his body. Even then, the scratches would have to be on his face or other place where scratching wouldn't normally occur during consensual sex. Scratches on his back would only serve to prove his point, not hers.

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31 May 11 - 01:27 PM (#3163174)
Subject: RE: BS: How can DNA convict a person?
From: GUEST,Eliza

Is anyone here in favour of a National DNA Register? (ie everyone has to have their DNA recorded for identity purposes)

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31 May 11 - 02:06 PM (#3163198)
Subject: RE: BS: How can DNA convict a person?
From: MGM·Lion

Yes ~~ & fingerprints. Don't know why it hasn't always been done, the same for everybody and then how could anybody object ~~ unless they were up to something dodgy?

~M~

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31 May 11 - 02:16 PM (#3163203)
Subject: RE: BS: How can DNA convict a person?
From: gnu

AND, such a registry could have aided in such cases as OJ's.

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31 May 11 - 03:15 PM (#3163245)
Subject: RE: BS: How can DNA convict a person?
From: Bill D

Fingerprints, blood types, DNA, retinal patterns....all can help identify someone accurately in useful ways. DNA is now being used to map the patterns of human migration out of Africa for 2-3 million years.

Just identifying babies mixed up in hospitals is worth the effort.

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31 May 11 - 03:17 PM (#3163246)
Subject: RE: BS: How can DNA convict a person?
From: open mike

DNA is used to convict criminals every day on CSI, Law & Order, NCIS and other t.v. shows....also fingerprints...also even ear prints...the new way to i.d. someone...

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31 May 11 - 04:10 PM (#3163277)
Subject: RE: BS: How can DNA convict a person?
From: DMcG

While I think Greg F overstates the problem, those are genuine concerns. In a perfect system there's a good case for a national database, but there are no perfect systems and so the question is whether the benefits outweigh the risks. Like Greg, I don't think they do

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31 May 11 - 04:59 PM (#3163301)
Subject: RE: BS: How can DNA convict a person?
From: GUEST,Eliza

Interesting viewpoints. When I've asked people this question (about a DNA Register), they seem either to be VERY enthusiastic or VERY anti. Myself, I can see the great advantages in many fields, but I still hesitate. It's the State filing away our very identity to use at will. On balance, I say "No."

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31 May 11 - 05:21 PM (#3163326)
Subject: RE: BS: How can DNA convict a person?
From: Bill D

"...unfettered access to our personal information !!"

Personal information...you mean like who we really are?
   Seems to me that what I do, and who I consort with and what is in my bank account are not exactly what a sample of my DNA will reveal. I don't want my personal activities to be monitored, but I really don't care if I can be identified in case of an accident.

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31 May 11 - 05:22 PM (#3163327)
Subject: RE: BS: How can DNA convict a person?
From: MGM·Lion

Remember, Greg ~~ irony can be a very two-edged weapon!

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31 May 11 - 08:14 PM (#3163398)
Subject: RE: BS: How can DNA convict a person?
From: gnu

"AND, such a registry could have aided in such cases as OJ's."

I don't think that particulasr poster advocated such as per Greg's inference. It was merely a statement of fact.

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31 May 11 - 11:54 PM (#3163504)
Subject: RE: BS: How can DNA convict a person?
From: katlaughing

No information is secure, these days. I would not want a national register because of that.

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01 Jun 11 - 12:15 AM (#3163511)
Subject: RE: BS: How can DNA convict a person?
From: GUEST,Alan Whittle

I'm in favour of it. It would be a way of catching the sort of rapist who jumps out and assaults.

If detection was a certainty - we could focus our minds as to why we are turning out so many of these strange criminals and murderers.

DNA used to be a luxury item in the detectives toolkit. When it was first used in the Pitchfork murder case in Leicestershire, England - it took weeks and was very expensive.

About twenty years ago - i was amazed when the police tested our house for the burglars dna (he had cut his finger breaking into our house). It was completely routine by then. The detectives first line of enquiry almost.

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