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

[LazyGun]

And actually MoJo I reject your comment that the example given was a bad one. It is a rarely occuring but perfect example of the shortfalls of DNA evidence and an excellent example of why DNA evidence needs to be considered in conjunction with other evidentiary sources....

[Hopkirk]

Try pressing it then try to accelerate.

If it won't go any faster, then there is your answer.

[Hopkirk]

Oops.

[LazyGun]

LoL @ Hopkirk.

[Canary42]

The weakness in the pro-DNA argument isn't the validity of the DNA testing, it's on whether the presence of one person's DNA points the finger to their guilt.

Just think where you have left traces of your DNA over the last few years, and how that could be transported on. Still feeling confident in the method ?

This was brought home to me several years ago when reading a library book in bed, and what looked suspiciously like a pubic hair dropped out of the book.

Now suppose that person was a victim of rape say, how would I explain their pubic hair in my bed ? Coincidence ? Would a jury accept that ?

[Canary42]

P.S. Incidentally, I am an identical twin.

[LazyGun]

@Canary Interesting, the identifcal twin thing. And of course, you are right - DNA can get anywhere and everywhere.

It is important to note that DNA testing, or forensic DNA profiling, is not the same thing as full genome sequencing ; rather they look for a pattern match in one or two specific regions, rather like forensic fingerprint analysis. This is fine for 99.9 % of DNA profiling, since there is usually enough variability in the areas they actually examine to make a positive match, but it is not infallible, nor is it whole genome sequencing, which is what i think some people imagine DNA testing actually is, from the various media representations.

[chakka35]

Identical twins have identical DNA because they are produced from the same fertilised egg, which splits into two, both halves containing the same chromosomes from mother and father. No scientist would attempt to different such twins by their DNA because he knows it’s impossible.

Everyone’s DNA is unique, so it would certainly be possible to be 100% sure if the whole of the DNA were tested. But that would be prohibitively expensive, so sufficient DNA is tested to give a result of millions to one against a mistake. This is as reliable as any sort of evidence and much more reliable than some – including eye-witness testimony, for example, which is notoriously unreliable.