I do Genetics me.
"Another issue is the statistics presented. A zero match would be enough to prove innocence but a 99.99% match doesn't prove guilt (even your brother will have a better match to you than that."
No match does not mean not guilty, it could mean several things:
- The sequence you're looking for in the DNA isn't present - try a different sequence.
- The DNA has been degraded with DNAse, and there's basically nothing left to identify.
- The person still comitted the crime, but this DNA isn't there's - find more samples!
And it doesn't work in a percentage-wize fashion. You wouldn't use inclusive matching to prove/elmininate a suspect, you would be exclusive - ie, you are either the person whos DNA this belongs to you or not. If the results aren't clear enough, then the lab will say so. They won't speculate.
(Fortunately, criminals leave more than enough DNA at the scene of the crime, which leads to very confident results, especially if the victim put up a fight - grow you're fingernails people!)
Your brother's DNA will be 99.99999% similar, but at the end of the day so is a chimps.
As you're only interested in that 0.000000001% difference, then you are technically only 50% similar (statistically) to your brother (50:50 chance of getting the same chromosome from your mother/father that your brother did) when it comes to the DNA that matters.
And if those odds arn't good enough for you, there are a whoooole bunch of other tests.
You can roughly tell a person's age from there telomere length, so if the brother is 60 and the suspect is 18, it will be very obvious that it isn't him - but a good DNA fingerprint test (which i assume you would use if someone's life is hanging on the results) will look for something in the sample that only one in a billion people will have - and perentage has nothing to do with it.
Just the random errors cells make when they undergo mieosis. These DNA replication errors are totally random.
Anyway, the point i'm trying to make is that DNA fingerprinting IS reliable and very effective if done correctly. You can't say that as 6 perps havn't been caught by fingerprinting, then the whole technique should just be dropped. It *has* caught hundreds of thousands of guilty people, and cleared thousands of other-wize convictable innocents.
Besides, it's not the technique that convicts people - a jury does that based on the evidence.
If they tried to convict someone who's DNA had appeared positive due to contamination, it would be quite obvious something has gone wrong.
"We found your DNA on the kitchen knife that killed your wife, and your family car which could have been used to transport the body was covered in it. A spade in your shed also contained your DNA, which could have been used to bury the victim's body."
"We also found sink and drain unblocker in your sinks and drains, presumably so you could eliminate the DNA evidence. What's more, our detectives noticed that you have Sky+ and have recorded several episodes of CSI Miami, dating as far back as season 1 recorded in 2003 - suggesting you have been planning the murder of your wife for at least 6 years..."
Forensics doesn't work like that. DNA fingerprinting is worthwhile, and really is very very accurate providing the source is of good quality.
So rest assured people, no one is going away for a crime they didn't commit simply because a lab assistant labeled a test tube incorrectly. If that ever does happen, a lab would quickly loose it's licence and go out of buisness when the inevitable re-trial comes around.
So i reiterate - grow your nails people!! xD