http://www.washingtonmonthly.com/features/2010/1003.bobelian.html
Dan Krane, a molecular biologist at Wright State University and a leading critic of the governments stance on DNA evidence, agrees. "There is a public perception that DNA profiles are black and white," he told me. "The reality is that easily in half of all casesnamely, those where the samples are mixed or
degradedthere is the potential for subjectivity."
Studies of DNA databases elsewhere have revealed similar findings. In 2006, for instance, Illinois officials searched the state's offender database, which at the time contained 233,000 profiles.
They found 903 pairs with nine or more matching DNA markers. Among geneticists and statisticians, these findings have eroded faith in the FBIs DNA rarity statistics, which were based on data from just 200 or 300 people and are used by crime labs across the country.
Heres my understanding and others may feel compelled to jump in. (No dna expert, just reading about it :dunno
. The fewer the markers, the more matches can be potentially found in these huge dna databases. It was a warning echoed by developer of dna profiling - Sir Alec Jeffreys. So if a match were found, and this was a sex offender who hated JR, and he could be traced to Denver/Boulder, well then a likely case could be built. Without the other evidence however, it may just be a chance match to someone who may have had nothing whatsoever to do with the crime.
And another dimension about transfer of dna:
http://www.lawofficer.com/article/needs-tags-columns/transfer-theory-forensic-dna-a Scientists do not yet know with certainty how much secondary or even tertiary transfer can or should be considered. Chances exist of putting an innocent persons dna somewhere it shouldnt be.