"Mr Yovich also referred to a contamination event that occurred in a UK lab in 2007, when an ex-employee’s DNA was detected on a DNA test negative control blank 16 months after they had resigned...Having read the incident report, Justice Hall clarified with Mr Yovich during his cross-examination of Dr Whitaker that the contamination could have occurred while the employee was still in the lab, but was not detected until later."
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The quote above is from an article on the Claremont murder case. Either way the DNA persisted. I would point to the Annie Le case as an example of DNA persisting in a less than pristine environment. Ms. Lee was a graduate student who was murdered by Raymond Clark. Another person's DNA was found on one sample from her underwear.
You have to separate two things, DNA of an employee and DNA of a sample.
Unlike samples, employees tend to walk around and deposit their genetic material on everything they touch. This is unavoidable, there is your DNA on your mouse, phone, on the door knob you used, toilet seat, everywhere. This is a fact of life and forensic laboratories are well aware this is a possibility (and is a major reason why DNA testing is such a great tool). What likely happened here is that the employee in question left their DNA on a piece of equipment or consumables that wasn't used until many months later. DNA can persist there, this is not a question, but the object in question was not used, that's why the contamination wasn't picked up until much later.
If I had to guess I'd say it might've been a pipette that the employee liked to use but other employees for whatever reason did not. Over a year later someone used that pipette for whatever reason and found the contamination. Other possibilities include consumables that were ordered in bulk and the employee in question stored (and hanlded them impropertly) or something of that nature. It wasn't something routinely in use that just happened to deposit their DNA 16 months later, but it could've happened at any time prior. This doesn't happen, something was done differently in that run to all the others.
Samples, by the virtue of being, well "not alive", are much more inert and do not deposit themselves all over the place. This is especially true for a known sample contaminating an unknown sample, because there are multiple safeguards aimed at preventing exactly that scenario.
The point is, an employee leaving a trace of their DNA is not an indication the same could happen to a sample. The contamination probably happened outside of clean area where samples are handled. In either event it is not an indication of a known sample contaminating another known sample as being possible, let alone likely.
Again, random chance should be mentioned. Suppose all this happens, what are the odds that a known sample contaminating something that would implicate this particular person? A laboratory processes hundreds of unknown samples each week, if this two events happened months apart you're talking about odds of well under 0.01% - and that is assuming the cross-contamination did happen (which is infitesemal). It's far more likely some random sample unconnected to the case would be contaminated than this particular sample would.
I mention this because for this to have any merit at all, one would expect it happens in the laboratory with some regularity before it becomes relevant. If there is no indication such a contamination is a problem the laboratory grapples with, then police framing is a better explanation than accidental contamination.
McHrozni
