Thinking about DNA, or lack thereof, on the bike, the helmet, and the dart sheath cap. I'm hoping someone with some expertise can chime in, but here are my thoughts with knowledge gleaned from Google.
Unlike DNA from bodily fluids, touch DNA is very very delicate and rapidly deteriorates as it comes from skin cells transfered to an object by contact. Such DNA is minute, and quickly degrades, or can be blown away when exposed to the elements for any length of time.
This would account for the lack of Barry's DNA on the bike, the helmet, and the tranquilizer dart sheath cap.
It's understandable that some trace DNA could/would be retained on a porous surface like the bike seat and the inside of the bike helmet, as such surfaces provide some level of protection. But on smooth surfaces like the metal on a bike, a dart sheath cap, or the hard shell of a bike helmet, there is nothing to hold the skin cells in place.
The fact that Barry's DNA was ONLY found on the bike seat (porous surface) does NOT mean that he only held the seat and therefore he couldn't have thrown it down the hill. He could have, and I think did, touch the bike in multiple locations without a trace of DNA remaining as the bike was left outdoors for hours. Remember, Barry supposedly removed the bike from the Land Rover and he wouldn't have done so by only handling the bike seat. Same with the helmet.
The tranquilizer dart sheath cap went through a cycle of the washing machine and the dryer. Had it done this alone, no DNA would be found. But in both cases, it had company. Bed sheets and Barry's shorts were both found in the dryer along with the sheath cap. Both would contain DNA, most likely left from bodily fluids. Had the sheath cap been in contact mostly with the sheets, which is the most likely scenario (sheets seem to swallow everything in the dryer), it would make perfect sense that the surviving DNA found on the sheath cap would have been transferred from the sheets. This would perfectly match the DNA results that were found on the sheath cap.
With this long winded post, I'm hoping to persuade those who use the lack of Barry's DNA found on those objects as exculpatory evidence against the prosecution's case. It's not, and I hope I've shown why.
MOO