Touch DNA does not mean BK touched the sheath.
“In 85 percent of the samples in this particular study we detected DNA on the object from individuals who did not have direct contact with the object. Their DNA was transferred to the object by the person they had direct contact with,” said Latham.
New study says your DNA can show up at a crime scene even if you were never there
2 cents
MOO
Below is from the second study linked by @CKS. Sorry, I’m not sure which one links above.
Snipped, italicized, and bolded by me.
Previously, she had found that shaking hands for two minutes could transfer one person’s DNA to an object by way of the other person’s hand. But many critics said that two minutes is a very long, awkward handshake.
For her new experiments, she shortened the handshakes to as little as 10 seconds. (That’s still two to five times longer than a typical handshake.) And even that brief contact transfers DNA, her data show.
After people shook hands, one person from each pair picked up a knife. Cale’s team then swabbed the knife handle and tested for DNA. Even after a 10 second handshake, people who never touched the knife were a major source of DNA on the handle some one in every 14 times. Their DNA had been transferred to the knife when the person’s handshaking partner had grasped the handle.
DNA was transferred just once in every 14 times, from a 10-second handshake (longer than a normal handshake), person touched object immediately, and object was then swabbed for DNA immediately.
Interesting, yes, but those are not real world
This is from 9 years ago and involved 2 students conducting an unofficial experiment. DNA is way past a decade old study.
The study was a project by graduate students Cynthia Cale and Madison Earll. It looked at secondary DNA– whether it can be transferred to objects from someone who never touched the smoking gun.
You are not showing me anything that says BK did not touch the sheath and the defense has not filed any motions we know of that says BK did not touch the sheath.
The defense cannot tell the jury BK did not touch the sheath without evidence to back it up. The judge will tell the jury to only consider the evidence they saw at trial.
For the defense to say "maybe" BK did not touch the sheath is not evidence for the jury to consider.
2 Cents
You should search for posts from @10ofRods. Fantastic posts explaining this DNA deposit from a professional's viewpoint. I won't even embarrass myself by trying to recreate them here, except to say the kicker here is WHERE they found the dna. It appears the sheath was carefully cleaned and the DNA was found inside the snap. Not just laying there on the leather, or top of the snap, but inside the snap. Not a place it's would be deposited through incidental contact, or by the sheath being handled after someone shook BK's hand or picked up something he picked up.
Going back to the 9 year old study for a moment. It would be interesting to know how long it was between test subject A touching test subject B, and then depositing only test subject B's dna on something. I can't imagine that test subject A is going to shake hands with test subject B, then go on about their day, working, driving, going to the grocery, etc., and hours later leave test subject B's DNA in that snap. I hope that makes sense. I need more coffee.
Agree with @maskedwoman. @10ofRods’ posts on DNA are well worth finding and reading. S/he explains the scientific in a way even I can understand.