DNA testing on the McStay Trooper, specifically the Steering Wheel, 4×4 gear shifter and AC/Radio controls, show Joseph McStay as a major DNA contributor, and Summer McStay was a trace DNA contributor.
Charles Ray Merritt was found to be a minor contributor to DNA on the steering wheel, & a trace contributor to DNA on the 4×4 gear shifter and AC/Radio controls.
(The McStay Isuzu Trooper was towed from the San Ysidro border four days after the McStays vanished. It is believed that the killer/s drove the Trooper from the McStay home and either immediately or later drove it to that location at the Mexican border . )
Charles Ray Merritt claimed he never drove the McStay Trooper, so the presence of his DNA is as a minor contributor on the steering wheel, and trace contributor on the 4×4 shifter, the radio and A/C panel was a clear red flag to SBC investigators. You see a ramp up in search warrants related to Charles Merritt, immediately following this discovery. See SBSW 14 1293 & 1294.
Below is the interior of a 1999 Isuzu Trooper. The 4×4 gear shifter is on the passenger side, as are controls for air and sound. For a tall guy sitting (6’2″) in the passenger seat which would normally be Summer’s seat (5’4″), it doesn’t seem all that remarkable that some of his DNA would end up in various places within the Trooper (not sure about the steering wheel). Merritt does reference the McStay Isuzu Trooper as the “small white car”, and given his height and weight, in comparison to both the McStays, this makes sense if Merritt is having to adjust the seat every time he rides as passenger. He’s a big guy, in a smallish space–he could have touched many items in the Trooper as he got the seat in the right position.
McStay and Merritt saw a lot of each other over the week the family went missing, and Merritt was the last person, living, who saw Joey, perhaps hours before he vanished. Could it just be that the two men shook hands or bro-hugged on departing from that last lunch at Chik fil a? Maybe the DNA is part transfer, part direct?
If Merritt’s fingerprints do not appear in the Trooper then how does his DNA get left behind? This was a cold time of year, it’s hard to imagine that skin cells sloughed off by way of perspiration.
It almost makes more sense that the presence of Merritt’s DNA was by way of contact with McStay, and that someone else drove that Trooper to the San Ysidro border, wearing gloves. And of course, that someone else could still be Merritt.
When people who spend this much time together touch similar objects or each other, it has been shown that DNA can show up on other objects, that one party never personally touched.
But how reliable is this evidence? What is touch DNA?