The Bra Clasp
Vecchiotti conceded in court that Sollecito's Y chromosome was on the clasp, although C-V beat around the bush in their report, and Hellmann ruled (against his own experts) that it could not be concluded to be Sollecito's profile.
Sollecito is a perfect autosomal match for the DNA found on the clasp. Ordinarily the autosomal profile alone is adequate to make a unique match. However because there is a such an overabundance of Meredith's DNA, some artifacts of her DNA are generated (e.g. stutter) which come close in height to the peaks which match Sollecito. In addition there *may* be fragments of DNA from other individuals on the clasp
These extra peaks allow C-V and others to argue that, theoretically, other people could be a match for the profile besides Sollecito. Depending on what is considered real and what is considered artifact, the number possible additional permutations varies widely. I don't know where the number 6-19 comes from. Vecchiotti suggested (wrongly) that the number is infinite. I'd be most comfortable between 0-2
The fact that we have a Y chromosome profile is very helpful, because Meredith's DNA is taken out of the equation along with any other female contaminants. The result is a profile that is much clearer.
Sollecito's DNA profile is vastly more abundant than any other male DNA on the clasp.
This disparity in peak heights makes additional permutations essentially impossible. So even though there is also some unidentified male DNA (versus artifact) on the Y profile, it is low copy number and we can forget about it.
The problem with Y chromosome DNA is that close male relatives usually share the same profile. Since the mom has no Y chrosome there is no reassortment of the Y chromosome during fertilization. It only changes through chance mutations during spermatogenesis