I don’t know that much about DNA tests. Hans from the Netherlands matched my first cousin on My Heritage. Because my own father had been overseas during WW2, I took the My Heritage and was NOT a match. Then he did Ancestry and found some matches. So I tracked them down and found his half sister. These companies do not share information.
So when they find DNA at a crime scene..who does the testing? How do they find family matches? Are they only in the company you paid to do it or do they have access? Too bad there wasn’t just one company that had all the information.
I think we have had very few killers tracked down by the DNA, so far. But, it is the beginning.
This is good enough for forensic DNA.
DNA Evidence: How It’s Done
For family matches, they need to translate the DNA (code) they have to the format that Gedmatch accepts, and then, download it into GEDMATCH. Formatting was a problem for the first (GSK) case, but not anymore. Then they load this kit (code, not material) into Gedmatch under a certain name and run it through, probably using "one-to-many" program, to find matches - relatives. The closer, the better. Then the genealogists build the tree upward for the match, to find a common ancestor (for me and my first cousins, it would be our grandparents, for example. For me and for my uncle once removed, it would be his grandparents and my great-grandparents...etc, for me and my second cousins, it would be our great-grandparents...you also need to know the line, so the closer the match, the better, as it is easier to sort things out). Usually depending on the percentage of the match, one can approximately tell the degree of the connection/separation.
AncestrySupport
When they come to the projected common ancestor, they start building a full tree for him, downwards. His kids, his grandkids, etc. And then try to sort out who of his descendants would fit - who lived in the area of the crime, was of the appropriate age, etc. I think for GSK, they found about three possible perps, and then collected DNA (from thrash bins) for all of them. One matched...
Now, I am not a genealogist, and I am straddled by my own tree (lack of information about one line, period). So I am far from the regular genealogy work, things might be more complicated. But this is how I understand it.
I wonder what happens if there is only a mitochondrial match. Meaning, that it is direct maternal line, as mitochondria get transmitted via the egg, so, from mother's side. It could rule out the suspects, if there is no match, and if the haplotype of the mitochondria DNA is rare, it probably would be very suspicious, if you get a match. But it is still complicated. As I have mentioned, my dad's mitotype is met in 1:20000 cases, if even that oftne. So the presence of a hair shaft of such a type at the crime scene would cast suspicion on him, his mother, his maternal grandmother, his sister, his sister's daughter, or her children. But not me, as I got my mitochondria from my mom. But if the mitotype is very common (like, pure H, without private mutations), chances are, many people might have it.