I will try and figure out a really simple graphic. I am just at the part of my semester where I can explain it to my students (we are 6 weeks in, with 4 weeks on genes and DNA, so that now they know what an SNP is, and which regions of the genome are specifically human (although...they really KNOW know that yet, they're trusting me when I say that's what we're looking at - the rest of the course is going to be week by week, slowly getting what it means to be genetically human.
So, it's not like it's easy to explain or reduce.
But here's a math example:
Partial DNA = 112 339 889
Matched to CODIS suspect find = 112 669 848
It doesn't actually MATCH the 9 digit series. It contains PART of the 9 digit series
(In real life, it's coded as letters and there are tens of thousands of digits to match). Finding the exact number of "digits" used by CODIS is a long term project for any scholar - as they change the formula occasionally, and, well almost no one has the whole human genome (billions of digits?) memorized.
CODIS doesn't look at the whole sample or the whole human genome (neither do any of the IGG companies). Instead, it's SNP's (the places in the human genome where we have the most individual variation.
Everybody gets their SNP variations from two and only two people (their parents), who will be "close matches" (and that's what I'd call siblings, cousins, grandparents, and so on. All of us have alleles (specific genes) from our ancestors. Some of those alleles go back to before we were even H. sapiens. Some of those alleles, we may share with other species.
I am a "partial match" for a certain celebrity (whom I've always admired), we are something like fourth cousins, 3X removed. Yay? And I"m a "partial match" for a famous basketball coach (second cousin, once removed). I am a "partial match" for an Australian aborigine family.
I do not know and have never met the people I just mentioned. And I could go on - my own IGG results have about 5000 people I've turned up in a database, to whom I am related.
Maybe this will help. All people (ALL of them) on Planet Earth who have blue eyes have one of a handful of alleles that cause the formation of eyes that appear blue (there is no blue pigment, it's actually a lack of melanin and the Rayleigh light scattering effect that makes eyes appear blue). So ALL blue-eyed people are a partial match for each other!
We know approximately when and where that blue allele first mutated and it was in Northern Europe, and using the math behind the study of adaptive radiation, we know the epicenter and first appearance of the allele was near what is now Pskov, Russia. It appeared at ~8000 BP (before present). By 6000 BP, it was also in Denmark. From there it spread to Scandinavia (and it sure looks like the early inhabits of Denmark/Sweden/Norway were ALL blue-eyed, so direct descendants of those people from Pskov). Later, the blue-eyed Vikings (Scandinavian explorers and pirates) would return (with a Y chromosome pattern that's unique to them - but closely related to the other blue-eyed people) and go down the riverways of Eastern Europe, bringing blue eyes to those regions (and Northern Germany, and later, during the times of Rollo and Peppa (my own ancestors), into Alsace, Holland, Belgium - and during the Norman Conquest, into England. Vikings arrived earlier in Ireland and Scotland, btw, so slightly different (but closely matching) allele for those blue eyes.
ALL of these people are "partial matches" for each other, insofar as the codons in the alleles are all very similar.
CODIS looks at 18 or so single regions of human DNA and if a suspect sample matches ALL 18, it's considered a CODIS match (so then, they can go look at each of the full matches - and see if they have alibis).
Partial matches on fewer than 18 SNPs are not sufficient to warrant an investigation, IMO. If we decide to go there, as a society, we're doomed. And the FBI and LE agree with me. It's my understanding that this particular partial match wasn't even to a known felon, but to rape kits/sexual assault evidence in three different cities (where a FULL profile was gained from some unknown rapist, so he can be matched to himself in three places, but he's a PARTIAL (cousin) match to the glove box DNA - who is likely the lowly technician who worked at the auto dealership, writing in or handling the owner's manual and perhaps putting it inside the glove box after).
There was also male DNA on the helmet (which she bought from a man so of course there would be).
Clear as mud? Or better?
