Thanks Carana, I haven't the time to read through loads of threads right now, can't you just describe what is the difference between alleles and markers?
I imagine most people have no idea, and unfortunately scientific explanations don't always help.
The easiest way to think of it is 1 marker = 1 pair of alleles.
Forensic DNA markers represent the specific sites (locations) used by labs to examine bits of genetic code, which are the alleles.
At each of these marker locations, there are two alleles (one comes from mummy and the other from daddy). What's confusing is that "marker" is often used to mean the pair of alleles.
In the UK, they examine 10 marker (locations), i.e. 20 alleles. In PT, they examine 15, i.e., 30 alleles.
They "measure" each allele and record its "length" (it's actually the number of times the bit of code repeats iteself). However, as the "length" will be within a tiny range of possibilities, even unrelated people will have a number of alleles of identical "length". In isolation, therefore, there is no way of knowing whether Allele A with an identical "length" (e.g. 9) is yours or mine.
Now, remember that there should be two alleles at each marker location. While you and I might share an identical Allele A, it is rarer that we will also share an identical Allele B, although that does happen.
If, at Marker 1, your reading is 7, 9 (the "length" of each of the two alleles) and my reading is also 7, 9, then we happen to share an identical marker.
So, move on to Marker 2. Your reading is 6, 7 and mine is 8, 9. That clearly differentiates us.
The problem arises when the sample is degraded (some of the alleles have gone AWOL) and / or contaminated (alleles from several people). You then get a soup of alleles some of which could be yours or could be mine and some that can't belong to either of us. And, evidently, family members will share far more of them than total strangers. And that was the issue in bootgate.