Modern DNA profiling doesn't read your whole genome. It reads a handful of specific, highly variable locations — and turns them into a short string of numbers that is, in practice, unique to you.
STRs: the markers we count
Profiling targets short tandem repeats (STRs) — places where a short DNA motif repeats a variable number of times. At each location (locus), the number of repeats is the "allele". Standard systems read around 16 or more loci plus a sex marker; older ones such as SGM+ used ten.
Reading the electropherogram
The laboratory separates the amplified DNA fragments by size and plots them as a graph of peaks — an electropherogram. Reading it is straightforward once you know the rules:
- each peak is an allele; its position gives the repeat number,
- two peaks at a locus mean the person inherited different alleles from each parent (heterozygous),
- one peak means they inherited the same allele from both (homozygous).
What a profile is
Because you have two copies of each locus (one per parent), each locus gives two numbers. Across all the loci, that's a couple of dozen numbers plus a sex indicator — and that short list is the DNA profile stored and compared.
What a "match" means
If two profiles differ anywhere, they are excluded — different people. If they match at every locus, the profile is so rare that, identical twins aside, they almost certainly share a source. Just remember the crucial caveat: the accompanying statistic is the frequency of the profile, not the probability of guilt.