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Forensic Biology · Tools
Combine STR allele frequencies into a random match probability.
heterozygote 2pq(1−θ), homozygote p² + p(1−p)θ, RMP = 1 ÷ Π(locus frequencies)
Allele frequencies per locus
0 applies plain Hardy–Weinberg. NRC II suggests 0.01 to 0.03 for structured populations.
Random match probability
1 in 39,62,784
Profile frequency = 2.523e-7 across 6 loci
Per-locus frequency
Multiplying across loci assumes they are in linkage equilibrium. That holds for the standard STR kits in general use, which is why they were selected.
The default frequencies are illustrative, not a population database. Real reporting requires allele frequencies from the relevant population, and the RMP says nothing about mixtures, relatives, degraded or partial profiles, drop-out, or laboratory error, each of which changes the interpretation entirely.
A random match probability answers one question: how often would this profile be expected in a population of unrelated people? Under Hardy-Weinberg equilibrium a heterozygous genotype has frequency 2pq and a homozygous one p². Because the standard STR loci are on different chromosomes or far enough apart to be in linkage equilibrium, the per-locus frequencies multiply.
Real populations are not perfectly randomly mating, so NRC II recommends a theta correction, typically between 0.01 and 0.03, which makes homozygotes slightly more common and the overall estimate more conservative. Multiplying across 15 to 24 loci produces the very large numbers seen in casework reports.