Short Tandem Repeat (STR)
Definition
Short repetitive DNA sequences scattered throughout the genome, with the number of repeats varying between individuals at each locus. DNA profiling compares repeat counts at multiple STR loci simultaneously, producing a profile whose random match probability is typically below one in a billion for a full panel.
- Structure
- Short repeat units (2-6 bp) repeated in tandem
- Variation
- Repeat count differs between individuals at each locus
- Typical panel
- 13-24 core loci, depending on jurisdiction's CODIS/DNA-17 standard
- Discrimination power
- Full-panel random match probability typically below 1 in a billion
- Detection method
- PCR amplification followed by capillary electrophoresis
Common questions
Why did STR analysis replace older serological methods?+
STR typing needs far less sample, works on degraded material, and gives a numerical match probability far more discriminating than ABO or enzyme typing, which could only exclude, not individualise, a source.
What happens to an STR profile when DNA is degraded?+
Larger loci amplify less reliably as DNA fragments shorten, producing partial profiles with dropout at some loci. Laboratories address this with mini-STR kits targeting shorter amplicons, or by reporting the profile as partial with an adjusted statistic.
Can two unrelated people share an STR profile by chance?+
For a full modern panel the random match probability is astronomically small, but partial profiles, close relatives, or complex mixtures raise the chance of a coincidental match, which is why match statistics are always calculated for the specific profile obtained, not assumed from the full-panel figure.
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