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.
Related terms
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- Anagen Phase
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- Chromosome
- A discrete, condensed structure consisting of a single long DNA molecule complexed with histone proteins. Humans normally carry 46 chromosomes in 23...
- Degradation
- The breakdown of DNA by enzymatic, chemical, or physical processes after biological material is deposited. Degradation produces fragmented DNA that may not...
- Discriminating Power
- The statistical ability of a typing system to distinguish one individual from another in a given population. Expressed as the probability that...
- External Contamination
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- Gene
- A segment of DNA that encodes a functional product, most commonly a protein but also non-coding RNA molecules. Protein-coding genes occupy roughly...
- Hair Shaft Cortex
- The main structural layer of the hair shaft, composed of tightly packed keratin filaments. It contains the melanin granules responsible for hair...
- Heterozygosity
- The proportion of individuals in a population who carry two different alleles at a locus. A locus with high heterozygosity is more...
- Leukocyte
- White blood cell. The forensically critical component of blood because it contains a nucleus with a full complement of genomic DNA. Leukocytes...
- Locus (Plural: Loci)
- A defined physical position on a chromosome. In forensic genetics, 'locus' usually refers to a specific STR site used in profiling. The...
Explained in these topics
- Blood as Biological EvidenceRepetitive DNA sequences at defined chromosomal loci that vary in the number of repeat units between individuals. Forensic STR profiling compares allele length...
- Chromosomes, Genes and the Human GenomeA DNA sequence consisting of a short motif (typically 2 to 6 base pairs) repeated in tandem multiple times. The number of repeats varies between individuals, m...
- Hair as a Source of DNA and Toxicological InformationRepetitive sequence motifs at specific chromosomal loci used for nuclear DNA profiling. STR profiles are individual-specific (except in identical twins) and ar...
- The Transition from Serology to DNA Profiling in Forensic Science