STR (Short Tandem Repeat)
Definition
A genomic locus where a 2, 7 base-pair unit is repeated in tandem a variable number of times between individuals. The variation in repeat number produces alleles that differ in length by one repeat unit; alleles are resolved by capillary electrophoresis. The operational standard for forensic identification from 1994 to the present.
- Motif size
- 2 to 6 base pairs
- Gold standard since
- 1994 to present
- Resolution method
- Capillary electrophoresis
- Basis of variation
- Variable number of repeats at the same locus
Common questions
What exactly is an STR?+
An STR is a stretch of DNA where a short motif of 2 to 6 base pairs repeats itself multiple times in a row. Different people have different numbers of these repeats at the same location, which makes STRs useful for telling individuals apart.
How do forensic labs use STRs to identify people?+
Labs analyze STRs at multiple locations across the genome. The pattern of repeat numbers at each location creates a profile that is nearly unique to each person. This profile can link biological evidence from a crime scene to a specific individual.
How are STR profiles analyzed in the lab?+
The number of repeats at each STR location differs by length. Capillary electrophoresis separates DNA fragments by size, allowing forensic scientists to count the exact number of repeats and determine each allele.
Related terms
- CODIS
- The FBI's Combined DNA Index System, the US national STR-based forensic DNA database. Contains profiles from convicted offenders, arrestees, and unidentified crime-scene...
- ABO Blood Group System
- Classification of red blood cells by the presence or absence of A and B antigens on the cell membrane, determined by alleles...
- Amelogenin
- A gene on both the X and Y chromosomes used for sex determination in forensic STR kits. The X-allele amplicon is approximately...
- Autosomal vs Mitochondrial DNA
- Autosomal DNA is nuclear, biparentally inherited, present as two copies per cell. Mitochondrial DNA is cytoplasmic, maternally inherited, present in hundreds to...
- BOLD Systems
- Barcode of Life Data System (University of Guelph, Canada). Holds over 9 million COI reference barcodes linked to voucher specimens, with mandatory...
- Capillary Electrophoresis
- Size separation of fluorescently labelled PCR products through a polymer-filled capillary on an instrument like the ABI 3500. Output is an electropherogram.
- Chromosome
- A discrete, condensed structure consisting of a single long DNA molecule complexed with histone proteins. Humans normally carry 46 chromosomes in 23...
- COI (Cytochrome C Oxidase Subunit I)
- The 648 bp mitochondrial gene region proposed by Paul Hebert in 2003 as the universal barcode locus. Its combination of interspecific variation...
- DNA Barcode
- A short, standardised sequence from a defined locus used to identify a specimen to species by comparison against a curated reference library....
- ElePhant Database
- A curated STR reference database for African savanna and forest elephants and Asian elephants, used to assign ivory to individual animals or...
- ESS (European Standard Set)
- The set of STR loci required for profiles entered into EU national databases participating in Prüm cross-border DNA exchange. Currently aligned with...
- Heterozygosity
- The proportion of individuals in a population who carry two different alleles at a locus. A locus with high heterozygosity is more...
Explained in these topics
- DNA Barcoding, STR Profiling, and Individual IdentificationA repetitive DNA sequence at which individuals differ in the number of repeat units. STR profiling at multiple loci generates a near-unique genotype that can l...
- DNA in Forensic Science: Structure, Genetic Markers, Extraction and ProfilingStretches of 2 to 6 base-pair motifs repeated head-to-tail. Length polymorphism at STR loci is the basis of modern forensic profiling.
- History and Development of Forensic BiologyShort repeated DNA sequences (typically 2 to 6 base pairs per repeat unit) found at specific loci throughout the genome. The number of repeats at each locus va...
- Polymorphism, Heredity and the Markers that Identify a PersonA genomic locus where a 2, 7 base-pair unit is repeated in tandem a variable number of times between individuals. The variation in repeat number produces allel...