Short Tandem Repeats (STRs) are segments of DNA consisting of short sequences, typically two to seven base pairs, repeated multiple times. The number of these repeats at specific locations varies considerably between individuals. This variability makes STRs a valuable tool for distinguishing one person’s DNA from another.
The Genetic Basis of STRs
Short Tandem Repeats are DNA sequences, typically 2 to 7 base pairs, repeated directly adjacent to each other. For example, a common repeat unit like ‘GATA’ might appear multiple times in a row. These repetitive segments reside at specific locations on a chromosome, known as a locus.
Most STRs are located in non-coding regions of the DNA, meaning they do not contain instructions for making proteins. Variations in their repeat numbers in these non-coding areas do not cause harm or disease. An ‘allele’ in the context of STRs refers to the specific number of repeats a person has at a given locus.
Inheritance and Individual Variation
Each individual inherits two alleles for every Short Tandem Repeat locus, one from each biological parent. For instance, if a mother has alleles with 7 and 9 repeats at a particular locus, and the father has alleles with 8 and 10 repeats, their child could inherit combinations like 7 and 8, 7 and 10, 9 and 8, or 9 and 10. This Mendelian inheritance pattern means a child’s STR profile is a direct combination of their parents’ profiles.
When scientists analyze multiple STR loci, the combined pattern of alleles creates a unique genetic profile for an individual. The chance of two unrelated people sharing the same complete STR profile across many loci is extremely small, often estimated to be less than one in a trillion trillion when over 20 loci are compared. This high variability allows for precise individual identification, with the exception of identical twins who share virtually identical DNA profiles.
Forensic DNA Analysis
Short Tandem Repeats are widely used in the criminal justice system for DNA analysis. The process begins with collecting a biological sample, such as blood, saliva, or skin cells, from a crime scene. DNA is then extracted from this sample.
Specific STR loci are then targeted and amplified using Polymerase Chain Reaction (PCR). PCR creates millions of copies of the STR regions, even from very small or degraded DNA samples. The amplified STR fragments are then separated by size, typically using capillary electrophoresis, which allows scientists to determine the exact number of repeats at each locus. This information generates a unique STR profile, which is then compared to DNA profiles of suspects or searched against large criminal DNA databases, such as the Combined DNA Index System (CODIS) in the United States.
Paternity Testing and Genealogy
Short Tandem Repeats are also used in establishing biological relationships, such as in paternity testing. Since a child inherits one allele for each STR locus from each parent, comparing the STR profiles of a child, mother, and alleged father can confirm or exclude a biological link with high accuracy. A match across a sufficient number of STR markers indicates a strong probability of paternity.
This principle extends to genetic genealogy, where STR analysis can help identify more distant relatives. By comparing STR profiles, individuals can find others who share common ancestors, particularly through paternal lines when Y-chromosome STRs are analyzed. While autosomal STRs are used for general relationship testing, Y-STRs specifically track male lineage, as they are passed down from father to son.
Role in Genetic Disorders
While many STRs are found in non-coding regions, some are located within or near genes. When the number of repeats in these specific STRs expands beyond a certain threshold, it can disrupt gene function and lead to genetic disorders. This mechanism differs from the harmless variation seen in non-coding STRs used for identification.
One well-known example is Huntington’s disease, a neurodegenerative disorder caused by an excessive number of ‘CAG’ repeats within the huntingtin gene. Most individuals have fewer than 26 CAG repeats, but exceeding approximately 36 to 40 repeats can lead to the disease, with more repeats often correlating with earlier onset and increased severity. Another condition, Fragile X syndrome, results from an expansion of ‘CGG’ repeats in the FMR1 gene, which has fewer than 45 repeats. When these repeats exceed 200, it can silence the gene, leading to intellectual disability and other developmental challenges.