STR polymorphism refers to the natural variation in the number of repetitive DNA sequences, known as Short Tandem Repeats (STRs), found at specific locations (loci) within the genome among different individuals. This variation makes STRs incredibly valuable tools in genetics for identification and relationship testing.
Understanding Short Tandem Repeats (STRs)
Short Tandem Repeats (STRs) are segments of DNA where a short sequence of two to six base pairs is repeated multiple times in a head-to-tail fashion. These repetitive sequences are typically non-coding, meaning they do not provide instructions for making proteins.
For example, a common STR might involve the sequence 'GATA' repeated several times:
...GATAGATAGATAGATAGATA...
The Concept of Polymorphism in STRs
The term "polymorphism" simply means "many forms" or "variation." When applied to STRs, it highlights that the number of times a particular short sequence repeats at a specific genomic location can vary significantly from one person to another.
A short tandem repeat polymorphism (STRP) occurs when homologous STR loci (corresponding positions on paired chromosomes) differ in the number of repeats between individuals. For instance, one individual might have 10 repeats of a specific STR sequence at a particular locus, while another individual has 15 repeats at the same locus.
Consider the following illustrative example for an STR locus:
| Individual | STR Locus Example | Number of Repeats |
|---|---|---|
| Individual A | (AGCT)7 | 7 repeats |
| Individual B | (AGCT)12 | 12 repeats |
| Individual C | (AGCT)9 | 9 repeats |
This difference in repeat numbers is what constitutes the polymorphism, making each person's genetic profile unique at these specific STR locations.
How STR Polymorphism is Utilized
The highly polymorphic nature of STRs makes them exceptionally useful in various applications, particularly those requiring individual identification. By identifying repeats of a specific sequence at specific locations in the genome, it is possible to create a genetic profile of an individual.
Key applications include:
- Forensic DNA Fingerprinting: STR analysis is a cornerstone of modern forensic science. DNA collected from crime scenes (e.g., blood, hair, saliva) can be analyzed for STR markers, and the resulting profile can be compared to profiles of suspects or entered into national databases like CODIS (Combined DNA Index System) in the United States, to identify individuals.
- Paternity and Kinship Testing: By comparing STR profiles between a child and potential parents, it's possible to determine biological relationships with a very high degree of certainty. A child inherits one copy of each STR locus from each parent, so their STR profile should be a combination of their parents' profiles.
- Human Identification in Mass Disasters: In cases of mass casualties or unidentified remains, STR analysis can help identify victims by comparing their DNA to samples from relatives or personal items.
- Population Genetics Studies: Researchers use the variation in STR frequencies within and between different populations to study human migration patterns, historical relationships, and genetic diversity.
Key Characteristics of STRs
- High Polymorphism: They exhibit a high degree of variation among individuals, making them powerful for discrimination.
- Abundant: STRs are widely distributed throughout the human genome.
- Small Size: The relatively small size of STR loci (typically less than 500 base pairs including flanking regions) makes them amenable to amplification by Polymerase Chain Reaction (PCR), even from degraded DNA samples.
- Co-dominant Inheritance: Both alleles (copies) inherited from each parent can be detected, allowing for a more complete genetic picture.
STR polymorphism forms the basis of highly accurate and reliable methods for individual identification, revolutionized fields from criminal justice to medical diagnostics and genealogical research.
