STR (Short Tandem Repeat) markers are units consisting of two or more nucleotides that are repeated directly next to each other. The number of repeats is variable; a certain number of repeats thus determines a corresponding expression (allele) of the marker. The assessment of the alleles of a defined number of STR markers is the basis of identity determination and parentage assessment.
Examples of STR motifs
Di-repeat: --------CACACACACACACACACACACA---------- (11 units)
Tri-repeat: ------GATGATGATGATGATGATGATGATGAT------(9 units)
Tetra-repeat: --------GATGATGATGATGATGATGATGAT----- (8 units)
If there are more than 2 expressions at a particular genome locus within a population, this is called 'multiple allelicity'.
This is typical for STR markers.
The number of repeats is variable, a certain number of repeats thus implies a defined extension of the marker. The length is measurable and defines the corresponding allele of the marker.
Allele 1: -------CACACACACACACACACACACA---------- di-repeat (11 units)
Allele 2: -------CACACACACACACACACACACACA---------- di-repeat (12 units)
Allele 3: -------CACACACACACACACACACACACACA---------- di-repeat (13 units)
The different expressions found in a population are the result of mutations that are passed on through the generations and are found with certain frequencies (allele frequencies) in a population over time.
An individual is either homozygous (2 identical alleles) or heterozygous (2 different alleles) at a marker.
STRs have been the method of choice for individual identification and parentage assessments since their discovery.
At the population level, they can be used to reconstruct the migration and evolutionary history of species as well as to assess biodiversity at different levels of biological organisation.
Other applications, which are increasingly being replaced by array analysis, are:
- the creation of genetic maps (mapping)
- the localisation of genes
- Genetic linkage analyses