Because of their varying repeat lengths, STRs are characterized with large number of alleles, which in turn make them being highly variable between populations (Lee et al, 1994). In addition, these microsatellite sequences account approximately for 3 percent of the human genome and occur on average every 10,000 nucleotides (Collins et al, 2003). Finally, among other types of biological material, STRs are characterized with consistency and stability, which practically means that any cell contains the very same profile of SRTs. From the forensic perspective, it means that regardless of type of biological material obtained in the crime scene, appropriate STR profile proving source attribution can be conducted using any type of cell without compromising the result.
From the practical perspective, STR profile constitutes simply a panel of numerical designations of each STR allele repeat number based on their electrophoretic sizing information. The electrophoretic mobility of DNA is inversely proportional to the length of the fragment; the longer allele with more repeats migrates slower than the shorter alleles containing fewer repeats. If a snapshot (electropherogram) is taken at a time point when two alleles of different lengths are electrophoretically migrating through a gel, the shorter allele will appear further from the origin than the longer allele. When looking at multiple alleles simultaneously, the STR electropherogram pattern (profile) of an individual is different from that of another individual whose alleles may be longer or shorter and hence assigned with different allele numbers. The variation in STR allele sizes results in distinctive DNA profiles with a uniqueness of 1 in a trillion using the 13 core STR loci in the FBI’s national database known as COmbined DNA Index System (CODIS). These 13 loci were chosen for their unique characteristics, such as number of alleles and repeat sequence, present in each one of them. Their adoption in the