STR GENOTYPING IN FORENSICS

Short Tandem Repeats (STRs) are efficient tools that allow to map specific traits and follow the pattern of genetic material in a given population. STRs are composed of short lengths of di, tri, tetra, or penta nucleotide sequences that are commonly present in eukaryotic organisms.

What is STR Genotyping?

STRs are short strands of DNA usually 6-10basepairs long. These show high levels of polymorphism. STR Genotyping is done by purifying genomic DNA and amplifying the STR regions of interest using a Polymerase Chain Reaction (PCR). the amplified fragments are then analysed on a Polyacrylamide gel. The result of the electrophoresis is detected by using fluorescent markers. This is done by the traditional Sanger sequencing method.

Advantages of STR Genotyping

The number of repeats is highly variable among individuals which offers a high power of discrimination when analyzed for identification purposes.
A Y-STR based genotyping can help detect the male component in the mixture of male and female DNA.
It is an effective tool for identification purposes using DNA.
It has various advantages like verifying tissue sample origins, authenticating cell lines, detecting tissue or cell mixtures, determining twin zygosity, and tracking genetic mutations in research studies of diseases such as cancer.
The ability to determine biogeographical ancestry from forensic STRs has also been studied.

Use of STR Genotyping in Forensics

STR genotyping is mainly employed in the field of forensic diagnostics due to high levels of polymorphism detected in the alleles. The DNA profiling of individuals is done by using PCR and STRs.

The STRs are present in the non-coding section of the DNA which is highly different to every individual. These STR loci are targeted by using specific primers and then allowed to amplify in a PCR. they are then separated by using any one of two different types of electrophoresis - Capillary Electrophoresis and Gel Electrophoresis. This is the base for using STRs to differentiate between sets of unrelated DNA. This has been used to establish the identity of missing persons, and confirm familial relations and persons of interest to respective crime scenes.

The earliest STR genotyping used in forensic casework was in 1994 in the UK. In 1997, the Federal Bureau of Investigation (FBI) provided a 13 autosomal STR loci to form the core of a database system called CODIS which stands for Combined DNA Index System. This consists of all the data from the state and federal forensic laboratories. The markers in the CODIS were explicitly selected as they did not code for any physical or medical traits and they were located in the non-coding region of the genome.

The conventional Sanger sequencing technology in forensics has some disadvantages like low throughput, high cost and operation difficulties. These have limited its use in modern day forensic analyses which are now carried out using NGS technology.