The human genome is full of repeated DNA sequences, occurs usually 5-50 times. The repeated sequences come in various sizes (6-10 bp in length) and are known as Short Tandem Repeats or Microsatellites. STR occurs at thousands of locations in an organism’s genome. STR is widely used in cancer diagnosis, paternity testing and in forensic investigations.
STRs can be easily amplified due to their small size. Every individual inherits single STR from their parent of variable sizes. The number of repeats of STR markers varies in each individual. So STR is considered to be significant diallelic markers in human Identification. The markers are stable due to low mutation rates.
The advantage of STR in Forensic DNA Typing is that the results are highly reproducible. The resolution of even a single base pair is attained over the range of analysis. They are stable due to low mutation rates and less sensitive to matrix effects in the analysis procedure.
In 1996, the Federal Bureau of Investigation, FBI launched a forensic effort to establish core STR loci known as CODIS (Combined DNA Index System).The CODIS loci are CSF1PO, FGA, THO1, TPOX, VWA, D3S1358, D5S818,D7S820, D8S1179,D13S317,D16S539, D18S51, D21S11. These 13 CODIS loci are internationally recognized for resolving forensic cases. Forensic laboratories use 13 CODIS loci and additional loci (for sex determination) as a powerful technique in solving crimes.
John M. Butler,2009