The DNA Identification Act of 1994 formalized the FBI's authority to establish a National DNA Index System (NDIS). The CODIS is different from the NDIS in that NDIS is the national database whereas CODIS is used today by more than 40 law enforcement laboratories in over 25 countries.( FBI, 2009) The CODIS system requires > 4 RFLP markers and/or 13 core STR markers. Now, what are STR's
Most of the DNA is identical to the DNA of others in the human genome. However, there are inherited regions of our DNA that may not necessarily code for proteins termed as introns, which can vary from person to person. A variation in the DNA sequence between individuals is termed as "polymorphism". Sequences which display the highest degree of polymorphism are very useful for DNA analysis in forensics cases and paternity testing. Forensic DNA testing is based on analyzing the inheritance of a class of DNA polymorphisms known as "Short Tandem Repeats "or STRs. STRs are short sequences of DNA, normally of length 2-5 base pairs, that are repeated numerous times in a head-tail manner, for example the sequence GATA occurs anywhere between 5-16 repeats at the locus D7S820. The number of repeats within an STR is referred to as an allele. Thus there are 12 different alleles possible for the D7S820 STR. Because there 12 different alleles for this STR, there are therefore 78 different possible genotypes, or pairs of alleles. Specifically, there are 12 homozygote's, in which the same allele is received from each parent, as well as 66 heterozygote's, in which the two alleles are different. Norrgard,K.(2008)
After an extensive validation study, 13 STR loci were selected to form the core (i.e., required) genetic markers for CODIS. The methodology involved in the STR analysis is as follows. The DNA is isolated from the sample. The STR region in the DNA is amplified using PCR and suitable primers. The various companies manufacturing STR kits typically vary in the primer they employ. PCR enables even nanogram amounts of sample to be sufficient for evaluation. Only when all the 13 markers match in the source and the felon sample is the convict included in the source of crime. Even after all the 13STR's are matched a statistical test is carried out to analyze the overall STR frequency. It is assumed that the STR's follow law of independent assortment which implies that each member of a pair of homologous chromosomes separates independently of the members of other pairs so the result is random and that the population mates randomly by the FBI in analyzing the STR profiles. The Hardy Weinberg law was used to calculate gene frequency which states that the gene frequencies remain conserved from generation to generation. It is important to understand that this number is the probability of seeing this DNA profile if the crime scene evidence did not come from the suspect but from some other person. If all the STR's from the sample are not retrievable due to damaged or an extremely old source of DNA, then the STR profiling cannot be used as the sole evidence and other proof's have to be collected. Though the STR profiling is rapid the CODIS database is currently facing a backlog of >600,000 samples to be keyed in.
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