Basic Overview of Forensic DNA and STR Data
Basic Overview of Forensic DNA and STR Data
Forensic DNA and STRs
Approximately 99% of all human DNA is the same. In the one percent that differs, sequences of repeating nucleotides exist called Short Tandem Repeats (STRs). STR regions are highly polymorphic, meaning that they are very different between any number of people. The FBI has marked 13 locations (or loci) on the DNA for forensic testing and identification.
DNA analysis is an elaborate process. Once a DNA sample has been obtained, it is amplified using Polymerase Chain Reaction (PCR). PCR works as a sort of Xerox machine, making copies of the DNA sample in a lab. STR-containing regions are specifically amplified in a way that labels each with one of three different dyes, then separated and placed in a genetic analyzer, such as the ABI 310. The machine uses capillary electrophoresis to separate the STR segments by size, and measures their presence with a laser by recording the fluorescence of light. The raw data is stored on a CD as evidence.
Figure 1: GenoTyper output illustrating the presence of alleles, shown as peaks. The graph is broken into three sections, representing three different DNA locations (loci).
A CD of raw DNA sample data is not very useful because it requires further analysis. The current standard in analysis software is the Applied Biosystems (ABI) GeneScan and GenoTyper software package. GeneScan identifies the signal corresponding to each dye present in the sample. GenoTyper uses the GeneScan analysis to identify and label each peak. Operating the GeneScan and GenoTyper software is not necessarily difficult, but does require special training and is time consuming. DNA profiling experts often spend between five and ten hours to analyze the raw data in a case before even determining if important issues need to be addressed in greater detail.
The GeneScan and GenoTyper programs produce a considerable amount of output, usually in the form of printouts or Adobe Acrobat PDF files. GenoTyper produces the final output in the form of an electropherogram, a graph that looks similar to an EKG (see Figure 1). The graph is mostly flat with peaks representing the presence of STRs. DNA experts analyze the output by eye to identify problems and match multiple samples.
A significant amount of the expert’s time is spent running the software. The process includes selecting the samples to analyze, setting up the correct parameters, performing the analysis, and saving all of the output. A typical case can easily result in over a hundred separate output files, making analysis a sizable task. Genophiler® expands upon the state of the art by automating the task of operating the analysis software. When Genophiler® has completed, the analysis output is stored in PDF format in an easily-accessible organized format. The user can access any of the testing results through a single index found on the CD-ROM.
Questions? Contact help@bioforensics.com.
Last modified: 10/04/05
