DNA is present in nearly every cell of our bodies. Flakes of skin, drops of blood, hair, and saliva all contain DNA that can be used to identify us. In fact, the study of forensics, used by police departments and prosecutors around the world, frequently relies upon these small bits of shed DNA to link criminals to the crimes they commit. This fascinating science is often portrayed on popular television shows as a simple, exact, and infallible method of finding a perpetrator and bringing him or her to justice. In truth, however, teasing out a DNA fingerprint and determining the likelihood of a match between a suspect and a crime scene is a complicated process that relies upon probability to a greater extent than most people realize. Government-administered DNA databases, such as the Combined DNA Index System (CODIS), do help speed the process, but they also bring to light complex ethical issues involving the rights of victims and suspects alike. Thus, understanding the ways in which DNA evidence is obtained and analyzed, what this evidence can tell investigators, and how this evidence is used within the legal system is critical to appreciating the true ethical and legal impact of forensic genetics.
Although, majority of the human genome is identical across all individuals, there are regions of variation. This variation can occur anywhere in the genome, including areas that are not known to code for proteins. Investigation into these noncoding regions reveals repeated units of DNA that vary in length among individuals. Scientists have found that one particular type of repeat, known as a short tandem repeat (STR), is relatively easily measured and compared between different individuals.
To perform a forensic DNA analysis, DNA is first extracted from a sample. Just one nanogram of DNA is usually a sufficient quantity to provide good data. The region containing each STR is then amplified by PCR and resolved according to size, giving an overall profile of STR sizes (alleles). The 13 core STRs vary in length from 100 to 300 bases, allowing even partially degraded DNA samples to be successfully analysed. Depending on the complexity of the repeat unit, the different alleles of an STR can vary by as little as a single nucleotide.. Because of the need to differentiate single-base differences, PCR products are typically resolved using automated DNA sequencing technologies with software that recognizes allele patterns by comparison to a known "ladder."