Forensic DNA testing serves a number of useful purposes. It can be used to track down criminal suspects who have left behind biological evidence, exonerate individuals who have been falsely accused of committing crimes, identify individuals who have fallen victim to violent crimes or disasters, and connect crimes that share biological evidence. It can also be used to identify the father of a child conceived through rape or incest.
Law enforcement officers and attorneys rely on a variety of forensic testing services to help them solve crimes and prosecute or defend criminal suspects. Some tests, for example, are able to detect the presence of blood, semen, or other biological substances in evidence left at crime scenes. If biological material is found, other forensic tests can be performed to determine whether the samples contain viable DNA.
Once viable DNA is detected, it can be analyzed using one of several DNA testing methods, such as STR typing, Y-chromosome STR (Y-STR) typing, or mitochondrial DNA (mtDNA) sequence analysis. The testing method used depends on the type of sample available, as well as its condition.
STR typing can be used for most forensic samples. STRs, or short tandem repeats, are DNA markers that are highly variable among individuals and therefore quite useful for human identification purposes. In addition to their variability, their small size and low rate of mutation make STRs desirable for forensic analysis. In 1998, in fact, the Federal Bureau of Investigation’s Combined DNA Index System (or CODIS, for short) established the use of 13 core STRs as an international standard for human identification testing.
Y-STR typing is useful for testing samples that contain a mixture of male and female DNA. This is because Y-STRs are found on the Y-chromosome, which only men have. Commonly used in sexual assault investigations, Y-STR typing is able to isolate a male assailant’s Y-STRs, which are variable among unrelated males and therefore useful in discriminating between male suspects.
Unlike the STR and Y-STR typing methods mentioned above, mtDNA sequence analysis is performed on mitochondrial DNA, which exists in the mitochondria of the cells, not in the nucleus. (STRs are found on the chromosomes of nuclear DNA.) Mitochondrial DNA is useful in forensic analysis because, like STRs, it is highly variable among individuals. Moreover, it exists in high quantities. This means that when a piece of evidence is too old or degraded to contain viable nuclear DNA, it may still contain viable mtDNA that can be used for human identification purposes. Hair, teeth, and bone samples are commonly used in mtDNA sequence analysis.
Once a DNA profile has been obtained from a forensic sample, it can be uploaded into CODIS. Developed by the FBI, CODIS is a national database containing DNA profiles generated at the national, state, and local levels. It allows laboratories to exchange and compare genetic profiles, which, in turn, enables crimes to be linked to convicted offenders and to other crimes.
CODIS contains two indexes. The Forensic Index contains DNA profiles generated from biological evidence left at crime scenes. The Offender Index contains DNA profiles generated from criminals convicted of sex offenses and other violent crimes. Profiles can be compared within each index and between the two indexes. When the database makes a match, the DNA laboratories that generated the associated profiles work together to confirm or deny that a true match has been made. In this way, investigations can be assisted and crimes can be solved.