What is DNA?

DNA

– deoxyribonucleic acid – is a molecule that contains genetic information. It is found in an organism’s cells and is the blueprint for that organism’s functions. DNA can be found in many biological materials because it is contained in every organism’s cells. In criminal cases, DNA is often found in biological evidence like blood, saliva, sweat, semen (in the sperm), hair, and skin. A person’s genetic code is inherited and unique, so each DNA profile is distinct (except in identical twins).

What is forensic DNA testing?


Several tests have been developed that can find DNA profiles in biological evidence. Forensic scientists can identify individuals from a biological sample by comparing DNA profiles. For example, if an intruder cuts himself on a window while committing a burglary, forensic scientists would try to obtain a DNA profile from the blood on the window. That profile could then be compared to the profile of any suspect.

DNA testing, when properly performed, is a powerful form of evidence. Like all scientific tests, however, DNA testing is only as reliable as the scientists conducting the testing. The Innocence Project has several recommendations that states can adopt to ensure that proper measures are taken in crime laboratories to reduce the chances of error.

What kinds of DNA testing are available?


The current standard for forensic DNA testing is the Short Tandem Repeat (STR) test. This test includes elements of the Polymerase Chain Reaction (PCR) method, which allows for very small and degraded samples to be tested successfully.

Another form of DNA testing is mitochondrial DNA testing. Also used in genealogical research, mitochondrial DNA testing looks for profiles from another kind of DNA that is found in mitochondria, small bodies that are found in cells and store their own kind of DNA. Mitochondrial DNA profiles are different, in that they are shared by people with the same mother and are passed down matrilineally, i.e. siblings with the same mother would exhibit the same mitochondrial DNA profile.

A relatively new form of testing is Y-Chromosome STR testing. Y-STR testing allows scientists to target only the DNA left by male contributors. This ability to target only male DNA can play a crucial role in cases with mixed sex samples or multiple male profiles, as Y-STR results can also help determine how many male contributors there are in any given sample.

How long does DNA testing take?


The time it takes to perform DNA testing varies, depending on the kind(s) of testing being performed and the number of items being tested. This time period also varies by laboratory. Some laboratories experience delays because they are so backlogged with requests for testing.

What is the legal status of DNA testing?


DNA testing was first allowed into court as evidence in 1988 (People v. Wesley). Since then, several new methods of DNA testing have been found to be reliable forms of evidence. In 1994, New York became the first state to pass a post-conviction DNA testing statute, allowing inmates to appeal if DNA testing was appropriate. Since then, more than 45 states have passed post-conviction DNA statutes.

What is a DNA databank or database?


The federal DNA databank,

called the

Combined DNA Index System (



CODIS)

, contains the DNA profiles collected in state and territory databases (except for those only arrested), as well as those of certain federal offenders.  Each state also maintains its own database, determining for itself what criminal convictions require DNA profile collection. Some databases include those arrested but not convicted of crimes, which is the subject of considerable debate.

Where appropriate, DNA found at crime scenes is compared to the profiles in these databases in an effort to identify the perpetrator(s) of the crime(s). Backlogs in processing these profiles plague many states, however, and this impedes the effectiveness of these databases in many law enforcement investigations.