READ: DNA Evidence

Combined DNA Index System (CODIS)

 CODIS, or " Combined DNA Index System", is a collection of databases of stored DNA profiles. The profiles are obtained from evidence samples in unsolved crimes and from known individuals convicted of particular crimes. Contributions to this database are made through State crime laboratories and the data are maintained by the FBI. Using 13 standardized short tandem repeat sequences known as CODIS core loci, CODIS provides support to federal, state, and local crime labs for use in comparing DNA evidence from violent crime investigations.

Recall that the purpose of comparing the STRs, such as in the CODIS core loci is to generate a DNA Profile. DNA Profiles are the result of determining the relative positions of DNA sequences at several locations on the molecule. Each person (except identical twins) has a unique DNA profile when used in the context of the CODIS database which evaluates those 13 specific DNA locations. All 50 states have mandated the collection of DNA profiles from all offenders of certain types of crimes including sex offenders.

When comparing DNA profiles, results may be returned which result in exclusion or inclusion. Exclusion is a DNA test result indicating that an individual is excluded as the source of the DNA evidence. However, in a criminal case, "exclusion" does not necessarily equate to "innocence." Inclusion, also known as "failure to exclude" is the inability to exclude an individual as a possible source of a biological sample. This occurs when all types from a specific location in the DNA of a known individual are also present in the types for that specific location in the DNA obtained from an evidence sample.

Terms

When using CODIS, there are terms that investigators must be familiar with in order to understand the results. These terms differentiate between the various types of profiles, matches, and results that may be obtained through CODIS:

Forensic Profile - These are DNA profiles submitted to CODIS with crime scene evidence such as blood or saliva.

Convicted Offender Profile - DNA profiles submitted to CODIS of those convicted of various crimes.

Arrestee Profile - DNA profiles of people who are arrested; not all states allow this.

Missing Person Profile - DNA profiles of missing persons to use as reference if DNA evidence is found.

Unidentified Human Remains Profile - DNA profiles kept of any unidentified human remains found.

Biological Relative DNA Profile - DNA profiles of any biological relatives that are submitted to CODIS to aid in missing persons cases. These are given voluntarily and are generally used to help identify unidentified remains if needed.

Types of Hits

Forensic Hit - A CODIS match between two or more crime scene profiles. In this type of hit, samples of evidence from two or more crime scenes are found to have matching DNA profiles, but the source of the DNA is still unknown.

Offender Hit - A CODIS match between a crime scene profile and an offender profile. In this type of hit, the name of the offender is matched with an evidence sample from a crime scene.

Georgia Virtual, DNA Analysis, CC BY-NC-SA 3.0