CODIS

The DNA-profile databases maintained by local, state, and federal law enforcement authorities by statue, as well as the software to compare these DNA profiles, make up the Combined DNA Index System (CODIS).[1] CODIS is a tiered system, where local accredited laboratories upload the original DNA profiles, state DNA index systems allow for the coordination of laboratories within states, and the National DNA Index System (NDIS) enables states to compare DNA profiles with each other (Fig. 16).

Figure 16. The Combined DNA Index System (CODIS) is a tiered system of forensic DNA databases. CODIS combines local accredited laboratories with DNA index systems (LDIS), state DNA index systems (SDIS), and the National DNA Index System (NDIS). Local, state, and federal systems vary in their laws and regulations for collection, upload, analysis, and expungement of DNA information.

CODIS is built upon state and federal systems that vary in their laws and regulations for collection, upload, and analysis of DNA information. For example, although the Supreme Court ruled in Maryland v. King[2] that collecting DNA from those arrested, but not yet convicted, was reasonable for identification purposes, states differ in whether they have adopted this practice.[3] States also differ in their policies on expungement of profiles and destruction of samples from those who were arrested, but later acquitted or had their case dismissed.[4]

CODIS allows for a forensic DNA sample to be compared against a database of DNA profiles for a match (Fig. 17A). Since 2017, the DNA profiles uploaded into CODIS have used genotypes from twenty STR loci across the genome.[5] There are a sufficient number of different alleles for each STR locus that the probability of any two random individuals matching across all twenty loci is incredibly small.

Figure 17. Forensic DNA samples can be searched in law enforcement DNA databases for full matches or partial matches. A) Forensic DNA samples are searched for full matches within the CODIS system. B) Partial matches between forensic profiles and database profiles can lead investigators to look at close family members, who share genetic sequence with the partial database match. This process of extended familial searching is not standardized across databases or investigator offices.

Unlike biomedical or DTC databases, where DNA samples are carefully collected from known individuals, DNA profiles obtained from forensic samples may be partially degraded and yield only a fraction of the twenty core STR loci or may contain DNA from more than one individual and generate additional alleles at STR loci.[6] In order to deal with this variability, the FBI sets and updates standards for appropriately searching NDIS. These standards usually require an exact match between profiles but allow partial matches to overcome degraded or mixed samples.[7]

Familial Searches

Allowing for partial matches also enables searches for related individuals through familial searching (Fig. 17B).[8] Because closely related individuals share more DNA with each other than with a random individual, if an unknown sample partially matches someone in a law enforcement database, investigators may turn their attention to close relatives of the partial match (Fig. 17B).

Familial searching raises several ethical and methodological issues.[9] For example, close genetic relationships do not necessarily indicate close family ties; a stepchild does not share any genetic material with their stepparent, and an adopted child may have no contact with their biological parents. Also, considering the disproportionate representation of racial minorities in law enforcement DNA databases, certain individuals have a higher risk of being targeted for investigation simply because genetic relatives are represented in the CODIS database.[10]

Familial searching practices vary by state and are even explicitly banned in some states.[11] There is, however, a proposed standard for familial searching (circa December 2023).[12]

Pending public comment, this standard will define how many core loci must match between individuals, or what level of genetic relatedness is appropriate to consider. The amount of shared DNA between relatives diminishes rapidly for relationships beyond a second cousin (Table 2). Performing extended familial searching beyond close relatives is likely to generate false leads and bring individuals into unnecessary contact with law enforcement.[13]

Table 2. Average Percent DNA Shared Between Relatives

Relationship Between CODIS STR Loci and SNP Data

The STR loci chosen for identification purposes as part of CODIS were intended to be uninformative about health, appearance, and behavioral traits. However, new research has developed statistical methods which could be used to translate uninformative CODIS STR loci into highly informative, genome-wide SNP data.[14] Translating the data could enable CODIS profiles to be searched in third-party DTC databases more easily and could enable predictions about health, appearance, and behavior. This could even be compromising medical privacy and would contradict the rationale for collecting uninformative STR data as part of a national DNA-profile database.[15] Furthermore, the quality of these searches or trait inferences would be unpredictable, since the translation of STRs to SNPs is probabilistic and likely to introduce unanticipated errors.