DNA

 Ongoing advancements in DNA extraction methods and improved analytical tools have significantly increased the sensitivity of forensic DNA processing. These innovations allow laboratories to generate DNA profiles from increasingly smaller amounts of biological material and from samples that are highly degraded, aged, or previously considered unsuitable for testing. As a result, evidence that once yielded no usable DNA profiles may now be successfully retested, expanding the potential for analysis in cold cases and other challenging investigations.

Below are basic definitions to help in the understanding of DNA testing opportunities. Further expertise and assistance can be garnered from public and private labs that adhere to FBI’s QAS (quality assurance and proficiency testing standards) and are accredited for submitting DNA profiles to the National DNA Index System (NDIS), the national database of CODIS. CODIS is a complex system of databases and indices and is explored further under “Databases” and on our Blog.  

Serological testing is utilized to screen evidence for bodily fluids such as blood, urine, semen and saliva for the extraction of DNA evidence. However, DNA can be extracted from other places as well, including bone, hair and teeth. 

How DNA is utilized

The Combined DNA Index System (CODIS) is a network of local, state, and the national DNA databases that store DNA profiles in multiple indices. Where a DNA profile is entered determines the types of comparisons that can be made. Forensic DNA collected from crime scenes can be compared to other forensic profiles to identify related or serial offenses, as well as to known offender profiles to help identify or exonerate potential suspects. DNA profiles from missing persons or their family members can be compared to profiles from unidentified human remains also to assist with identification. DNA testing techniques are chosen based on the condition, quantity, and quality of the biological sample, and each technique yields different forms of evidentiary information.

Nuclear DNA (nDNA) contains the genetic information that directs the structure and function of cells and organisms. It is inherited from both biological parents, with half coming from the mother and half from the father. Because of natural variations in the genetic code, an individual’s DNA profile is unique (except in the case of identical twins). This makes nuclear DNA a powerful tool for precise individual identification and for associating individuals with crime scene evidence.

Mitochondrial DNA (mtDNA) is inherited from the mother and shared by individuals in the same maternal lineage. MtDNA testing is used when nuclear DNA is unavailable or too limited, such as with naturally shed hairs, bones, teeth, or with highly degraded samples. Because mtDNA is present in greater quantities than nuclear DNA, it can often be recovered from older or compromised evidence, making it useful in cold cases, missing persons investigations, and mass disasters. However, mtDNA analysis cannot uniquely identify an individual, as the same profile may be shared by multiple people within the same maternal family line.

Y-chromosome (Y-STR) testing is a type of nuclear DNA analysis that targets genetic markers on the male Y chromosome. Because the Y chromosome is passed from father to son largely unchanged, males within the same paternal lineage share the same Y-chromosome profile. As a result, this testing cannot uniquely identify an individual. However, Y-chromosome testing is particularly useful for isolating and identifying male contributors in complex, degraded, or mixed samples, and is especially valuable in sexual assault investigations.

Forensic Investigative Genetic Genealogy (FIGG), also known as Investigative Genetic Genealogy (IGG), uses consumer DNA testing combined with traditional genealogical research to develop investigative leads in violent crime and unidentified human remains cases. Unlike CODIS, which relies on short tandem repeat (STR) markers for direct identification, FIGG analyzes single nucleotide polymorphisms (SNPs). These SNP profiles are compared to DNA data voluntarily uploaded to genealogy websites to identify potential biological relatives. 

CODIS profiles are built from a limited number of DNA markers at each of 20 specific locations known as “core loci.” These STR markers come from non-coding regions of DNA and are used solely for identification purposes. Genetic genealogy profiles are far more detailed. They examine hundreds of thousands of SNPs spread across the entire genome. This extensive data allows investigators to identify close and distant relatives, estimate ancestry, and, in some contexts, infer certain traits or health-related risks.

Both STRs and SNPs are inherited from both parents, meaning each can reveal some familial relationships. However, because SNP profiles contain significantly more genetic information, genetic genealogy databases can identify indirect and very distant family connections that CODIS cannot, allowing investigators to develop investigative leads in cases where traditional forensic DNA analysis has failed to identify a suspect.