How is DNA used to identify human remains found in the backcountry?

Months or years after someone goes missing hiking or walking outdoors, human remains can be discovered, even if areas previously thoroughly searched. The question often remains - to whom do the remains belong? The challenge is especially significant if the remains are badly degraded, bones are the only clue, and there are no teeth or dental records.

What is DNA forensics, and how is a DNA fingerprint created?

Our DNA is a genetic code made up of 4 letters (A, T, G, C), called DNA bases, that are interpreted by our cells to make the molecules and structures that allow the human body to operate, Regions of DNA that encode molecules known as “proteins” are called genes. The unique code in every person results in physical differences, e.g. hair and eye color, and height, between different people.

The vast majority of DNA (99.9% on average) between two individual humans is the same, but the 0.1% is the key to identifying us.

Forensic laboratories look at 20 DNA regions that vary between individuals, called short tandem repeats (STRs), to create a DNA “fingerprint”. These STRs are located in stretches of DNA between gene-coding regions and consist of short DNA sequences that are repeated different numbers of times in other people. The number of repeats at each of these STRs means the probability can be calculated that the DNA from a body sample can be matched to a specific person. The chance that two people who aren’t closely related have the same DNA profile is tiny, i.e. 1 in billions.

Forensic DNA typing can identify biological samples if sufficient DNA can be recovered, even when the human remains are fragmented and the DNA is degraded. Identifications are made by comparing the DNA profile of reference samples with those from the human remains.

The acquisition of DNA from skeletal remains can be a delicate process. Extraction of total genomic DNA from nearly any skeletal element is possible with improved extraction buffers that provide complete demineralization of the bone material.

A tissue, bone, or blood sample is sent to a testing facility. In the United States, the University of Texas Health Science Center conducts the analysis :

Two types of DNA profiling can be used on a body that has been dead for a while:

1. Autosomal DNA Profiling (DNA Fingerprinting) usually refers to testing STR (short tandem repeat) markers found in Autosomal DNA. STRs are short fragments of DNA, usually 2 to 6 base pairs in length, repeated repeatedly in a defined location of the autosomal DNA. This helps identify unique characteristics, e.g. eye color.

2. Mitochondrial DNA profile. The analysis of mitochondrial DNA (mtDNA) fills a niche in forensic genetics since it is superior to standard nuclear DNA (nDNA) typing when samples have to be identified that do not contain enough nDNA or need to be evaluated concerning their maternal relatedness. Since mtDNA is only passed from mother to child, all siblings share the same amount of this DNA.

Next-generation sequencing (NGS) sequences or reads through many small fragments of DNA simultaneously, giving results much more quickly and at a lower cost than older methods. As a result, the number of regions used in STR analysis increased from 13 to 20 in 2017, increasing the accuracy of DNA testing.

Methods to analyze samples with mixed DNA samples have also been developed. Sophisticated software uses probabilistic genotype matching to determine the chances that two samples came from the same person. A likelihood ratio is calculated, which measures how much more likely someone matches the sample than someone randomly from the general population.

These analyses are then uploaded to a database, e.g. CODIS or NamUS in the US. In December 2018, the commencement of the Criminal Justice (Forensic Evidence and DNA Database System) Act 2014 in the U.K. meant that forensic evidence, such as DNA profiles and fingerprints, could be exchanged and compared with databases in other European Union Member States.

U.S. DNA databases

The Combined DNA Index System (CODIS) is a tool for linking DNA to violent crimes. It enables federal, state, and local forensic laboratories to exchange and compare DNA profiles electronically, linking serial violent crimes to each other and known offenders.

The National Missing and Unidentified Persons System (NamUs) is a national clearinghouse and resource center for missing, unidentified, and unclaimed person cases throughout the United States. They are funded and administered by the National Institute of Justice through a cooperative agreement with the University of North Texas Health Science Center for Human Identification. All NamUs resources are provided to law enforcement, medical examiners, coroners, allied forensic professionals, and family members of missing persons.