Printer Friendly| Forward to a Friend| Share this |
Forensic mitochondrial DNA analysis of hair shafts and naturally shed hairs is a tool to enhance the investigation of cold cases; this form of evidence historically has had severely limited utility. Many cold cases have been re-opened in hopes that DNA profiling of evidentiary material may strengthen a case against an existing but weak suspect or identify new leads and new suspects. “Cold hits” are made when nuclear DNA (STR) profiles of semen, blood, or saliva crime scene samples are linked to convicted felon DNA profiles that are stored in the national DNA database (CODIS). A lesser-known form of DNA testing, however, also is being used for cold case investigation. In the 1990s, mitochondrial DNA (mtDNA) analysis was introduced for samples unsatisfactory for STR profiling. The earliest use of mtDNA analysis was for the identification of human skeletal remains that contained insufficient or degraded nuclear DNA, but sufficient mitochondrial DNA to aid in matching an individual to his or her maternal relatives. Since 1993, the Armed Forces DNA Identification Laboratory in Rockville, Maryland, has been using mitochondrial DNA to return the skeletal remains of military dead to their families. Identification of missing persons is also aided by this technology. DNA can be difficult to recover from very small or environmentally challenged samples. While nuclear DNA is present in only two copies per cell, the small circular mitochondrial DNA molecule (Figure 1) is present in hundreds to thousands of copies per cell and is therefore a naturally abundant DNA molecule. During the 1990s, forensic scientists learned that while naturally shed human hair roots and hair fragments lacking any root at all do not contain sufficient nuclear DNA for routine STR typing, they contain abundant mitochondrial DNA (Figure 2). Today, the ability to perform mtDNA analysis on virtually any head or body hair is a bonus technique in the investigation of criminal cases. The first mitochondrial DNA analysis that aided in a criminal conviction involved a suspect charged with the 1996 rape and murder of a young child in Tennessee. Since then, hundreds of cases have been examined using mitochondrial DNA analysis, and dozens have successfully been tried in the courtroom. A sizeable body of peer-reviewed scientific literature on forensic mitochondrial DNA analysis is available, and courtroom admissibility hearings have uniformly allowed its introduction worldwide.
Advantages and Limitations of mtDNA Analysis
A nuclear DNA match of the 13 core STR loci permits little doubt that a questioned sample has come from a known individual, except in the case of identical twins. However, because mitochondrial DNA is maternally inherited, all a woman’s offspring, her siblings, her mother, and other maternal relatives will have the same mitochondrial DNA profile (Figure 3). Mitochondrial DNA, therefore, is not a unique identifier and the test’s conclusion can be only whether or not a known individual is excluded as the donor of the questioned sample. While non-uniqueness is a limitation, there are thousands of different mitochondrial DNA types, and the relative population frequency of almost any type is low. In most cases, at least 99% of the population will be excluded as contributors and the pool of random individuals who could have contributed the sample is less than 1% of the population. In many cases, well over 99.9% of the population may be excluded. The inability to use mtDNA in quite the same way as nuclear DNA highlights its value as a “piece of the puzzle,” meaning that mtDNA almost always supplements other information in the theory of the crime and that evidence tested by this...