ABSTRACT: Mitochondrial DNA (mtDNA) typing is increasingly being offered in criminal jury trials as proof that the defendant is a possible contributor of DNA found at a crime scene. As a prerequisite to introducing such evidence, the prosecution typically must estimate the frequency in the general population of the mtDNA sequence found in the defendant and the crime scene so that jurors can evaluate the probative value of the defendant's inclusion as a potential contributor. The government estimates sequence frequencies by comparing the observed sequence to sequences listed in a racially categorized mtDNA database developed and maintained by the Federal Bureau of Investigation and the Scientific Working Group on DNA Evidence. While mtDNA evidence has significant potential as a law enforcement tool, the SWGDAM database is currently too small and insufficiently representative to provide meaningful estimates of sequence frequencies. Most importantly, the database fails to account for historic and recent human migration patterns that, because mtDNA is maternally inherited and not recombinant, have resulted in significant regional differences in sequence frequencies. With further sampling and study, large regional databases may prove to be an effective and feasible improvement upon the current forensic database for the calculation of meaningful frequency estimates. However, until such databases and meaningful frequency estimates exist, mtDNA evidence is not yet ready for admission in criminal cases to permit inferences that suspects left mtDNA at crime scenes.
Although the public and the legal community are now familiar with nuclear DNA, first admitted as evidence in United States courts almost twenty years ago, (1) nuclear DNA has a less famous counterpart found in the mitochondria of human cells, known as mitochondrial DNA ("mtDNA"). In recent years, law enforcement has increasingly used mtDNA evidence as a tool of both exclusion and inclusion of individuals as suspects in criminal cases. Specifically, because mtDNA exists in greater copy numbers per cell than nuclear DNA, mtDNA is becoming the primary type of forensic evidence extracted and reported from hair shaft samples and degraded DNA. A person may be excluded as a suspect if his mtDNA "profile, " i.e., his mtDNA sequence in particular regions of his full mtDNA strand or "genome, " differs from the profile of the crime scene sample. Such exclusion evidence may be relevant both at the pretrial investigation stage--to prevent wrongful accusation and conviction of innocent persons--and in post-conviction proceedings, such as Innocence Project DNA exonerations, to correct wrongful convictions. Use of mtDNA typing for exclusion purposes has been widely embraced in the scientific and legal communities because it preserves law enforcement resources by removing red herrings from investigations and focusing attention on the true perpetrator(s). (2)
As a tool of inclusion, however, mtDNA typing is more controversial. In criminal cases involving mtDNA, the prosecution typically reports that a defendant is included as a suspect if his mtDNA profile is consistent with, or "matches, " (3) the profile in a crime scene sample. (4) Most courts also require the prosecution to present an estimate of this shared mtDNA profile's frequency in the relevant population, (5) on grounds that, without such an estimate, jurors cannot meaningfully assess the probative value, if any, of the defendant's inclusion as a potential contributor. To estimate such frequencies, an analyst typically compares the suspect's mtDNA profile to a forensic reference database compiled and maintained by the Scientific Working Group on DNA Evidence ("SWGDAM"), a group sponsored by the Federal Bureau of Investigation ("FBI"). (6) In comparing a suspect's sequence to the SWGDAM database, the analyst counts the number of times the sequence appears in various sub-databases organized by the self-reported "race" (7) of the sample contributors, then uses this number to estimate the true frequency of the sequence in each race-based population. …