Chapter 23: Mitochondrial DNA Profiling

Mitochondrial DNA differs fundamentally from nuclear DNA in several critical ways: it exists as a small, circular molecule present in hundreds to thousands of copies per cell within the mitochondrial organelles, and it follows a maternal inheritance pattern, meaning individuals inherit their mitochondrial genomes exclusively from their mothers, though paternal mitochondria may occasionally enter the oocyte before being selectively eliminated through degradation processes. The forensic utility of mtDNA profiling centers on two hypervariable regions, designated HVI and HVII, located within the displacement loop or D-loop control region, which exhibits substantial sequence variation among individuals and populations. A critical complication in mtDNA analysis is heteroplasmy, the phenomenon wherein a single individual carries multiple different mtDNA sequences simultaneously due to the large number of mitochondria distributed throughout the cell, potentially creating interpretive challenges when analyzing degraded or mixed biological samples. The analytical workflow proceeds through several stages: preliminary screening assays establish whether mtDNA evidence warrants further investigation, followed by comprehensive sequencing of the target regions using polymerase chain reaction amplification to generate sufficient DNA for analysis, typically followed by Sanger sequencing or next generation sequencing methodologies to determine the exact nucleotide sequence. Once sequence data is obtained, analysts assign a mitotype, the specific mtDNA profile of an individual or evidence sample. The three possible conclusions from comparative analysis are exclusion when sequences demonstrably differ, cannot exclude when sequences match supporting a potential maternal relationship, or inconclusive results when data quality or heteroplasmy prevents definitive interpretation, with all conclusions guided by rigorous standards established by professional organizations including SWGDAM and ISFG.