Heteroplasmy is when an individual exhibits more than one mtDNA (mitochondiral DNA) type. This phenomenon is usually observed as a single difference at one position exhibiting either two or more bases or a length difference often seen in a polycytosine rich region (C-stretch). Single base heteroplasmy has been documented, most notably in the identification of Tsar Nicholas II.¹⁵   Length heteroplasmy in the D-loop region has been described in both HV1 and HV2.^16, 17, 18 Whereas heteroplasmy may complicate interpretation, it actually becomes a "signature" of the donor's mtDNA haplotype. Heteroplasmy can, therefore, strengthen the identification of a sample.

Point mutations or deletions in mtDNA usually increase with age. Defects in mitochondrial function produce a wide range of human diseases and can be caused by mutations within the mtDNA. The first mtDNA mutation discovered to be the cause of a disease was Leber's hereditary optic neuropathy (LHON).¹⁹ Identified in 1988, LHON is a maternally inherited form of adult-onset blindness due to death of the optic nerve. Point mutations are relevant because the person's mtDNA type can change with age.

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