Mitochondrial DNA (mtDNA)

Unlike nuclear DNA, which is passed from both mother and father to the offspring, mitochondrial DNA (mtDNA) is maternally inherited. At fertilization, the mtDNA, contained in the tail end of the spermatozoa, is never allowed to enter the ovum, deleting the male mtDNA information from the offspring's genome.

Mitochondrial DNA is useful for forensic purposes because of two properties. First, part of the mitochondrial genome is highly polymorphic, making it useful for human identification. The two most variable regions known as HV1 and HV2 are usually amplified and sequenced to compare the difference between the evidence and reference samples.

Secondly, although mtDNA comprises less than 1% of the total DNA within a cell, its genes exist in high amounts. Because mtDNA is present in high copy number, it is very useful when analyzing degraded samples or samples that lack nuclear DNA. One example is hair, which is a common item of evidence, especially when there is little or no root is present to test. The examination of mtDNA in evidence such as bone and teeth, which may contain degraded DNA, can produce a satisfactory profile because of the high copy number of mitochondrial sequences.

Mitochondrial DNA testing was performed in a number of high-profile cases such as the Boston Strangler, the Green River murders, and the Laci Peterson homicide. It is also used for historically important cases like the Romanovs and the unknown Vietnamese soldier.