When viewed with a microscope, mitochondria are among the most prominent organelles within
human cells. They are primarily known for the central role that they play in the generation of metabolic
energy. In humans (and most animals), mitochondria are exclusively inherited through the mother because
eggs (and not sperm) are the major contributor of cytoplasm to zygotes. A typical human cell contains
between 1,000 and 10,000 mitochondria to satisfy its energy- production needs. Each of these mitochondria
contains a copy of the mitochondrial genome which is very small in comparison to the nuclear genome
where STR loci are found (16,569 bp vs. 3.2 billion bp for the genome overall). Within that relatively
small genome is a stretch of nucleotides called the “mitochondrial D-loop” that tends to differ in its
particular sequence of nucleotides (but not its length) from one maternal lineage to another. Analyses of the
mitochondrial D- loop sequences have been very useful to biologists studying the migration patterns of
humans another mammals. From a forensic perspective, the presence of 1,000 to 10,000 more copies of
mitochondrial DNA than nuclear DNA per cell gives analyses of it a distinct advantage in situations where
a sample is not expected to have much DNA associated with it (i.e. a hair shaft or a fingerprint) or the DNA
within a sample is badly degraded (i.e. after cremation). The utility of mitochondrial DNA sequencing in
forensic casework, however has been limited due to:
1) the fact that a single cells fairly frequently contain
more than one kind of mitochondria (a situation known as “heteroplasmy”);
2) differences between
mitochondrial DNA are not easily detected differences in length like those for STRs and must be
determined by comparatively costly and subjective DNA sequencing;
3) like Y-STRs, the rarity of
mitochondrial sequences must be determined by empirical studies and the associated statistics are (and will
remain) far less impressive than those generated with STR testing;
4) all maternally related individuals are
expected to have the same mitochondrial DNA sequence(s); and
5) the ease with which samples are
contaminated and cross- contaminated with mitochondrial DNA.
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