Via Wired;

Mitochondria are found by the hundreds in every human cell. They convert glucose to energy, and possess their own tiny genomes, separate and distinct from the organismal genome found in each cell nucleus.

That, and it comes in really handy when law enforcement officials state that the odds that they got the wrong guy are “one in a quadrillionzillion!”

In the mid-1990s, law enforcement added mitochondrial DNA comparison to its forensic genetic toolkit. Because there are so many mitochondria in each cell, readable copies of their genomes can often be found even when the nuclear genome has been damaged. This is especially useful for old, highly degraded biological samples.
Mitochondrial DNA-matching is based on the assumption that it doesn’t vary much in an individual: Aside from a few inevitable mutations, mitochondrial DNA are generally supposed to be the same in, say, heart cells and hair cells. But when Papadopoulos’ team applied the latest in gene-sequencing technology to mitochondrial genomes from nine tissue types in two people, that’s not what they found.
Instead, each person seemed to have a mixture of mitochondrial genotypes. One DNA variant, for example, was found in about 7 percent of a person’s skeletal-muscle mitochondria, but 90 percent of their kidney mitochondria. That spread was typical.
“It’s more than was thought, and was present in almost every tissue we looked at,” said Papadopoulos. Further research into these variations is needed, but forensic specialists should be careful to compare the same types of tissue, he said.

And don’t get me started on the practice of basing “evolutionary clocks” on the stuff.

Upon learning of the paper, John Planz, associate director of the DNA Identity Laboratory at the University of North Texas Health Science Center, cautioned that further studies are needed.

I’ll bet he did.