MtDNA

Mutations in mtDNA cause human diseases.
A number of human diseases are caused by mutations in genes in our mitochondria:
cytochrome b
12S rRNA
ATP synthase
subunits of NADH dehydrogenase
several tRNA genes ...

mitochondrial DNA (mtDNA)
Circular or linear DNA of the mitochondria. It codes for only a small but essential part of the mitochondrial proteins and for genes for tRNAs and rRNAs.

Mitochondrial DNA (mtDNA) comparisons serve as a model for genome evolution and as a tool for reconstructing evolutionary relationships. Relative to the nuclear genome, this system has several advantages for a comprehensive sampling across life.

"Harvesting the fruit of the human mtDNA tree". Trends Genet. 22 (6): 339-45. doi:10.1016/j.tig.2006.04.001. PMID 16678300.
^ a b Garrigan D, Hammer MF (2006). "Reconstructing human origins in the genomic era". Nat. Rev. Genet.

Critics raised an important point about the inferences based on the mtDNA studies: it is based on only a single, independently evolving gene region.

In fact, all of your mitochondria come from your mother and since mitochondria contain their own DNA (mtDNA), this is known as maternal inheritance.

Wayne and Jenks, in a recent Nature, present a study of the mtDNA(mitochondrial DNA -- it's maternally inherited) of the endangered red wolf, Canis rufus. This species, once extending over a large range in the southeast, is now extinct in the wild.

Amerind mtDNA suggests two waves of migration (one perhaps as old as 21-42 thousand years ago). The genetic model confirms the accepted ideas about human migration into the Americas and suggests a possible fourth wave.

Granular, rod-shaped, or filamentous self-replicating organellae in cytoplasm. Consist of an outer and inner membrane and function in cell respiration and nutrition. They have their own DNA (mtDNA) and ribosomes and are mostly maternally inherited.

See also: Mitochondria, DNA, Organ, Trans, Cell