Mitochondrial DNA (mtDNA) is the DNA located in cellular organelles called mitochondria, which are structures within eukaryotic cells that convert chemical energy from food into a useable form, adenosine triphosphate (ATP). Mitochondrial DNA is only a small portion of the DNA in a eukaryotic cell; most of the DNA (e.g. the chromosomes) is found instead in the cell nucleus.
In humans, mitochondrial DNA can be considered as the smallest chromosome, coding for 37 genes and containing approximately 16,600 base pairs. Human mitochondrial DNA was the first significant part of the human genome to be sequenced. In most species, including humans, mtDNA is inherited solely from the mother.Wikipedia
More than 500 point mutations in mtDNA have been implicated in various diseases so far. Because a single cell can contain hundreds to thousands of mitochondria, it also carries multiple copies — and sometimes, variants — of these maternally inherited genomes. Pathogenic mutations can co-exist with healthy mitochondrial DNA within a cell or group of cells; clinical signs of disease usually only occur when a mutation is in 60% or more of the mtDNA copies.
Mutations in one or more mtDNA copies are known as heteroplasmies, and they appear to be common. Approximately 90 percent of healthy participants in the 1000 Genomes Project harbored at least one heteroplasmy, and 20 percent bore mitochondrial genome mutations implicated in diseases (without exhibiting those diseases).[PMID 25002485]
A list of all mtDNA SNPs currently in SNPedia can be found here.