What makes mitochondrial DNA special?

Information about Mitochondrial DNA :

• Found in the cytoplasm, outside the nucleus.
• Only passed through the mother. During fertilization, only nuclear DNA from the sperm enters the cell; thus the father does not make any contribution to the mitochondrial DNA.
  Abundant in number. While your nuclear chromosomes are only found in 2 copies per cell, one from your mother and one from your father, mtDNA is found much more abundantly—anywhere between hundreds to thousands of copies per cell.
• Small and circular. While nuclear DNA is made up of much longer strands of DNA that are tightly coiled into chromosomes, mitochondrial DNA is found as a small, circular chromosome.
• Mitochondrial DNA codes for proteins that are involved in cellular respiration. In addition, certain regions of the mtDNA are useful for ancestry research. These regions are called hypervariable regions (HVR).

HVRs are the site where the most mutations occur in mtDNA. On average, one letter in the sequence of mtDNA changes every 1,500 years. This change is then passed on to succeeding generations. The changes in an individual’s mtDNA comprise his or her haplotype—the mitochondrial DNA profile – that allows scientists to assign a person to his or her haplogroup, a group of people with similar haplotypes. The haplogroup is associated with a historical and geographical origin.