http://www.bradshawfoundation.com/journey/eve.html
To say that we get exactly half of our DNA from our father and half from our mother is not quite true. One tiny piece of our DNA is inherited only down the female line. It is called mitochondrial DNA because it is held as a unique circular strand in small tubular packets known as mitochondria that function rather like batteries within the cell cytoplasm.
So each of us inherits our mtDNA from our own mother, who inherited her mtDNA intact from her mother, and so on back through the generations hence mtDNAs popular name, the Eve gene. Ultimately, every person alive today has inherited their mitochondrial DNA from one single great-great-great-. . .-grandmother, nearly 200,000 years ago. This mtDNA provides us with a rare point of stability among the shifting sands of DNA inheritance.