Mitochondrial D-Loop
All the mitochondria of an individual are originating from the mother. This means that if the mother is afflicted by a mitochondrial disease, the offspring will also be affected. This is why the three parent model is being considered in some cases (1). 

(1) Pritchard, C. (2014, September 1). The girl with three biological parents. BBC News. From http://www.bbc.com/news/magazine-28986843