Heteroplasmy vs. Homoplasmy
Replication and random segregation of heteroplasmic mtDNA mutations
A cell can have some mitochondria that have a mutation in the mtDNA and some that do not. This is termed heteroplasmy. The proportion of mutant mtDNA molecules determines both the penetrance and severity of expression of some diseases.
Homoplasmy refers to a cell that has a uniform collection of mtDNA: either completely normal mtDNA or completely mutant mtDNA.
A unique feature of mtDNA is that, at cell division, the mtDNA replicates and sorts randomly among mitochondria. In turn, the mitochondria sort randomly among daughter cells. Therefore, in cells where heteroplasmy is present, each daughter cell may receive different proportions of mitochondria carrying normal and mutant mtDNA.