The presence within a cell, tissue, or organism of mitochondria with different genetic constitutions. Mitochondrial genomes have a high mutation rate, and mutations probably occur in all individuals. Such mutations can be of two types, either altering the sequence of the nucleotides or varying the total length of the mitochondrial DNA. They are subject to selection in cells and can be lost or transmitted to progeny cells at cell division. More rarely, heteroplasmy results from inheritance of paternal mitochondria. Generally, an organism inherits its mitochondrial DNA from its mother via the egg cell, with no contribution from the male gamete, and therefore all its mitochondria are genetically identical. However, in some species there is ‘leakiness’ from the male line. For example, in mice about 1 in 1000 mitochondria originate from the male parent. Exceptionally, in some organisms (such as mussels) mitochondria are apparently inherited equally from both parents. Compare homoplasmy.