Mitochondria are the power generators of the cell. Each cell has thousands of them, and each has its own genome. The DNA it possesses is needed for survival, and it has to be replicated to generate new ones. This means that mitochondria that can replicate better can out-compete others with replicative disadvantage and, in extreme cases, take over the entire cellular compartment. Since, the cell is blind to such competition, 'bad' or non-functional mitochondria can take over, to their own benefit and to the cell's detriment; thereby leading to 'selfish behavior'.
This is of particular importance to current human health. UK's decision to allow three-parent baby is a monumental step in curing a set of congenital diseases. In this the defective mitochondria is replaced with that from a healthy donor. But even if a few defective ones remain behind (from among thousands), this also creates a competition among the two populations. And if the selfish drive of the defective one is strong enough, it might again take over the cell, thereby increasing the chance of pathogenesis. A good donor should not only be healthy, but also strong in its selfish drive. To understand how such competition is accomplished, please listen to the interview with Hansong Ma.
For more information, please refer to:
Ma and O'Farrell. Nature Genetics 48, 798–802 (2016).
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