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heteroplasmy The presence within a cell or organism of mitochondria with different genetic constitutions. 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.