Heteroplasmy is the presence of a mixture of more than one type of an organellar genome (mitochondrial DNA (mtDNA) or plastid DNA) within a cell or individual. Since every eukaryotic cell contains many hundreds of mitochondria with hundreds of copies of mtDNA, it is possible and indeed very frequent for mutations to affect only some of the copies, while the remaining ones are unaffected.

Symptoms of severe heteroplasmic mitochondrial disorders frequently do not appear until adulthood because many cell divisions and a lot of time is required for a cell to receive enough mitochondria containing the mutant alleles to cause symptoms. An example of this phenomenon is Leber optic atrophy. Individuals with this condition often do not experience vision difficulties until they have reached adulthood.

The incidence of heteroplasmy in human mtDNA is unknown, as the number of individuals who have been subjected to mtDNA testing for reasons other than the diagnosis of mitochondrial disorders is small. One notable example of an otherwise healthy individual whose heteroplasmy was discovered incidentally is Nicholas II of Russia, whose heteroplasmy (and that of his brother) served to convince Russian authorities of the authenticity of his remains.

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