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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsHeteroplasmy means that a cell contains more than one version of mitochondrial DNA (mtDNA). Because cells hold many mtDNA copies, the versions can exist in different proportions—and those proportions can shift between cells, tissues and generations. The key is that mtDNA copies are replicated and distributed unevenly, while biological selection can also favor or disfavor particular variants.
What is mitochondrial DNA heteroplasmy?
Each mitochondrion contains mtDNA, and cells generally have many mtDNA copies across their mitochondria. When those copies include different genetic variants, the cell is heteroplasmic. If the copies share the same genotype, the cell is homoplasmic. A heteroplasmy percentage describes the proportion of measured mtDNA copies carrying a particular variant in the sample tested; it is not a percentage of a person’s cells that have the variant.
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Why can mitochondrial DNA vary between cells?
As cells grow, mtDNA copies replicate, mitochondria turn over, and copies are distributed among organelles and daughter cells. This distribution is not perfectly even. Random sampling can therefore raise the proportion of a variant in one cell and lower it in another, a process often described as stochastic segregation or drift. Over many rounds of cell division, this can create differences among cells and tissues.
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Random change is not the only influence. Selection can favor or disfavor certain variants, shifting their proportions over time. The direction and strength depend on the variant and biological setting; a variant is not guaranteed to rise or fall in every person or tissue. A 2026 review describes cell-to-cell variation as arising from stochastic processes, with selection mechanisms also capable of driving directional shifts (Ryall, Chinnery and van den Ameele, 2026).
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Can siblings inherit different levels of a mitochondrial mutation?
Yes. Human mtDNA is predominantly inherited from the mother, but a mother does not pass an identical fixed proportion of each mtDNA variant to every child. During development of the germline, the mtDNA variants transmitted through eggs can shift substantially. This mitochondrial genetic bottleneck, followed by further segregation, helps explain why siblings can inherit different heteroplasmy levels from the same mother (Zhang, Burr and Chinnery, 2018).
This is not ordinary Mendelian inheritance, in which a predictable copy of a gene is transmitted from each parent. The proportion in a mother’s sampled tissue cannot by itself specify the proportion in each egg or predict the level in a particular child.
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Does heteroplasmy change over time or between tissues?
It can. Different tissues have different cell populations and histories of mtDNA replication, turnover and selection. As a result, a variant’s proportion may differ between tissues and may change over time. A measurement from blood, for example, should not be treated as the level in muscle, brain or every other organ. Reviews of mtDNA disease emphasize tissue and cell-type differences and the importance of variant-specific context (2024 review).
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For some disease-associated variants, cellular function may be affected when the variant’s proportion exceeds a threshold. That threshold is not universal: it depends on the particular variant and biological context. A percentage alone therefore cannot establish a diagnosis or determine an individual’s prognosis.
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What does a heteroplasmy percentage mean?
It is an estimate of the fraction of mtDNA copies in a particular sample that carry the variant, interpreted using a particular assay. To understand what the number can and cannot say, consider:
- Sampled tissue: The result applies to the tissue tested, not automatically to the whole body.
- Assay and interpretation: Sequencing can detect and quantify variants, but the assay’s detection and interpretation limits matter.
- Variant and clinical context: A measured proportion has no universal disease cutoff; its significance depends on the variant and the tissue involved.
One technical complication is that nuclear DNA contains segments resembling mitochondrial DNA, known as nuclear mitochondrial DNA segments (NUMTs). These can be mistaken for mtDNA variants unless the assay and analysis distinguish them carefully. A 2021 review discusses NUMTs as a potential mimic of true heteroplasmy (Plazzi and colleagues, 2021).
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A heteroplasmy result is one piece of genetic evidence, not a standalone diagnosis or forecast. Its meaning depends on the tissue sampled, how the test measured the variant, and what is known about that specific variant. For personal medical decisions, results should be interpreted with a qualified clinician or genetic counselor who can put the finding in its clinical and family context.
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