Mitochondrial inheritance · USMLE Step 1

What is “heteroplasmy” in the context of mitochondrial genetics?

  1. Presence of both mtDNA and nuclear DNA in one cell
  2. A mix of mutant and wild-type mtDNA in one cell
  3. Inheritance of mitochondria from both parents equally
  4. Complete deletion of the mitochondrial genome
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Correct answer: A mix of mutant and wild-type mtDNA in one cell

Heteroplasmy refers to a mixture of mutated and normal mitochondrial DNA within a cell or organism—because each cell has many mitochondria and many mtDNA copies. This mixture influences disease severity and penetrance.

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