Mitochondrial heteroplasmy refers to the presence of more than one type of mitochondrial DNA (mtDNA) within a single cell or organism. Mitochondria, the energy-producing structures in cells, have their own DNA, which can vary due to mutations. When a cell contains a mix of normal and mutated mtDNA, it is considered heteroplasmic. This condition can affect how cells function and may lead to various health issues, depending on the proportion of mutated mtDNA. The severity of symptoms often correlates with the percentage of mutated DNA present, influencing conditions like mitochondrial diseases.