A 28-year-old woman presents with recurrent stroke-like episodes, progressive encephalopathy, and elevated serum and CSF lactate levels. Genetic testing confirms a mitochondrial DNA mutation. Her two children (one son, one daughter) are both asymptomatic on examination. Her brother, who carries the same mutation, has two children who are unaffected. The inheritance pattern shown in the diagram at **A** (Maternal Mitochondrial Inheritance) explains why the proband's children are at risk, but her brother's children are not. Which of the following best explains the pedigree pattern observed in this family?