Pedigree — Mitochondrial Heteroplasmy with Variable Expressivity (MELAS)

A 28-year-old woman presents with recurrent stroke-like episodes, seizures, and elevated serum lactate. Muscle biopsy shows ragged red fibers. Genetic testing confirms m.3243A>G mutation in her mtDNA. Her two siblings have the same mutation but with markedly different clinical severity — one is asymptomatic, the other severely affected with early-onset diabetes and deafness. The inheritance pattern marked **A** in the pedigree diagram demonstrates mitochondrial inheritance with heteroplasmy. Which of the following BEST explains why her asymptomatic sibling has not yet developed clinical features despite carrying the same pathogenic mutation?