Tuberous Sclerosis Pedigree

A 3-year-old boy presents with a 6-month history of infantile spasms and developmental delay. On examination, he has multiple hypomelanotic macules on his trunk and facial angiofibromas. Brain MRI shows cortical dysplasias and subependymal nodules. Genetic testing confirms a de novo TSC2 mutation. The inheritance pattern marked **B** in the pedigree diagram indicates this is an autosomal dominant disorder with a high de novo mutation rate. Which of the following best explains why approximately two-thirds of tuberous sclerosis cases present as sporadic (de novo) mutations rather than inherited from an affected parent?