Hereditary Spastic Paraplegia (X-linked SPG1/SPG2)

A 9-year-old boy presents with progressive spastic gait, lower-limb spasticity, hyperreflexia, bilateral extensor plantar responses, and adducted thumbs held tightly across the palms. MRI reveals agenesis of the corpus callosum and hydrocephalus. His pedigree shows affected maternal uncles and male cousins (through carrier females), but NO affected females and NO male-to-male transmission. Every affected male is connected through a carrier mother or grandmother. The inheritance pattern marked **A** in the diagram is most consistent with this presentation. Which of the following genetic mechanisms best explains why affected fathers CANNOT transmit this condition to their sons?