Duchenne Muscular Dystrophy DMD X-linked Recessive

A 4-year-old boy presents with progressive proximal lower-limb weakness, toe-walking, and frequent falls over the past year. His mother reports he walked normally until age 2 years. Serum creatine kinase is markedly elevated at 45,000 IU/L. Genetic testing confirms a deletion in the DMD gene on chromosome Xp21.1 with loss of dystrophin expression. The inheritance pattern marked **A** in the pedigree diagram shows the characteristic X-linked recessive transmission. Which of the following best explains why this boy's father is clinically unaffected despite carrying the same mutation?