Pedigree — X-Linked Recessive (DMD)

A 4-year-old boy with Duchenne muscular dystrophy (DMD) presents to the pediatric clinic with progressive proximal weakness and Gowers sign. His father is unaffected. The pedigree shows the inheritance pattern of X-linked recessive disorders. The feature marked **C** in the diagram—No Male-to-Male Transmission—is critical to understanding why this boy's father cannot have passed the DMD mutation to him. Which of the following best explains the biological basis for the absence of male-to-male transmission in X-linked recessive inheritance?