A 32-year-old man with a 15-year history of episodic burning pain in hands and feet, hypohidrosis, angiokeratomas in bathing trunk distribution, and recent lacunar strokes is diagnosed with Fabry disease. His mother had chronic kidney disease requiring hemodialysis at age 55 and also reported episodic burning foot pain in youth. His maternal uncle died at age 42 from renal failure with confirmed Fabry disease. His sister (age 28) reports occasional foot burning and elevated proteinuria. Genetic testing confirms a GLA gene missense mutation. The inheritance pattern shown in this pedigree is best described by the structure marked **B**. Which of the following BEST explains why both the patient's mother and sister show clinical manifestations despite being female?