A 4-year-old boy with blue sclerae, dentinogenesis imperfecta, and recurrent low-impact fractures presents with his third long-bone fracture. His father, paternal uncle, and paternal grandmother are all affected with osteogenesis imperfecta of varying severity. The pedigree shows the pattern marked as **A** in the diagram—vertical transmission across three generations with affected males passing the condition to both sons and daughters. Which inheritance pattern and genetic basis best explains this family's presentation?