Familial Retinoblastoma RB1 Autosomal Dominant

A 14-month-old boy presents with leukocoria and strabismus. His father survived bilateral retinoblastoma in infancy and later developed secondary osteosarcoma. Examination reveals bilateral retinoblastomas in the proband. A three-generation pedigree shows affected individuals in consecutive generations with male-to-male transmission, and an unaffected aunt who carries a pathogenic RB1 mutation. The inheritance pattern marked **A** in the diagram is most consistent with which of the following genetic characteristics?