Alagille Syndrome JAG1 Autosomal Dominant Pedigree

A 3-month-old male infant presents with prolonged neonatal jaundice, conjugated hyperbilirubinemia, and pale stools. Clinical examination reveals characteristic triangular facies, a harsh systolic murmur in the pulmonary area, and posterior embryotoxon on ophthalmologic examination. Skeletal imaging shows butterfly vertebrae. Liver biopsy demonstrates bile duct paucity. The pedigree shown displays the inheritance pattern marked **A**, with the affected mother showing only mild GGT elevation and subclinical skeletal findings, while her son has severe multisystem involvement. The maternal grandfather had end-stage liver disease. Which of the following best explains the inheritance pattern and clinical presentation observed in this family?