Pedigree — Genomic Imprinting (Prader-Willi vs Angelman Syndrome)

A 3-year-old boy presents with severe developmental delay, absent speech, and a characteristic ataxic gait with jerky arm movements. His parents note he has frequent inappropriate laughter and a happy demeanor despite his severe disability. EEG shows high-voltage 2–3 Hz delta waves with posterior triphasic activity. The pedigree shows this is a de novo case. The chromosomal region marked **A** in the diagram (15q11-q13 imprinted region) is affected. Which of the following BEST explains the clinical presentation and the parental origin of the lesion in this child?