Angelman Syndrome - 15q11-13 Maternal Deletion Imprinting

A 2-year-old girl presents with severe global developmental delay, absent speech, and a jerky ataxic gait with characteristic arm posturing. She displays frequent inappropriate laughter and a happy demeanor, with hand-flapping stereotypies and attraction to water. EEG shows high-amplitude 2–3 Hz rhythmic delta activity with superimposed spikes, maximal anteriorly. Her parents are unaffected with no family history. Genetic testing reveals a 5–6 Mb deletion at 15q11-13 on the maternal allele. The proband marked **B** in the pedigree diagram represents this clinical presentation. Which of the following best explains why loss of the maternal 15q11-13 allele causes Angelman syndrome rather than being compensated by the paternal allele?