Which syndrome is due to maternal chromosome 15 involvement and presents with intellectual disability, language impairment, epilepsy, microcephaly, stereotypies, and a happy demeanor?

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Angelman syndrome arises when the maternal copy of genes in the 15q11-q13 region is lost or not expressed, because the paternal copy in this region is normally silenced by imprinting. This leads to a lack of the UBE3A gene function in the brain, causing severe intellectual disability, language impairment or absent speech, seizures, microcephaly, ataxia, and characteristic hand-flapping with a puppet-like gait. A defining feature is a happy, excitable demeanor with frequent smiling or laughter, even in the context of profound disability. This pattern contrasts with Rett syndrome, which is due to an X-linked MECP2 mutation and features normal early development followed by regression and hand-wringing; Down syndrome involves trisomy 21 with different facial and developmental features; and Prader-Willi involves paternal chromosome 15 loss of function, presenting with hypotonia and later obesity, not the happy demeanor.