•  Imprinting—one gene copy is silenced by methylation, and only the other copy is expressed → parent-of-origin effects.
  • Prader-Willi syndrome
    • Maternally derived genes are silenced (imprinted). Disease occurs when the Paternal allele is deleted or mutated. Results in hyperphagia, obesity, intellectual disability, hypogonadism, and hypotonia.
    • Associated with a mutation or deletion of chromosome 15 of paternal origin.
    • 25% of cases due to maternal uniparental disomy.
  • AngelMan syndrome
    • Paternally derived UBE3A gene is silenced (imprinted). Disease occurs when the Maternal allele is deleted or mutated. Results in inappropriate laughter (“happy puppet”), seizures, ataxia, and severe intellectual disability.
    • Associated with mutation or deletion of the UBE3A gene on the maternal copy of chromosome 15.
    • 5% of cases due to paternal uniparental disomy.