Karyotype of Angelman syndrome (AS), computer illustration. AS is a genetic disorder characterised by arm flapping, excessive laughing (even when hurt), a jerky gait and learning problems. It is caused by genes being deleted or inactivated (switched off) on the copy of chromosome 15 that is inherited from the mother. Roughly one in 15,000 individuals have the condition. Life expectancy is normal, however those with AS are unable to live independently. If the same defect is on the paternal chromosome instead, it causes Prader-Willi syndrome. Labelled version of the illustration.