Angelman syndrome is characterized by generally severe intellectual disability, severe speech impairment, gait ataxia and/or tremors of the extremities, and seemingly happy behavior. It is caused by an abnormal methylation pattern at 15q11.2-q13 or by mutations in the maternal allele of UBE3A.
Prader-Willi syndrome is a milder intellectual disability syndrome, accompanied by significant hypotonia, lack of appetite in early childhood, followed by excessive eating and gradual development of morbid obesity. Hypogonadism, short stature and a characteristic behavioral phenotype also stand out, with tantrums, stubbornness, manipulative behavior and obsessive-compulsive behaviors. It is caused by an abnormal methylation pattern at 15q11.2-q13.