[11] The main characteristics of Angelman syndrome are severe intellectual disability, ataxia, lack of speech, and excessively happy demeanor.

Angelman syndrome results from a loss of gene activity in a specific part of chromosome 15, the 15q11-q13 region.

In most cases (about 70%)[citation needed], people with Angelman syndrome have a deletion in the maternal copy of chromosome 15.

People with paternal UPD for chromosome 15 have two copies of the UBE3A gene, but they are both inherited from the father and are therefore inactive in the brain.

In a small percentage of cases, Angelman syndrome may be caused by a chromosomal rearrangement called a translocation or by a mutation in a gene other than UBE3A.

Angelman syndrome can be hereditary, as evidenced by one case where a patient became pregnant with a daughter who also had the condition.

Prader-Willi syndrome is caused by the loss of active genes in a specific part of chromosome 15, the 15q11-q13 region.

Rarely, the condition is caused by an abnormality in the DNA region that controls the activity of genes on the paternal chromosome 15.

A larger isodicentric chromosome 15 can result in weak muscle tone (hypotonia), intellectual disability, seizures, and behavioral problems.

[13] Signs and symptoms of autism (a developmental disorder that affects communication and social interaction) have also been associated with the presence of an isodicentric chromosome 15.

In some cases, several of the chromosome's DNA building blocks (nucleotides) are deleted or duplicated.