A genetic disease which appears to be to be the genetic counterpart to Prader-Willi Syndrome, with the opposite pattern of imprinting (maternal imprinting for Angelman, paternal for Prader-Willi).

Clinical Features
The Angelman phenotype was first described by French doctors Bower and Jeavons in 1967 as "happy puppet" children ('marionette joyeuse'), due to the unusual gait and mannerisms of patients. The gait ataxia seems to be the result of unusual electrical activity in the brain, somewhat like low-grade epileptic seizures. Some of the earliest described patients had convulsions and episodes of flapping their arms up and down with elbows bent (like a puppet on strings, leading to the name of the disease), as well as uncontrollable laughter. Other general features of the disease are hypopigmentation compared to relatives, a tendency of the tongue to protrude, and severe mental retardation (speech absent, or limited to 6 words or less).

Molecular Basis
Angelman Syndrome is inherited through maternal imprinting of the long arm of chromosome 15. The most common mechanism of transmission is as a deletion of bands 15q11-q12 inherited from the mother (note, this is the same exact region as Prader-Willi Syndrome, but with maternal imprinting).

Because the disease phenotype is the result of chromosomal abnormalities, most cases are sporadic in nature, arising without previous family history. As such, the risk of recurrence is very low, considered to be about 1 in 1000.

As an aside, the mouse model for AS has a very interesting phenotype, exhibiting many of the neurological defects of AS sufferers. The mouse is highly sensitive to sound, going into seizures at any sharp noise. In fact, the lab in which I worked accidentally killed the very first Angelman mouse we made in this exact manner while video taping it. The camcorder tapping against its cage caused the mouse to seize uncontrollably until it died.