Angelman Syndrome is caused by variety of factors, all involving the UBE3A gene located on the maternal copy of Chromosome 15. Because of this variation, Angelman syndrome usually occurs sporatically within the population. However, it has been shown have an inheritance component as well. In most cases, a microdeletion occurs. The second most likely cause is a mutation on that gene. Rarely, the disorder is triggered by the inheritance of two paternal copiesinstead of one from each parent. This is referred to as paternal uniparental disomy.

 http://ghr.nlm.nih.gov/gene=ube3a

In most body tissues, both the paternal and maternal regions specific to this gene are activated. This is not so in the brain. Therefore, an individual may have a fully functional paternal chromosome 15, and still exhibit abnormalities in neurological function.This is why inhereting two parental copies of the gene is bad; no maternal copy, no genetic material recognized by the brain for performing a specific task.

The UBE3A gene codes for enzyme that breaks down proteins in what is known as the ubiquitin pathway.This enzyme, ubiquitin protein ligase E3A,helps to regulate cellular function by breaking down damaged or unneeded proteins from inside the cell. One such protein regulates cellular growth. Without the degradation of these proteins in braincells, neurons misfire and the phenotypic characteristics appear: developmental delay, impaired motor skills, seizures. 

http://www.uky.edu/~jasmal3/images/UbqPathway.jpg

Another gene deleted within this same segment is OCA2, a determining factor in hair and skin pigmentation. Deletion of this gene  leads to fair skin and blonde hair, a characteristic common among of those who have AS.     ^[1]                          

http://www.asf-wac.org/ourstories.html

All children are affected by Angelman syndrome, and also show the OCA2 deletion.