Six persons with the classical Angelman syndrome (AS) phenotype and de novo 
deletions of chromosome 15q11-q13 were studied to determine the parental origin 
of the chromosome deletion. Four of the 6 patients had informative cytogenetic 
studies and all demonstrated maternal inheritance of the deletion. These 
findings, together with other reported cases of the origin of the chromosome 15 
deletion in AS, suggest that deletion of the maternally contributed chromosome 
leads to the AS phenotype. This contrasts with the Prader-Willi syndrome (PWS) 
in which a similar deletion of the paternally contributed chromosome 15 is 
observed. In deletion cases, a parental gamete effect such as genomic imprinting 
may be the best model to explain why apparently identical 15q11-q13 deletions 
may develop the different phenotypes of AS or PWS.