When a person receives both chromosomes from the same parent it is called "uniparental disomy."
When a person receives both chromosomes from his or her mother, it is called "maternal uniparental disomy."
Maternal uniparental disomy for chromosome 15 leads to PWS because the genes on chromosome 15 that should have been inherited from the father are missing, and the genes on both the chromosome 15s inherited from the mother are imprinted.
PWS caused by maternal uniparental is sporadic.
If a child has PWS due to maternal uniparental disomy, the chance the parents could have another child with PWS is less than 1 percent.