Table 1.

Molecular defects and recurrence risks in PWS

Genetic defect	Proportion of cases [7]	Recurrence risk
De novo deletion of 15q11q13 on the paternal chromosome	70–75%	<1%a
Maternal uniparental disomy (UPD) of chromosome 15	25–30%	<1%b
Imprinting defects (without an imprinting centre deletion)	1%	<1%
Imprinting centre deletion	≈10–15% of patients with an imprinting defect	50% (if present in a non-mosaic state in the father)

^aIf paternal karyotype is normal

^bIf parental karyotypes are normal