Which chromosomal mutation occurs in alagille syndrome?

Alagille Syndrome is: Caused by an autisomal dominant mutation in the jagged-1 gene on the short arm of chromosome 20, 20p12. Because of "variable penetrance" some parents don't have noticeable symptoms, especially if they have mosaicism, i.e., some, but not all, of their cells carry the mutation. Therefore, although 15% of cases are "sporadic, " i.e., from new mutations, it is recommended that parents be tested.