Why everyone with Hereditary Sperocytosis doesn't have an Affected Parent?

Hereditary spherocytosis is a disease inherited in autosomal dominant manner. Therefore, theoretically everyone with this condition should have at least one affected parent, but this is not the case in 25% of the patients with this particular condition. This is because 25% of the patients who develop this condition get is as a result of spontaneous mutation of the genes responsible for the development of the red cell membrane.