The Inheritance of NPD
Types A and B NPD are autosomal recessive disorders. Since both forms of the disease are caused by abnormalities in the same gene (i.e., ASM), they are considered to be "allelic." Types C and D NPD are caused by mutations in a distinct gene, and despite their similar name and some similar biochemical and clinical findings, are completely distinct disorders from Types A and B NPD. For autosomal recessive disorders such as these, in order to have an affected child both parents must carry one copy of the abnormal gene. However, such individuals (called carriers or heterozygotes), have no signs of the disease. Children with Types A and B NPD have two copies of the abnormal gene (one each coming from the mother and father), which leads to the reduced ASM activity and disease symptoms.
Each time carriers have a child, there is one chance in four that the child will be affected with NPD and one chance in two that the child will be a carrier.