Mode of inheritance

Most individuals with adFEVR inherit the altered gene from a parent, although the parent may not have any symptom of the disease. Offspring of an affected individual has a 50 % risk of inheriting the mutation, but more than 50 % (up to 90 %) of individuals with adFEVR can be asymptomatic because of reduced penetrance.

Individual with arFEVR inherits the disease from both parents (each of them carries one copy of the mutated gene) but they do not have the disorder.

When FEVR is caused by mutations in the NDP gene, which is located on the X chromosome, in male individuals (who have only one X chromosome), one altered copy of the gene is sufficient to cause the disease. In females (who have two X chromosomes), a mutation would have to occur in both copies of the gene to cause the disorder. A striking characteristic of X-linked inheritance is that fathers cannot pass the X-linked traits to their sons. Although very rarely, both Norrie disease and XFEVR can occur in women.