How is familial Mediterranean fever (FMF) inherited?
FMF is almost always inherited in an autosomal recessive manner.This means that to be affected, a person must have a mutation in both copies of the responsible gene in each cell. The parents of an affected person usually each carry one mutated copy of the gene and are referred to as carriers. Carriers typically do not show signs or symptoms of the condition. When two carriers of an autosomal recessive condition have children, each child has a 25% (1 in 4) risk to have the condition, a 50% (1 in 2) risk to be a carrier like each of the parents, and a 25% chance to not have the condition and not be a carrier. As many as 1 in 5 people of Sephardic (non-Ashkenazi) Jewish, Armenian, Arab and Turkish heritage are carriers for FMF.
In rare cases, this condition appears to be inherited in an autosomal dominant manner.This means that to be affected, a person only needs a change (mutation) in one copy of the responsible gene in each cell. In some cases, an affected person inherits the mutation from an affected parent. Other cases may result from new (de novo) mutations in the gene. These cases occur in people with no history of the disorder in their family. A person with FMF inherited in an autosomal dominant manner has a 50% chance with each pregnancy of passing along the altered gene to his or her child.
In some cases, FMF may appear to be autosomal dominant when it is actually autosomal recessive. This phenomenon is called pseudodominance. This may happen in families if one parent is an unaffected, unknown carrier (with 1 mutation) and the other parent is affected (with 2 mutations). It may appear that an affected child inherited FMF from only the affected parent, when in fact he/she inherited one mutation from each parent.
Last updated: 12/8/2015
What is the chance that a sibling of someone with familial Mediterranean fever will inherit this condition?
A sibling of an individual with familial Mediterranean fever may be at risk for FMF. Their level of risk depends on the genetic status of the parents. This condition is inherited in an autosomal recessive manner, so the parents of an affected individual can either be affected by this condition (they have two copies of the disease-causing mutation), or they are carriers (they only have a single copy of the disease-causing mutation). Carriers do not have symptoms of FMF. The risk for siblings depends on whether both parents are carriers or if one parent has FMF. Genetic testing of the parents may be necessary.
If both parents are carriers:
Eachsibling of an affected individual has a 25% chance of being affected, a 50% chance of being a carrier (no symptoms of FMF), and a 25% chance of being unaffected and not a carrier.
If an individual is unaffected, their chance of being a carrier is 67%.
If one parent is affected and one parent is a carrier:
Each sibling of an affected individual has a 50% chance of being affected and a 50% chance of being as carrier (no symptoms of FMF).
If an individual is unaffected, their chanceof being a carrier is 100%.
Last updated: 6/29/2012
Is genetic testing for familial Mediterranean fever (FMF) available?