How is Marfan syndrome inherited?
Marfan syndrome is inherited in an autosomal dominant manner. This means that there is not sex bias to the inheritance of the condition. Marfan syndrome is caused by the presence of one genetic change (called a mutation) in the FBN1 gene. We all have two copies of the FBN1 gene, one from mom and one from dad. A person has Marfan syndrome when a mutation on one copy of the FBN1 gene prevents it from working, even though the second copy of the FBN1 gene is normal. A person with Marfan syndrome has a 50% chance of passing the copy of the FBN1 gene with the mutation to each of their children who would then also have Marfan syndrome, and 50% to pass down the normal copy of the FBN1 gene to each of their children who would not have Marfan syndrome.
75% of individuals with Marfan syndrome have a parent who also has Marfan syndrome, meaning it was inherited. However, the other 25% of individuals with Marfan syndrome do not inherit the genetic change from a parent. The genetic changes that cause Marfan syndrome in these individuals are de novo, meaning they happened spontaneously or randomly only in that individual, likely around the time they were conceived.