Each person has two copies of most genes in their cells. When a couple has a child, each

parent transmits only one copy to the child. If there is a mutation, or change, in one copy of a gene that produces a medical condition it is called a dominant condition. Marfan syndrome is a dominant condition in which one copy of the FBN1 gene has a mutation that affects the function of the protein, fibrillin-1, produced under the direction of the gene. If a potential parent has Marfan syndrome, each child born to that parent has a 50 percent chance of inheriting the mutated copy of the gene and, therefore, of having Marfan syndrome.

There are several instances when genetic testing may be beneficial:

• When adults with Marfan syndrome are considering having children.
• For a child with a parent who has Marfan, even before features are present in the child.
• In someone who has features of Marfan and several related conditions
• When someone has some features of Marfan, but do not have a definitive diagnosis.
• In families with a history of aortic aneurysm or dissection.

A gene is made up of DNA, which has four building blocks, called nucleotides (known by their abbreviated names: A, C, G, and T). The cell uses the nucleotides as instructions to make a specific protein. If there is a mutation or change in the nucleotide sequence, the protein may not function properly and cause a medical condition such as Marfan syndrome. Genetic testing looks for mutations or changes in the nucleotides. If a mutation or change is found in a gene, that person may have the medical condition associated with that gene mutation. If no mutation is found during the gene sequencing (genetic testing) and a person has clinical features of the disease, then a whole part of the gene could be missing. This is called a deletion. Further testing is required for confirmation.