What is the probability that normal parents will have a child with marfan's syndrome?

1/4 of cases. The syndrome is inherited as a dominant trait, carried by gene fbn1, which encodes the connective protein fibrillin-1. People have a pair of fbn1 genes & because it is dominant, people who have inherited one affected fbn1 gene from either parent will have marfan syndrome. The impact varies from mild to severe. Approximately one-quarter of the cases occur as a result of a spontaneous mutation.