If one parent is normal but the other has marfan's, what is the probability that their child will develop marfan's syndrome?

50% The mutation in marfan syndrome is dominant, so whenever one mutation is present, the person will have the disease. A marfan syndrome parent has one normal gene and one mutated gene. Half of his or her children will get the mutated gene and thus the disease.
Autosomal dominant. As an autosomal dominant disease, there is a 50 percent chance in each pregnancy that the newborn will inherit the gene.