What are the chances of a person with marfan syndrome passing it to their offspring?

Have a 50% chance of passing it on to there offspring.

This is called an autosomal dominant gene. What that means is, considering each human has TWO sets of chromosomes (1 from mom, 1 from dad), it takes only ONE copy of the mutated gene being passed on for the child to inherit the syndrome. Because for the conception of each child there is a chance of passing one or the other gene on, there is a 1 in 2 or 50% chance of giving the child Marfan Syndrome.