Marfan syndrome is hereditary, which means it can be passed to a child from a parent who's affected.
In around three-quarters (75%) of cases, Marfan syndrome is inherited from one parent. The syndrome is autosomal dominant, which means a child can inherit it even if only one parent has the syndrome.
There's therefore a one in two (50%) chance that the child of a parent with Marfan syndrome will inherit the syndrome.
The gene defect leads to abnormal production of a protein called fibrillin, resulting in parts of the body being able to stretch abnormally when placed under any kind of stress.
The defective fibrillin gene also causes some bones to grow longer than they should. This means a person with Marfan syndrome may be tall because their arms and legs grow longer than normal.
In the remaining quarter (25%) of cases, neither parent has the syndrome. In these cases, the fibrillin gene changes (mutates) for the first time in the parent's egg or sperm. The mutated gene can be passed on to the child, who will then go on to develop the syndrome.