Because the gene for Marfan's syndrome is dominant, parents with the disorder have a 50% chance of passing the condition on to their children. Patients tend to be tall and thin with very long thin fingers, and hypermobile joints. There may be problems in many organ systems, but in the knee the issues involve patella subluxation and dislocations and cruciate ligament problems.
When flexibility is not necessarily a virtue: a review of hypermobility syndromes and chronic or recurrent musculoskeletal pain in children. Cattalini M, Khubchandani R and Cimaz R. Pediatr Rheumatol Online J. 2015; 13: 40.