Whether or not Michael Phelps has the signs for Marfan syndrome, doesn't necessarily mean that he has the disease. The reasoning for this is very simple. Lets say that he does have a family history of the disease. Not that I would know this, but lets just use this as an example. Offspring of a parent that passes on either the dominant or recessive gene for a genetic disorder can pass on the signs regardless. Although Marfan is an autosomal dominant disorder, if he inherited the normal recessive gene, and the recessive gene has an increased ability to produce proteins that result with a stronger expression of this recessive gene (no signs of Marfan), co-dominance can occur. With co-dominance, only some side effects might result from the disorder. Example, Michael Phelps is a very tall individual, with an evidently longer arm span, longer hands/feet, increased flexibility, etc. However, he does not show the other mandatory side effects, such as ocular defects, back curvature of the knees (that I am aware of), as well as the aortic dilation, etc. So, due to the fact that he has some signs of the disease and not others, Michael Phelps might present co-dominance with those specific genes. That is to say, only if he actually has a family history or the disorder, as well to diagnose this disorder it would be difficult to establish if no family history is known, or if there simply is no family history revealing Marfan, especially since the gene is dominant.