For example, consider James' first degree relatives. Their risk depends on their age: at birth the risk is 50%, but it declines as a person ages without having suffered from deep vein thrombosis.
Knowing that they are at risk, James' relatives may want to be tested for antithrombin III levels to see if they have James' trait or not.
If they do have an antithrombin III deficiency, they can modify their behavior to reduce the risk of having a deep vein thrombosis.
Finding out that they do not have Jame's condition may also be beneficial since it may relieve them of unnecessary worry.