Riddle me this

Here's an old riddle. I am using the cognitive scientist and author Douglas Hofstadter's version, which appeared in Scientific American in 1982. If you find you can't solve it, contact me and I'll email you the answer.

A father and his son were driving to a ball game when their car stalled on the railroad tracks. In the distance a train whistle blew a warning. Frantically, the father tried to start the engine, but in his panic, he couldn’t turn the key, and the car was hit by the onrushing train. An ambulance sped to the scene and picked them up. On the way to the hospital, the father died. The son was still alive but his condition was very serious, and he needed immediate surgery. The moment they arrived at the hospital, he was wheeled into an emergency operating room, and the surgeon came in, expecting a routine case. However, on seeing the boy, the surgeon blanched and muttered, “I can’t operate on this boy — he’s my son.”

What's the explanation? As Hofstadter wrote, "You'll know when you've got it, don't worry."