Yes, a 747 can be flown by one pilot. All large commercial aircraft including the Boeing 747 have 2 pilots in case one becomes incapacitated. Otherwise you would need 3 pilots on the flight deck.

First of all, I need to make this clarification. From the wording of the question, it implies that there is only one pilot flying with the assitance of a first officer. Both the captain and the first officer are pilots, fully qualified and trained exactly the same way. In fact, when the chips are down at the airline, captains sometimes get demoted to first officers. Keeping this in mind, heres my answer: Either pilot can perform the entire flight by himself and is 100% safe and qualified in doing so. On 3 pilot crews, even the second officer is qualified to fly the aircraft in all flight stages. I cannot emphasize enough that regardless of rank or command, they are all trained the same way and can fly the aircraft at any time. I hate the term co-pilot, because it implies that the captain is the one flying and the first officer is a helpful side kick to turn knobs, throw switches, and fly the plane while he takes a bathroom break. The only difference between the captain and first officer is the paycheck and epaulets. The captain and first officer actually alternate flying the plane. For instance, I was captaining a 5 leg flight last weekend. The first officer and I alternately flew each leg of the flight. He ended up flying 3 legs and I flew 2. If I had died of a heart attack during any one of those legs, he could have completed the flight without me. I jokingly say, the only problem of doing everything yourself is that some things require a little longer of a reach to get to. Since pilots alternate flying, they also alternate the other responsibilities. While one is rotating the aircraft to the correct heading, the other one may be communicating with ATC to confirm a heading or request a clearance. While one is hand flying an approach, the other is putting down the gear, setting the flaps, and on the radio asking for clearance. Even though this helps the flight go smoother, both pilots know how to do all of these things and are qualified to do them, so even by themselves either pilot can do everything. Lastly, note the captain is the one in command and makes all final decisions. If he should be incapacitated, this responsibility transfers to the first officer.