Your hive has become defensive. It has become aggressive, mean, painful and just not a pleasure to work with. We call them defensive hives because we never want to call a hive aggressive. Civilians just do not like to hear of aggressive hives. It brings up images from the movie, "The Swarm." I like to call them hot hives.
There are reasons other than genetics for a hot hive. The hive may be under attack from raccoon or skunk. You may be going into the hive too often. The nectar flow is over and the hive is now under pressure from robbers. Try to correct these possibilities before assuming the cause is genetic.
Having said this, the problem is probably genetic. Your nice docile bees have lost their queen and the replacement queen mated with drones with defensive genes. Enough of the new queen's offspring have emerged to change the hive behavior.
Hot hives are difficult and unpleasant to deal with. Re-queening is the solution; the problem is finding the old queen. The best thing you can do is move the hive a short distance, more than 10 feet, and set up a new hive in the old location. The field bees and guard bees will return to the new hive where you can kill them or just let them die in a few weeks. The best time to do this is in mid-afternoon on a nice day when most field bees are out. Moving the hive can be a painful experience, but it is less painful than trying to find the queen where the hive used to be. With the hive population now reduced and consisting only of house bees, you can go through the hive to find the queen. A new queen can be then introduced. Make sure the hot hive does not have or create any queen cells.