Why does my Cat attack her grown up kitten?

I took in a stray about 2 years ago unaware at the time she was pregnant. Well she had 5 kittens, I found homes for 4 of them and kept a little male. Well now mommy cat has gotten to where she attacks her son all of the time. He is little over a year old now and both of them have been spayed and neutered. They are both inside/outside cats and after they finish breakfast I have to let her out the front door and him out the back door. If he tries to follow her out she turns and attacks him. He will just lay down and take it. I feel bad for him. He is so much bigger than she is but he won't fight back at all. How can I get her to stop?

This is quite typical of the momcat to do this. The only thing you can do is add cat condos, or put her in a room by herself for a few hours a day so he can have a rest from the attacks. Add cat ramps, and places where he can get away from her, but really this is quite common, and not much can be done about it.

Just to add, with wildcats, the mother cat will often chase her kittens away once they are ready to be out on their own, especially male kittens. This is an inbuilt thing so that inbreeding is avoided and mum does not have to provide for them. Your momma cat is probably trying to tell him that he's old enough to be out on his own now and to stop hanging around her. He doesn't fight back because she is the dominant cat. You might want to try a feliway diffuser. Feliway is a substance that helps to create harmony and calm for cats. You plug the diffuser into an electric outlet in the room most often used by the cat. More information here: http://www.feliway.com/html/detail.php3/pID=2

Angel does this sometimes with Sagwa, Sarobi, and Cringer. Usually they are fine...all cuddled up and grooming each other. But once in awhile Angel will be sitting by herself and if one of the "kids" comes by she attacks them. I think its the kitty equivalent of "Leave me alone"!