Ms Mucket, I am no scientist or gene specialist for sure. LOL But you might try using a different meat bird for your rooster. Say one from Nolls, or the one down in Alabama which are both totaly different stains. This cross might put some hybrid vigor back into the birds and you would get growth as good or possibly better than the parents. In cattle it is called hetorosis (SP) and according to MARK (Meat Animal Research Center) in Neb. the best possible cross is a 4 way cross with one being 50% each of two breeds and the other being 50% of two different breeds. The resulting calf grows faster, and has more imunities than either of its parents. Heck its worth a shot.