Cross-breeding is fine, inbreeding is not. When you identify A+B makes a taller plant, you start breeding the children C with each other to isolate the "tall"

Cross-breeding is fine, inbreeding is not. When you identify A+B makes a taller plant, you start breeding the children C with each other to isolate the "tall" gene. Soon you are inbreeding the descendent's of A+B. Most "breeds" of dogs are horribly inbred so that they maintain their traits. Same goes for "heritage" vegetables, "fancy" rats, or any other organism that humans have selectively cultivated.