This is done by cross-breeding. As many fruit trees require as many as six years to begin producing fruit, crossbreeding is a timely and often costly process. Some trees may resist cross-pol- lenization, and the quality of hybrids is difficult to predict.
另一方面，過去流傳至今的許多品種，都是經過多次自然雜交的成果，遺傳因子非常複雜，很難預測或控制下一代的品質。