You might grow corn, or you might grow defective unusable corn and/or any number of other things like locusts or fungi or other plants that decide to grow in the place where you planted corn. Sure, the corn seeds will not produce ball bearings. Genius observation. There are about an infinity of other things that can and do happen besides that.

Planting is merely setting up the conditions. We didn't write the dna, we couldn't write the dna if we wanted to because we are an infinity away from understanding all the actual processes that descend from the dna. And when we utilize the dna that we simply found and didn't and couln't hope to write, it's always, at best, a case of hoping it goes right again this time.