Question:

Why aren't Jews allowed to interbreed plants? Why aren't we allowed to grow fruits like clementines or tangerines?

Answer:

First of all, it's important to realize that we can't understand the full reason for any commandment. There are different explanation and beautiful insights into the concepts and workings of each one, but as for the ultimate why—that's a mystery. We observe the commandments because that's what G‑d wants. He wants us to eat a certain way and dress a certain way, plant according to these instructions, and harvest according to others. With even the most complex and fascinating explanations, we'll never be able to say—here, this is the reason why G‑d wants this.

Having said that, an explanation offered for the prohibition against interbreeding is as follows: G‑d created His world and all creatures in it for a specific purpose. Each species of plant and animal is exactly as it should be—masterfully designed with a unique goal in mind. By mixing species, we detract from their original designs, and the perfection with which they were created in order to serve its purpose. Thus, we are not allowed to interbreed either plants or animals.

For some of the parameters of this prohibition – where and under what circumstances it applies, and the status of interbred produce – see Is genetically grafted produce kosher?

Malkie Janowski for Chabad.org