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For the case of genetically altered crops. Pros: They can be made to have higher yeild, higher/better nutritional value (healthier), higher tolerance to disease and/or pests (and even pesticides), easier and safer to grow and harvest and you could make it taste better too if you wanted to. Cons: You will have a homogenetic crop without much if any genetic variation, making it very susceptible to the diseases etc that still can infect it. It might also spread to the wild where it might outcompete a naturally occuring crop and thus destabilize the ecosystem. It might also cross fertilize with local flora and create a "superweed" that might also collapse the local ecosystem and/or become an enormus pain to control. In addition to this there is also a chance that the genetic manilpulation will have a negative side effect that was not intended, like if you introduce a protein that facilitates one specific chemical reaction in the cell it might also facilitate another unwanted (and for the consumer harmful) chemical reaction that you wont notice since you only looked for the good stuff. But most importantly, we currently lack effective laws to regulate genetically grown crops, we have a law system that is not designed to handle the current level of genetical manipulation. If you want to read up on controversies regarding genetic manipulation and outdated laws you can start to read up on the corporation Monsanto.