Only in the 20th century did genetic engineering reach a point where it began to present active dangers to humans as the world as a whole. Before this time, humans used natural laws to manipulate genes, a method that began with technically with Mendel's pea plant experiment in the back-gardens of an abbey but has in fact been a part of human history since we started growing plants for nutrition and raising animals. When we began choosing more tasty, hardier breeds and developing useful, inherited characteristics through careful seeding and breeding we tampered with genetics—albeit a useful and usually harmless tampering.

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Then the latter half of the 1900s dawned, and scientists began dabbling in earnest, starting small with viruses and bacteria and finding ways to isolate genes or alter DNA, identifying key enzymes and putting the pieces together to form a picture of how DNA works. The 1970s saw widespread genetic research, while institutes (in America, at least) such as the National Institute of Health began to put restrictions on experimentation until satisfied it could be conducted safely. Unfortunately, the NIH spoke about 30 years too soon.
Our reach exceeds our grasp, they saw, and science proves this best. Up through the 1990s, genetic engineering produced surprising resources, such as the famous genetically engineered insulin and interferon medicine products, vaccines, healthier plants, and bug-resistant vegetables. Not only the NIH but the FDA, the Department of Agriculture, the US Patent and Trade Office, and the National Academy of Sciences got behind altering DNA to advance human medicine and agriculture. Eventually, when slight genetic enhancements in everyday products were becoming fairly common, science caught up with itself and researchers began discovering that even slight genetic manipulation resulted in long-term, irreversible, and possibly dangerous results, not just for biology but for economics and industry, too.
Here are five examples of dangers our just-in-time genetic engineers are beginning to consider: