What are GMOs?
GMO, or Genetically Modified Organism, is a term usually used to describe crops that have had one or more genes added to them. These genes can come from related or distant species: for example, a genetically modified white potato could have a gene from a wild potato added to its DNA. It can also have a gene from a bacteria added to its genome. These genes give the crop new characteristics or traits.
Scientists give crops new traits to help farmers or to add features that consumers want. An example of a crop engineered to help farmers is the Rainbow papaya, which was modified to be resistant to a virus that impacts the growth of the fruit. Traits such as pest resistance decrease the amount of pesticides that farmers use. An example of a crop that was modified to satisfy consumers is the Arctic Apple, which was engineered with a gene from the apple itself so that it does not bruise or brown. This reduces food waste from farm to table.
There are only a handful of genetically modified crops on the market. A list of crops is available here and a more detailed database of engineered crops, their regulatory approval status globally is available here. Each country has a different process for the approval and testing of GMOs. In the United States, this process takes years while the FDA, USDA, and other entities ensure that the crops pose no greater risk than other crops. An excellent article outlining the nuances of this process are outlined here.
Despite the persistent myth, there are no crops with fish genes in them currently on the market. All organisms on the planet have DNA and due to evolution, the genes that make up our DNA can be quite similar. In fact, 60% of our genes are similar to those in fruit flies. Consequently, adding a gene from a distant species does not make this process immediately risky or harmful, and this process does occur naturally in crops.
To learn more about GMOs, see this Q&A put together by the Royal Society in the UK.