A transgenic organism is an organism which has been modified with genetic material from another species. The genetic modification is accomplished by inserting DNA into an embryo with the assistance of a virus, a plasmid, or a gene gun. The embryo is allowed to develop, and the mature organism will express the DNA which has been inserted into its genome. Transgenic organisms can also pass the modification on to future generations by breeding with other members of the same species.
Organisms may be genetically modified for any number of reasons. Genetic modification may be performed to make organisms more vigorous, to add resistance to specific threats, or for the goal of expressing a particular trait. It can be used for everything from adding nutrition to staple crops for the benefit of people in the developing world to making fish glow so that they can be sold as novelty pets.
While species cannot interbreed, as a general rule, DNA from one species can express in another. This is because DNA codes for the same thing no matter where it is, so if a researcher can determine which part of an organism's genome codes for a particular trait and the trait is compatible with the organism being modified, the DNA can be successfully inserted to cause the trait to be expressed in a different species.
One of the most common reasons to develop a transgenic organism is in agriculture, where the development of genetically modified crops has led to a number of advances in the way in which agriculture is practiced. This organism may grow in areas where other members of the species cannot, may be easier to harvest and handle, may be hardier, may contain more nutrition, and so forth. In some cases, the organisms are rendered sterile so they cannot interbreed, for the purpose of protecting patents and to prevent transgenic organisms from cross-breeding with wild relatives.
Another area in which some transgenic organisms are used is in scientific research. Transgenic mice, for example, are used for a variety of studies in which researchers want to be able to study traits from one species in a more convenient setting. Mice can be modified with human DNA for the purpose of testing medical treatments and seeing how they might behave in a human.
There is some controversy over the practice of genetic modification. Some advocates are concerned that interbreeding between transgenic and wild organisms could have unforeseen consequences, and others worry that consuming things like transgenic organisms could be dangerous.