A transgene is a section of genetic material from one organism which appears in the DNA of another organism. Depending on a number of factors, the transgene may fail to express, may express in a way different from that observed in the original organism, or may express in the new organism in exactly the same way it did in the original one. A gene is identified as a transgene when it has been sequenced in one species and later identified in another.
Some transgenes occur naturally. For example, bacteria are very adept at exchanging genetic material, even across bacterial species, because this allows them to adapt rapidly to changing environments. This is one reason why drug resistance is a concern, because bacteria can not only pass drug resistance on to members of their species, but also potentially to members of other species.
In other instances, transgenes are inserted through genetic engineering. Genetic engineering is used to do everything from creating transgenic mice for laboratory research to developing new crops which will be resistant to drought. In this case, a gene of interest is identified and inserted into the genome of the target organism in the hopes that it will express as desired. For example, headlines were made in the early 21st century when a man claimed to produce a rabbit which glowed in the dark by inserting jellyfish genes into the genome of a rabbit.
A transgene can be passed on to future generations, which is sometimes a cause of concern for researchers. Some people fear that transgenic organisms made in the lab could weaken wild populations of the same animals, or that inserting transgenes could have unintended consequences which will only become apparent when it is too late. In response to this concern, sometimes sterility is created during the process of genetic engineering, as with transgenic crops which are not supposed to be able to reseed themselves.
Transgenes can be used and studied in a number of ways. A transgene may be used, for example, for a purpose such as introducing human DNA into lab animals in order to study human pathology without experimenting on humans. The exploration of gene expression or lack thereof can also be accomplished with the use of transgenes and transgenic organisms. People may be curious, for example, about genes which appear dormant in some animal species, wondering if these genes are relics of ancestors or if they can be induced to express.