Horizontal gene transfer or HGT, sometimes also known as lateral gene transfer, refers to the movement of genetic information between cells and organisms by means other than sexual reproduction1.
In simpler terms, we humans exchange genetic information when we have sex leading to reproduction; but bacteria and many other species don't have sex, yet under certain circumstances manage to "mix their genes" anyway. Amazingly enough, this is sometimes even possible between species that are not sexually compatible or even drastically unrelated. Horizontal gene transfer has been observed between bacteria and rodents, for example.
It turns out there are three mechanisms by which this can occur:
- Transformation is the uptake and incorporation of "naked" DNA by a cell. In mammals, this is known as transfection. Some bacteria do this fairly actively, and this ability in bacteria is known as competence.
- Conjugation is a more specialized form of gene exchange, practiced by most bacteria. Bacteria reproduce asexually by fission but can exchange genetic material with other bacteria when in direct contact.
- Transduction requires viruses or bacteriophages (bacterial viruses) as a carrier. The virus incorporates the genetic material in its own genome during infection and can later transfer it into other cells.
My main reference1 is a trial deposition about "Risks posed by horizontal gene transfer
with regard to Chardon LL maize" for a court hearing. It is perhaps not a coincidence that this is the first useful reference I found, because there are rising concerns about the risks of gene technology and its possible cross-species effects.