Symbiogenesis is a rare process where an organism reproduces by merging with another organism to create a hybrid. The process usually occurs with microcellular organisms, such as the Andorian amoeba. However, the process occurred on the USS Voyager in 2372 when Lieutenant Commander Tuvok and Neelix found themselves combined to form a new individual they named Tuvix.
Neelix and Tuvok had been collecting samples of a rare orchid when they were beamed up. The orchid enzymes reacted with the separated microcellular state the crewmen were in, combining their DNA. As a result, there was only one pattern, the combined one, to rematerialize.
The combination was a success, and Tuvix had no ill effects physically or mentally. Eventually, The Doctor found a way to separate them by using a radio-isotope which would attach itself to one of the original species' DNA patterns, but not the other, allowing the transporter to separate them, despite Tuvix's protests.