A hybrid either describes:
- The interspecies reproduction between two or more animals, alien races, or plants; or
- The intraspecies reproduction between two or more races or ethnic populations within the same species.
As most humanoids share a common genetic ancestor, the ancient humanoids who seeded the Milky Way Galaxy with genetic material, interspecies reproduction can occur between many races. (TNG: "The Chase") Interspecies reproduction can be difficult between some species as not all species have their genitals in the same place. (Star Trek VI: The Undiscovered Country)
Hybridization is not without risk. Several anatomical, physiological and psychological abnormalities can occur. Kowalczyk's Syndrome affects the nervous system in hybrids from two or more telepathic species. (Star Trek: Shadowstar Station: "Through the Looking Glass") Vulcan hybrids could suffer from Vulcan Hybrid Societal Maladjustment Syndrome. (Star Trek: The Cantabrian Expeditions: "Bitter Sweet") They could also suffer Jaren's Hybridic Lupus, a manageable but incurable disease in the 24th century. (Star Trek: Liberty: "A Room of Her Own")
Some species are also nearly or totally incompatible. Interspecies reproduction between some races requires genetic engineering to assist the process. Without such assistance, reproduction between a Klingon and a Trill would be impossible. (DS9: "Tears of the Prophets") Bolians have very different blood chemistry than most other species, and extremely different blood chemistry than Vulcans. (VOY: "Prototype") Sexual relations between a Bolian and a human would have unfavorable symptoms for the human, including fatigue, nausea and joint inflammation. (VOY: "Life Line")
- It can be assumed, and has been discussed in several Pocket Books novels, that a Vulcan-human hybrid would require at least some amount of genetic assistance, due to the incompatibilities between copper- and iron-based blood.
Genetic hybrids sometimes have heightened strength or senses, gaining the advantages of both parent species without most of their weaknesses. Occasionally, the opposite is also true. Attempts to create a breed of enhanced hyrbids were made by the Klingon Empire and Romulan Star Empire in the 23rd century on the planet Chal. The result was a hybrid species called the Chalchaj 'qmey, or "Children of Heaven" in the Klingon language. (TOS novel: The Ashes of Eden) A century earlier, the Klingons experimented with human Augment DNA resulting in a viral mutation that effectively created a sub-species of human/Klingon "fusions." (ENT: "Affliction", "Divergence")
In addition to the potential physiological difficulties, some hybrids suffer from the cultural prejudices and taboos of their parents' societies. The hybrid children of Bajorans and Cardassians were often rejected by both sides, due to the strained relations borne from the Occupation of Bajor. (DS9: "Indiscretion", "Return to Grace")