Inter-Species Reproduction

In science-fiction the concept of inter-species reproduction is common and widespread. Although there is little to no scientific evidence that such a thing could even be remotely possible, often though people are able to suspend their disbelief.

In real life, the idea of inter-species reproduction with an extraterrestrial race is also moderately widespread, with many claiming to have experienced such occurrence. Said cases have been subject of investigation by ufologists.

Sexual
Sexual reproduction occurs when two or more members of separate races engage in sexual act, this is the most common type of inter-species reproduction seen in science fiction, and like-wise the most unlikely to occur.


 * Actual Universe – Occasionally reported with Greys; more rarely with other species. May also involve artificial means for conception.
 * Star Trek Universe - This is incredibly prevalent in the Star Trek universe. Almost every sapient race that could be considered even vaguely humanoid can reproduce with the others.
 * Klingons have been demonstrated to mate with, Betazoids, Romulans (Thus they are likely compatible with Vulcans), and Humans.
 * Betazoids have been shown to reproduce with humanity.
 * Cardassians have been show to be able to reproduce with Kazan, Bajorans, and it is implied they are biologically compatible with Humans.
 * Perhaps the most prevelent example is Star Trek's humans, humanity is shown to be able to mate with almost all of the most prominent races of the series.
 * Star Wars Universe - Inter-species reproduction is common, but often occurring between two species who are closely related (Such as the Kiffar and Humanity), there is also mention of species who are distinctly incapable of inter-breeding.
 * The Theelin are perhaps the most interesting example of this, the race suffers from genetic incompatibility with its own race. They are forced to have to seek mates from outside their species.

Non-Sexual
Not all reproductive acts are performed via sexual acts. Some species utilize other methods to accomplish their needs.


 * Amphibiosans' reproductive method bridges the gap between species in a very simple and direct way: they must wear gloves at all times, as their hands utilize a permeable membrane that can absorb the DNA of another to lead to pregnancy. The individual whose DNA was used becomes known as the smizmar and is typically the lover of the Amphibiosan although this is not always true.
 * While sex may be involved, the true reproductive method of the Asari is a mental linkage that transforms theirs and their partner's nervous systems into one system known as "melding" or "embracing eternity". Asari are a monogendered parthenogenetic species that may therefore breed with any (presumably sapient) species of any gender, with the only thing of the "father" crossing over being the overall personality style.

Invasive
There is a myriad of races that need separate organic hosts in order to further their own reproduction, this is not a consensual act and often either permanently damages the host-species, or outright kills them.

Not to be confused with assimilating races, which insert their genetic material (or equivalent, such as Borg nanoprobes) inside hosts and proceed to transform the host's body into one of their own (exemplified by viruses in real life; creatures such as Vampires and Werewolves in folklore and several alien species in fiction). These aren't considered cases of inter-species reproduction unless the parasite itself incorporates genetic material from the host, as happens with transgenic species.


 * The Xenomorphs of the Alien Series seem to be completely dependent on foreign hosts in order to facilitate full reproduction, for without foreign hosts new queens cannot manifest.


 * The Flood seem to require foreign bodies in order to spread, while there seems to be some manifestation of the flood that can occur without hosts, hosts generally seem to be required to spread.

Grey Area?
Many of the acts of so-called "Invasive" reproduction can actually occur consensually.


 * Star Treks Humans are shown to be able to carry Xyrillian offspring, the mother was not aware this was possible when she accidentally inseminated the father. While this example is invasive, the act is not inherently so, and if the couple was willing it could be carried out consensually.

Supernatural?
Some races seem to poses quasi-supernatural abilities, and often this allows a broad range of reproductive options to the race.


 * Star Trek's Q claims to be able to mate with practically any life form, note that it seems he still needs another life-form to reproduce.

Artificial
Some media portray inter-species reproduction in a more scientific-probable light by having it achieved by artificial means; usually done in occult laboratories by either human or alien scientists (or, in rare cases, both) and for villainous purposes.


 * The Syndicate and the alien Colonists (representing Greys in The X-Files universe) have successfully created Human-Alien Hybrids by artificial means; with the later intend on converting the entire population of Earth to create a race of Hybrid slaves. The Syndicate, although pretending to be allied with the aliens; use hybrid research as a way to try to make hybrids immune to the purity.

Hybrids
If successful, inter-species reproduction will result in hybrids, which may or may not be sterile. Human-alien hybrids portrayed in fiction often inherit practically all alien traits of their E.T. parents.
 * The X-Files Universe - In the aforementioned The X-Files, there are two kinds of artificially created hybrids. Those created by the Syndicate are rarely successful and rarely even survive. The Colonists, however, have created super soldiers from abducted humans; the creation of which involves surgical proceedings and infection with the purity/black oil. These have shown super human strength and endurance, as well as accelerated healing. It isn't clear whether the super soldiers actually have alien genetic material or are just augmented humans. Also unclear is the extent to which the purity merges with and/or controls the body of its hosts. If the connection is to a genetic level (which is likely, since it has been referred to as a virus), it is possible that anybeing infected with the purity becomes a hybrid in the technical meaning of the word.
 * Star Trek Universe - In Star Trek, humans tend to be depicted as one of the weakest and less sophisticated species compared to the stronger, more physically resistant and often psychic-capable alien races. As a result, hybrids such as Spock or Deanna Troi appear at the same time super-powered compared to humanity and weak compared to their alien ascendance (e.g. Spock having difficulty to control his emotions and Troi being only an empath, as opposed to the full Betazoids who are telepaths).
 * Doctor Who Universe - Despite being mentioned in the canonical 1996 movie, the revelation that the Doctor had a human mother is usually disregarded by fans. Still, Humans are known to be genetically compatible with Time Lords (even two Humans may produce a half-Time Lord offspring if the child is conceived aboard a TARDIS traveling through a time vortex), as well as with Peladonians, Catkind, Eve's Species, Flisk, Kronteps and Tandonians. There is also the artificially created Human-Dalek hybrids. By the time the sun goes supernova, the Human race will have been so integrated in the galactic community that pure Humans will no longer exist due to several generations of interbreeding (except for Cassandra O'Brien, although that only applies in a genetic level as she is horribly altered physically; while most of the hybrid descendants of humanity tend to retain the physical appearance of their Human ancestors). It is also known that Vespiforms are able to reproduce with Humans while assuming Human form; and the Silurians and Sea Devils are capable of producing hybrid offspring.
 * DC Multiverse - Humans have been shown to be genetically compatible with Kryptonians, Vuldarians, H'San Natall, Lexorians, and Yeti (although the later are probably not meant to be extraterrestrial).
 * Isaac Asimov's Universe - Asimov's 1940 short story Half-Breed and its sequel Half-Breeds on Venus deal with a race of human-Martian hybrids referred to as Tweenies, which suffer from social exclusion on Earth (and presumably Mars as well) due in part to their superior intellectual capacities. As the second story's title suggest, they eventually settle a colony on the (here depicted as tropical) planet Venus.