In a few science fiction series (the Star Trek franchise being the most notable), humans can mate with extra-terrestrial species and produce healthy children. This is, of course, absolutely ridiculous; species are defined by their inability to mate and produce fertile offspring with other species. Even taking into account the possibility of panspermia, a human would have more in common genetically with a daylily than with an extra-terrestrial.
Cross-species mating in Star Trek
In the Star Trek universe, Humans, Klingons, Vulcans, Romulans, and Cardassians are all cousins whose ancestors were deliberately sown across the galaxy, and can interbreed just as lions and tigers (or dogs and wolves) can on Earth, despite massive physiological (Klingons have two hearts) or biochemical (Vulcans have copper-based blood) differences. Because of Hodgkin's Law, not only Trek species but entire Trek cultures developed along similar paths. Notable products of cross-species matings include:
- Deanna Troi
- B'Elanna Torres
- Alexander Rozhenko
- Miral Paris
- Naomi Wildman
- Tora Ziyal
Of these, Torres, K'Ehleyr, and Wildman were able to give birth to healthy children, effectively demonstrating that Humans are the same species as Klingons and Bajorans.
Cross-species mating in other Science Fiction
- Edgar Rice Burroughs' John Carter was able to father half-Martian children, even though Martian women lay eggs.
- In an effort to reintroduce the genes for telepathy into the Narn population, Ambassador G'Kar proposed paying human telepath Lyta Alexander for her eggs and even suggested that a "direct mating" would be an option. (Babylon 5)