In some works, the children of Interspecies Romance are always, or almost always, the same species as one parent or the other. You can breed a cat and a dog, but you'll only get puppies and kittens rather than catdogs. Or, at least, if you do get a catdog, it'll be unusual and unexpected, rather than the normal standard expected result of reproduction between compatible organisms (weirdly enough). The organism's species is apparently controlled by one specific gene. This seems a bit weird genetically, but in a setting where interspecies reproduction is common it allows for recognizable species instead of having a bunch of catdogfishbirds running/swimming/flying around the place. It provides a sort of Hybrid Overkill Avoidance.
This is a Supertrope of Gender Equals Breed, but applies to cases where the offspring's breed doesn't necessarily match up to their genders. (Examples of Gender Equals Breed don't need to be listed here as well.) This is also a Super-Trope to Dominant Species Genes, where offspring always inherit the dominant species.
This is also more or less an inversion of All Genes Are Codominant. Contrast Patchwork Kids and Half-Human Hybrid. Compare Random Species Offspring, where the offspring's species is way different from their parents' species.
- In BNA: Brand New Animal it's explained that beastmen of different species will produce children of one species or the other, no hybrids, and humans without the beast factor gene can't become beastmen. Which is where Michiru, a Tanuki beastman who can change her limbs into parts from other species and used to be human, comes in. Though the villain turns out to be a "purebred" beastman who looks down on the other "hybrid" beastmen and is plotting their genocide.
- In One Piece, when fishmen and merfolk have children, the child will either be a pure fishman or a pure merman/mermaid, not a hybrid, due to the special genetics of their breed. Incidentally, it seems that are many more fishmen than fishwomen and many more mermaids than mermen. This is separate from either one's subspecies (i.e. what specific kind of fish they resemble), which is varied and random.
- In Archie Comics' Sonic the Hedgehog Jon, the son of Rob O'Hedge and Mari-An, is an Echidna, like his mom. Most other children of interspecies couples are one species or the other but follow Gender Equals Breed.
- In My Little Pony: FIENDship is Magic uses this trope to explain why Tirek, a centaur, has a brother who looks like a completely different species. They're the offspring of an Interspecies Romance between a centaur and a gargoyle, Tirek is a centaur like his father and Scorpan is a gargoyle like his mother.
- Lady and the Tramp ends with Lady and Tramp having three puppies that look like their pure-bred Cocker Spaniel mother and one that looks like their mutt father, all of unspecified sex. Lady and the Tramp II: Scamp's Adventure portrays the litter as straight Gender Equals Breed, while in Scamp one of the Cocker Spaniel pups is male.
- In Hotel Transylvania 2, vampire Mavis and human Jonathan have a child, Dennis, and the main focus is on what he'll turn out to be. He's a vampire.
- The Lord of the Rings: Humans and Elves can reproduce with each other, and a number of such unions took place over the setting's history. However, no true hybrids exist, in large part due to the two species being distinct in a metaphysical, as well as biological, sense — Elves are tied to the physical world and linger within it after death, remaining as shades in the Halls of Mandos until the world will be remade, whereas humans depart for... somewhere else, presumed to be the Timeless Halls where Eru (God) dwells, and leave the material world behind. Since no halfway point can be made between these two fates, children of Men and Elves remain in an "in-between" state for a while, but inevitably have to choose to join one species or the other, becoming either fully human or fully elven.
- The Elder Scrolls In-Universe Books: As said in Racial Phylogeny, a.k.a Notes on Racial Phylogeny:
After much analysis of living specimens, the Council long ago determined that all "races" of elves and humans may mate with each other and bear fertile offspring. Generally the offspring bear the racial traits of the mother, though some traces of the father's race may also be present.
- In the systemless Zodiacs campaign guide, it is mentioned that any child has roughly a 3 in 4 chance of being the same species as their mother.
- Hc Svnt Dracones:
- Vectors are, essentially, a genetically engineered race of Funny Animals (with minor variations allowing for both Little Bit Beastly and Talking Animals). However, their creators explicitly included "mutt reduction blocks" in their genetic code; a child of a cat and a dog has about a forty-five percent chance of being either a cat or a forty five percent chance of being a dog, and a ten percent chance of being either one of the other many races or having a mutation their parents didn't (which does include hybridization of the parent races as a possibility).
- Cogs, the Ridiculously Human Robots of the setting, have reproductive capabilities compatible with Vectors, either translating Vector genes into Cog blueprints or vice-versa. Generally, though, the kids have to be the same "substrate" as their mother, the "womb" mod was a PR disaster.
- Pathfinder: Hags — a humanoid species based off of fairytale witches — are Always Female, and thus need to mate with humanoids, monstrous humanoids or fey to reproduce. The female children resulting from these unions are always hags, while the male children are members of the father's species, albeit sterile and appearing... wrong, somehow. This is in contrast to Dungeons & Dragons, Pathfinder's Spiritual Predecessor, where the hags originate from and where the results of hag/other unions are either a) Always Female and always hags, or b) result in female children who become hags and male children who become hulking, misshapen monsters called hagspawn, with little resemblance to their father's species.
- Shadowrun: The offspring of two different metahumans is always one of the two parents, with no half-breeds existing. The lore mentions offhandedly that this can cause some problems if the male partner is a Troll.
- Pokémon: Most mons play by Dominant Species Genes with offspring the same species as the mother, but Illumise/Volbeat and the Nidoran lines play by this trope instead — attempting to breed the females of these species with any compatible mons has a 50% chance of producing offspring of the Spear Counterpart species, and attempting to breed the males with Ditto has a similar chance of producing the Distaff Counterpart.
- In Mass Effect's monogendered asari species, any reproductive pairing will end up with the child being asari as well. This is because asari do not reproduce sexually like other species, but instead use their finite control over their nervous system to copy part of their partner's genetic code into their daughters', which is then carried to a full-term pregnancy as would be expected of a near-human humanoid.
- In Solatorobo: Felineko and Caninu can interbreed, and the children are either Caninu or Felineko.
- This is generally how interspecies births work in The Sims. For example, if a werewolf and a vampire have a baby, the result is only one or the other. Glitches may result in faux-hybrids which don't function properly (missing interactions, UI, etc).
- In Precocious, Bud is a Scottish fold like his mother, while his older sister is a white American shorthair like their dad.
- Even though the four races (Spades, Hearts, Clubs, Diamonds) from Alice and the Nightmare can marry and have children, the children are neither more than one suit. For example, Dee and Dum and Edith are biological siblings, but Edith is a Diamond and Dee and Dum are Hearts.
- Original Life: Fisk and Elizabeth, a cat and mouse couple, have an older daughter and son who are cats while their younger daughter is a mouse.
- In Chakona Space Chakats, Skunktaurs, and Stellar Foxtaurs are all reproductively compatible but don't hybridize, rather their cubs are the same species as the mother (though they are all hermaphrodites to some extent). Though, Chakats sired by Skunktaurs tend to inherit their fathers' Psychic Powers to varying extents.
- In the game Dragon Cave, in most cases the offspring of different breeds will randomly be either of the parents' breeds. The only exceptions are the handful of pairings that can actually produce a hybrid, pairings that include a holiday dragon and a non-holiday (which will only produce the non-holiday breed unless bred during its holiday season), and Bright Pinks and Frills during the time when they were retired (which would also only produce the non-Frill/Bright Pink breed).
- In The Amazing World of Gumball, children whose parents are different species seem to be either parent's species at random. Hector's dad is mentioned to be a giant like him, but his mother is a human-sized witch. Gumball is the same species as his mom (a cat), his sister is the same species as his dad (a rabbit). Richard is a rabbit like his mom, but it turns out his dad is a rat (who otherwise looks extremely similar). Jamie's father is probably the same species as her (albeit with Bizarre Sexual Dimorphism) but her mother, the giant cube-shaped gym coach, is probably something completely different.
- Wakfu invert Gender Equals Breed with Evangelyne and Sadlygrove's children, as each kid is the same race as their opposite sex parent.
- In Bojack Horseman, offspring of human/Funny Animal couples (or different species of animals) follow this trope. Word of God has confirmed that in the series' world, there are no hybrids and children of Interspecies Romance couples will either be one species or the other.
- When Bojack Horseman dreams about having a daughter with his deer ex-girlfriend Charlotte, their child is a horse. But Charlotte's real-life children with her actual (human) husband are a deer daughter and a human son.
- When Diane (a human) has an (aborted) pregnancy due to her husband Mr. Peanutbutter (a dog), it's implied that she was expecting a puppy at the time.
- When Princess Carolyn (a cat) gets pregnant, she and her mouse fiance Ralph wonder if the baby will be a cat or a mouse.
- There's also Bojack's half-sister Hollyhock, who has a human mother, but she's a horse (because she and Bojack both share the same horse father, Butterscotch).
- Season 5 has a human woman giving birth to a porcupine.
- Some members of the four different races in The Legend of Korra can each "bend" (control) a classical element (Air, Water, Earth, or Fire), but only the Avatar is able to use all four elements. Children of mixed parentage can either control one element (each), or none at all.
- Best demonstrated by the children of Aang (an Avatar with Air as his native element) and Katara (a master of the Water element); who both have a non-bender son, a water-bender daughter, and an air-bender son. The lone son without elemental powers suddenly becomes an air-bender thanks to a worldwide supernatural incident, but this is apparently a coincidence.
- Mako and Bolin are two brothers born to a mother and father from the Fire and Earth nations respectively, and thus Mako is a fire-bender and Bolin is an earth-bender. Unlike the above example, there's no correlation between their sexes and elements.
- In Young Justice, this is apparently how Martian races work. Word of God says that M'gann's father is a White Martian while her mother (Martian Manhunter's sister) is Green; most of her siblings are Green, but she has one White brother, along with being White herself.