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 puppy-kittens (CatDogs?). Or, at least, if you do get a puppy-kitten, 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 Sonic the Hedgehog (Archie Comics) 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, any resulting children will be human, barring direct interventions by the Valar who can in exceptional circumstances allow someone with both parentage to chose Elf or Human. They only ever did it for one family.
- 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 Night World the offspring of a witch/lamia pairing will always be either a witch or lamia (a possible exception being Mal Harman, who's described in expanded material as a "vampire-witch hybrid"). For example, three of Maeve and Hunter's daughters - Garnet, Lily and Dove - were born fully lamia, while one daughter - Roseclear - was born a full witch, and neither display traits associated with the other species. The same possibly applies to werewolf/human offspring and shapeshifter/human offspring. The only known vampire/human offspring (Jez Redfern) is initially thought to be a case of this, but is actually a true hybrid.
- In the furry novel Windfall two side characters, a cat and a rabbit, are brother and sister. One of them later explains that their parents couldn't naturally interbreed so they got donor sperm and eggs from each other's species.
- Dungeons & Dragons:
- When trens and troglodytes interbreed, the resulting eggs will hatch into either full trens or full troglodytes, with an equal chance of each.
- Changelings are a race of shapechangers that can adopt any humanoid appearance, and are rather distrustful of each other in general. This naturally means plenty of interracial unions, but there are no such thing as a half-changeling. A changeling mating with another humanoid race will produce either changelings or the other race's offsprings. About a 50% chance of each — unless the changeling has the Racial Emulation feat and uses it during conception to match the partner's race, which will insure a full-blooded child of that same race.
- Eon: Humans and elves can produce viable offspring, but the offspring of such a union is either elf or human depending on whether or not they inherit the life magic field that grants longevity from their elvish parent. That said, the offspring of such a union will inherit traits from their parents as any child does, so an elf with one human parent will quite likely be stockier and have more body and facial hair than one with two elvish parents, and vice versa.
- 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.
- 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.
- 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).
- Implied to be how families with mixed species work in Kitaria Fables. The main character Nyan, who's an anthro cat, has at least a few anthro bunnies in their extended family, including their uncle, their cousin Kiki, who sells seeds in Paw Village, and their grandfather.
- 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).
- RWBY: Faunus genetics defy scientific understanding. When a Faunus and Human have a child, the offspring will be either one or the other, never a hybrid of the two; sometimes the child will initially look Human, but manifest the Faunus trait later on. Faunus' of the same type will always produce a child of their type, and a Faunus child born with one human parent will always have their Faunus parent's traits; however, the child of two different kinds of Faunus will have completely random traits; for example, if a Dog Faunus and a Cat Faunus have a child, their offspring might be a Snake Faunus.
- 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. Of course, in this world "race" is a misleading concept: individuals take an appearance based on the god they identify the most with, thus it's more of a case of the children leaning toward the same religion/occupation as a parent independently of their gender.
- 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 waterbender daughter, and an airbender son. The lone son without elemental powers suddenly becomes an airbender 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 firebender and Bolin is an earthbender. Unlike the above example, there's no correlation between their sexes and elements.
- There are three types of ponies in my My Little Pony: Friendship Is Magic: unicorn, pegasus, and earth pony. Whenever ponies of different types reproduce, they will never have an offspring who has magic from both races. They will be one or the other or, in rare cases, neither.
- In Young Justice (2010), 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.
- M.O.D.O.K. (2021): Of M.O.D.O.K.'s children, Lou is a normal human like their mother while Melissa is a Cephalothorax in a hover chair like their father.