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All Genes Are Co-Dominant

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A Mixed Ancestry trope.

In a work with an Interspecies Romance, the happy couple are very likely to pass on different alleles to the resulting child. This is normally where the laws of genetics are thrown out the window in favor of Hollywood Genetics. And in Hollywood, the three things that can happen are as follows:

  • Either/Or Offspring: The child will be either parents species with no hybridization.
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  • The Mary Sue route: The hybrid inherits both species' strengths and none of the weaknesses.
  • All Genes Are Co-Dominant: The hybrid gets a half deal on all traits in a kinda-sorta-but-not-really incomplete dominance way. They have half of everything — from physical appearance, to strengths, to weaknesses. This is probably an attempt at a more realistic aversion of the Mary Sue route above; but if one of the races has a severe imbalance of power compared with the other, it can have a similar effect overall.

This approach may fail in the realism department — it is somewhat improbable that none of the obvious physical traits will have a Mendelian inheritance pattern. This approach can also be an attempt at simplifying, as working out exactly which fantastic traits are dominant in your world, which traits are recessive, and which are codominant while still keeping the character's Competitive Balance in mind could distract you from figuring out more important plot points and leave viewers who don't understand genetics scratching their heads.


This can also be seen as a throwback to the pre-Mendelian view of inheritance where there was no idea of individual genes inherited from each parent but rather the notion that inherited traits will all simply blend, producing an offspring that's the average of its parents.

In real life, there are two or more allelesnote  for any one genenote . These alleles will determine which form of that gene the being will display in their phenotypenote . If the offspring gets two different alleles for the same gene, there are several different ways that it can go:

  • One allele is dominant, and the other is recessive, so the offspring will display the trait that the dominant gene codes for. For example in eyes, the "b" allele codes for blue colour, and "B" for brown. A person with blue eyes will have two copies of the recessive gene, making them bb. However, as B is dominant, you would have brown eyes if you were either Bb (heterozygous) or BB (homozygous). From this, heterozygous people can be 'carriers' of traits without displaying them, which may appear in their children if the other parent either is homozygous for the recessive trait (bb) or is also heterozygous for it.note 
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  • The alleles are incompletely dominant. Then, the offspring will display a trait that is in between the traits of their parents. (Cross red and white roses, and you'll get pink ones.)
  • The alleles are codominant. The offspring display both traits in full. The classic example is blood groups - cross a homozygous type A with a homozygous B and you get heterozygous blood type AB.

Even the Mary Sue route can be Truth in Television, as hybrids often(-ish) do inherit most of the strengths and none or few of the recessive-based weaknesses of the parents.

Mules, for instance, qualify for both types of this trope. At a glance, they are an intermediate between horses and donkeys in terms of size, coat, ears, and other obvious physical characteristics. They also tend to actually be more intelligent than either donkeys or horses, are stronger and able to pull and carry a slightly higher percent of their body weight, and have considerably more stamina than either parent. Many human cultures have bred lines of donkeys specifically to produce good mules, to take advantage of this "hybrid vigor".

Also important to the trope: many traits (most types of coloration, for instance) are determined by more than one gene, and the different genes may exhibit different types of dominance. Even if all the genes involved display classic Mendelian dominance, one can still get intermediate phenotypes if some of the pairings come up double-recessive.

Contrast Random Species Offspring when the offspring is biologically not like the parents at all.

Very prominent in fanworks.


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  • Super Bowl 2014 Commercial: the Doberhuahua.

    Anime & Manga 
  • Dragon Half: The daughter of a dragon and a human is a Cute Monster Girl with horns, wings, and fire breath. As well as inhuman physical strength and agility.
  • InuYasha:
    • Inuyasha is the son of a human princess and a Dog Demon Lord, and besides claws and strength he also inherited dog ears and white hair, but he is significantly weaker than his whole demon half-brother and does not possess the ability to transform into a giant dog. Then there are the times he turns completely human (during the new moon) which just makes things complicated... There are also times where he turns full demon. He doesn't turn into a dog, but his facial markings change, and he goes berserk.
    • There are other half-demons in the series who look more like one parent than the other. Jinenji's demonic heritage is unspecified, but he is over ten feet tall and has a beastial face, while Shiori the half-bat-demon looks almost completely human (aside from her white hair, purple eyes, and dark skin for a Japanese child who spends most of her daylight hours indoors), and almost nothing like her demonic grandfather whose powers and duties she inherited. This is complicated somewhat by the fact that some full demons have a human form and an animal form that they can magically switch between, and it's unclear if the vastly dissimilar phenotypes are influenced by inheritable differences.
    • It's explained that how a hybrid appears is really luck of the draw. Jinenji's father was a bishonen if you've ever seen one, yet their kid is more ugly than a Hutt with his face caved in. Some come out to be beautiful and humanlike, others come out horrifying.
  • Averted in Dragon Ball Z, where the human/saiyan hybrids end up being at least potentially more powerful than full-blooded Saiyans while inheriting their weakness (a tendency to go all weremonkey during a full moon). It's also been confirmed by Word of God that tails are specifically not codominant, with Gohan inheriting one but Trunks and Goten being born without.
  • Dhampyrs in Durarara!! appear to work this way, if Ruri Hijiribe is anything to go by. Being quarter-vampire, she has more common vampiric traits and weaknesses in a diluted form (for example, she hates direct sunlight and garlic, possesses a moderate level of Super Strength, heals noticeably faster than normal, and has a thing for rare meat with a high iron content).
  • Half-demons in Superior are portrayed as looking mostly human, but with demon eyes. However, a quarter-demon has one demon eye.

    Comic Books 
  • Kryptonian/Human Hybrids Follow this trope perfectly in any alt universe or future story that shows Superman with children. Superman's children with Lois have his powers and weaknesses at half strength. Their children (when they breed with humans) are 1/4th strength and so on. In one Elseworlds, Superman's ancestor landed during the American Revolution, crushing it and creating his own British empire. The story is set in the present where Kal-El's genetics are so watered down, that he has no powers at all and his father was only marginally superhuman.
    • And that's disregarding the mechanical difficulties inherent in human-Kryptonian intercourse.
      • That article disregards the fact that Kryptonian involuntary reflexes do NOT seem to be as strong as their voluntary movements, as neither Superman's blinks nor heartbeats nor digestive peristalsis seem to cause massive destruction of the surrounding terrain.
    • Averted in the 86 reboot where it was merely explained that Superman's Kryptonian DNA was incompatible with humans. Maxima tried to use this to her advantage to propose marriage to Superman explaining to him that unlike an Earth woman, she could provide him an heir. He declined.
    • Even more realistic is Jon Kent, the son of the Pre-New 52 Superman who was introduced during the events of DC Rebirth. Jon seems to have his powers like flight and heat vision almost to the same level as his dad. The rub comes in that Jon's powers are unpredictable and sporadic. at first he'll be invulnerable but then a tumble from a tree will give him a concussion. It seems that as he grows his gene's are fighting and shifting in order to find a more stabilized state. They do finally stabilize after the Black Dawn' arc, giving him an overall power level that is comparable to, but still weaker than, Superman himself, but his overall potential is a case of Hybrid Power, with Batman claiming that Jon's full powers will surpass his fathers and potentially grant him abilities that his father may never manifest. His unique genome does leave him deficient in other areas, particularly his control over his Solar Flares, which can potentially kill him.
  • Averted big time by the Rom Spaceknight villain Hybrid. Half human and half shape-shifting alien Dire Wraith, his true form is an absolutely hideous gloppy mess that doesn't look much like either, and he possesses psychic powers to rival Charles Xavier (note that while many Wraiths practice sorcery, they're not noted for being natural psychics). It might be possible to ignore his horrible appearance if he wasn't also sadistically evil.

    Fan Works 
  • In RainbowDoubleDash's Lunaverse, a foal will very occasionally be born who is a perfect balance of two of the tribes, and thus possesses attributes of both. An example would be Alula, who is a pegacorn (half Pegasus, half unicorn, has both horn and wings. Frequently mistaken for an alicorn).
  • Defied (for laughs) in Friendship is Witchcraft:
    Fluttershy: Dragon-ness is recessive.
  • This is subverted in Neither a Bird nor a Plane, it's Deku!. The Quirk Factor is dominant over the Metagene, as the latter requires a stimulus to manifest while the Quirk Factor will manifest with time. It is possible to have a natural affinity for magic (and thus be a Homo Magi) and have a Quirk, but it's very rare. Sudden mutations from freak lab accidents bypass all of the above, allowing those with existing Quirks like Barry Allen (better known as The Flash) to obtain a totally different power like Super Speed.

  • In Harry Potter, this appears to be J. K. Rowling's approach with the half-giant Hagrid. Normal giants are 20-25 feet tall, with extremely large bodies, and are not as intelligent as wizards. Hagrid is "Twice as tall as a normal man, and five times as wide," and somewhat bumbling. On the other hand, the other half-giant in the series, Madame Olympe Maxime, is just as large as Hagrid but more intelligent and graceful, and better at magic.
    • Although the better at magic thing could be explained by the fact that Hagrid was expelled in his third year.
    • Averted with the wizard/Muggle blood system. A person is either magical or not, and if they are magical then their level of ability is unrelated to the "purity" of their blood. Although the Death Eaters would love to believe otherwise.
    • Meanwhile, a strange trait pops up in pureblooded families every once in awhile, which results in the occasional Squib child, like Argus Filch or Arabella Figg. This seems to just be a rather rare and unlucky condition. However, Rowling has mentioned something to the effect that Muggleborn witches and wizards could in fact be born from Squib ancestors, suggesting that the same strange quirk that causes loss of magic in some babies also causes the magic gene to reactivate in their effectively muggle descendants. (Or that magic is recessive and that squibs just got unlucky mutations on those genes, Word of God to the contrary notwithstanding.)
      • One biology student wrote a paper explaining how genetics might produce all these traits.
    • Some magical traits, such as the ability to talk to snakes, are passed down hereditarily.
  • Both applied and subverted in Broken Sky. The half-Kirin, half-Dominion Jaan has grey skin and yellow eyes, making him physically different from both Dominionfolk and Kirins. King Macaan, on the other hand, is half-Kirin, but looks entirely like anyone else from The Dominions.
  • In the Kiesha'ra Series by Amelia Atwater Rhodes, the shapeshifters were all once humans who were either granted second forms by elementals, or created with the help of other species. Eventually, the monarchs of the Avians and Serpiente work together to bridge their 2000 year old war and the heirs to the thrones intermarry. It was not believed that they would be able to have children at first, but there apparently was enough human in their genes to make it work, and the child is a wyvern who has a pure Hawk form, a pure Cobra form, and a form that has traits of both.
    • This codominance of genes only works for these two breeds, since among the birds the Hawk gene is dominant, and among the snakes the Cobra gene is dominant.
    • The combination of the power of the two species makes it so that it would be dangerous for any wyvern (or anyone with Mixed Ancestry) to have children themselves, but they are not actually sterile.
  • Xanth sidesteps the issue of genetics with its "love springs," which cause the drinker to fall in love with whoever they see next, and also allow for the birth of offspring even from pairings that should create none. In extreme cases, a shapeshifter is born, but usually the child is just half-and-half. (Among other things, this is where centaurs came from — explorers led their mares to drink ....)
  • In Katherine Kurtz Deryni books, Deryniness is hereditary, but what happens with the offspring of mixed unions is unclear. Sometimes human blood seems to dilute Deryni powers, but sometimes mixed-blood offspring are as powerful as full Deryni.
  • In Vampire Academy, the vampiric Moroi are physically frail (which is compensated for by their elemental magic and healing factors), so they interbreed with humans in order to create dhampirs- which have the strength of full humans and a slight healing factor- to protect themselves from the Strigoi. Dhampirs cannot procreate with humans or among themselves, and do not have to drink blood like either vampiric races.
  • Red Moon Rising plays with this:
    • Played straight with half-human vamps, who are fairly similar to both sides of their family tree and share the weaknesses of both species.
    • Subverted with any half-wulves, where the wulf genes are dominant and the person is considered either a full-wulf or not a wulf at all depending on whether their parents can afford gene-therapy to suppress the wolf side of their biology (and avert the Change).
    • Danny is a subversion. His gene therapy was incomplete, meaning his wulf genes were weakened but not suppressed and his vamp traits are constantly battling it out. At the beginning of the book he's not as stocky as a wulf but not as tall as a vamp. He has wulf hair colour, but vamp eyes and intelligence. As he comes closer to the Change he grows excessive body hair like a wulf, but the Change itself leaves him mostly cognizant when he should be feral, and doesn't alter his appearance as a normal wulf.
  • In Warrior Cats:
    • According to feline genetics Hollyleaf should have been a tortie. There's much more, but in general cat genetics are a lot more complicated than the Erins know about. They even admit they don't know a thing about cat coat genetics.
    • There are multiple male tortoiseshells in Warriors, which are incredibly rare (about a one in three thousand chance). At least one of them, Redtail, is also a father, despite almost all male tortoiseshells being sterile.

    Live-Action TV 
  • Angel:
    • Doyle didn't even know about his father's demonic lineage until he turned twenty — presumably because of the spikes ejecting from his face. He prefers to pass as human, something pure-blood Bracken demons can't do.
    • The Groosalugg is basically a pink-skinned variant of Lorne's species, with navy blue eyes.
  • In Star Trek: The Next Generation, Deanna Troi has a (telepathic) Betazoid mother and a (non-telepathic) human father, and is herself empathic: she can sense emotions psychically but not read individual thoughts (usually).
    • Averted with the teeth of half-Klingons K'Ehleyr from TNG and B'Elanna Torres from Star Trek: Voyager, which are human Eternally Pearly-White Teeth instead of Fang Thpeak-inducing Klingon yuckiness. As for the Klingon forehead ridges, played straight with B'Elanna who has a halfway downplayed version, and averted with K'Ehleyr who has ridges indistinguishable from a full-blooded Klingon.
    • Averted with the infant son of Seska (Cardassian) and Maje Culluh (Kazon) in Star Trek: Voyager. He inherited a half-way Cardassian Rubber Forehead but not a Kazon Rubber Forehead, initially leading people to believe that his father was Chakotay, a human.
    • Played straight with Tora Ziyal in Star Trek: Deep Space Nine, half-Bajoran and half-Cardassian; she has a reduced, softened version of the Cardassian forehead ridges (and lacks their rigid shoulder shape) combined with Bajoran nose ridges, and her skin tone is also halfway between grey Cardassian and humanlike Bajoran.
    • Played with in one episode of Next Generation, where Worf finds himself on a planet where Klingons and Romulans were forced to coexist for mutual protection. He grows quite fond of a woman who looks like any other Klingon...until he brushes her hair back and sees her ears are pointed, outing her as half-Romulan.
  • In Star Trek: The Original Series, it is averted in the case of Spock, who is indistinguishable from full-blooded Vulcans—to humans (Vulcans seem to be able to tell the difference, somehow).
    • The smell, perhaps?
    • Or perhaps his telepathic 'aura'?
  • Averted in Farscape by Scorpius. Scorpius looks like something intermediate between a Scarran and a Sebacean, but his two halves are at war with each other, since Scarrans produce excess body heat naturally and Sebaceans are very intolerant of heat, entering a coma-like state when their internal body temperature (which they cannot regulate) gets too high. Scorpius compensates for this disability with an internal cooling apparatus. Scorpius also possesses an ability due to his hybrid nature that is unique to him and not present in either of his parents' species: he can see heat patterns and detect when a being is lying. Interestingly, Scorpius's appearance is similar to the more humanoid, upper-caste Scarrans, rather than the lower-caste Scarran who fathered him.
    • With D'argo's half-Sebacean son, Jothee, this trope is played straight. Jothee is essentially the Farscape version of a Klingon-Human hybrid: physically stronger than a Sebacean but weaker than a full Luxan, possesses his father's stinging tongue but isn't as coordinated with it, and has a weaker sense of smell than a full Luxan that is still superior to a Sebacean's. He even has somewhat intermediate features, though this is partly due to his attempts to make himself look more Sebacean by mutilating his face and tentacles.
    • When he returns in The Peacekeeper Wars, after having learned to accept his Luxan side, he looks more or less indistinguishable from any other Luxan. They might have done this to hide the fact that he was now being played by a different actor.

    Tabletop Games 
  • Dungeons & Dragons does this with many races and monsters.
    • 3rd and 3.5's Half-Orcs and Half-Elves were both fairly infamous for being weaker than either parent race in terms of stats and racial abilities. In 4E, they abandoned the idea of half-elves being "halfway to an elf," so to speak, and expanded on the idea that they were inherently versatile (which makes a lot less sense, but works a lot better). An example of Hybrid Vigor
    • Similarly, Dark Sun has Half-Dwarves called Muls who are midway in height and ability between both parent races. In keeping with the Grim Dark setting, it's established that mothers rarely survive a Mul pregnancy, and most Muls wind up as slaves.
    • Some variation of this is so dominant in D&D interbreeding (although, admittedly, once it gets to weirder crossbreeds the explanation tends to boil down to Magic Did It) — even the more mechanically diverged half-elves are usually still described as physically in-between human and elf — that the aversions tends to be notable. One Forgotten Realms source, for example, indicated that dwarf traits were so dominating over human traits that a half-dwarf, half-human for practically all purposes is a dwarf that had been born to two non-related parentsnote .
  • This is also the classic method of half-human stats in Shadowrun. Averted in the 3rd Edition of the game, where they stated that with magic being new to the Sixth World, the meta-races were still in a state of flux, such that the product of two different meta-races mating would result in offspring that was one or the other, not a mix of both. On the same token, it was still possible (though rare) for two parents of the same meta-race to produce offspring of a different meta-race.
  • In Munchkin, the Half-Race card allows the player to choose either all the strengths and none of the weaknesses from one race, or all strengths and weaknesses from two races.
  • In Ars Magica, characters with the "Faerie Blood" or "Magical Blood" virtues gain some useful minor powers with no real downside other than looking a bit different.

    Video Games 
  • Half-Human Hybrid Dante in Devil May Cry. He seems to fall into the first side of this trope, as he possesses all of the attributes that make humans so special, as well as the physical appearance of one, albeit with white hair, while also possessing superhuman strength, speed, and magical abilities; being able to best a Physical God in combat, and also, at times, assume the form of a demon.
  • Subverted in Warcraft, as a Half-Draenei Half-Orc will show that Draenei genes are almost all recessive save for the eyes.
    • The reason for this is that she started out being half-orc and half-? (with the implicit assumption being human), with an appearance appropriate for that; Draenei didn't even exist in the lore at that point. It was later realised that there wasn't really any way for her being half-human to make sense, so she ended up half-Draenei since it was the only thing she could be.
      • Her son, a half-Human, quarter-Draenei, quarter-Orc, clearly shows signs of all three races: he has glowing eyes, chin tentacles and small horns (draenei); a green skin, small pointed ears and sharp teeth (orc) and a humanlike body and face (ignoring the tentacles). He also uses the magic from those three races: Arcane magic for humans, Light magic for Draenei and shamanistic Earth magic for Orcs.
  • Averted in Dragon Age where a human-elf hybrid ends up looking completely human. May or may not have a slightly increased likelihood of being delicate or having magic, but there's no defined category of half-elfishness.
    • Played straight after their slight redesign in Dragon Age II, where full-blooded elves now are noticably lither than humans with slightly larger doe-like eyes, whereas they were merely short humans with pointed ears in the first game. One half-elf we see, Feynriel, looks like a subtle blend of both species, being taller and broader than an Elf, but lither and with more pointed features than a Human.
  • In Tales of Symphonia, half-elves have all of elves' longevity, physical appearancenote , and—despite humans naturally having no magical ability—even better magical power.
  • In Dragon Ball Online, Saiyan genes were largely dominant and Saiyan / human interbreeding has progressed to the point where a sizable portion of the human population can boast some degree of Saiyan ancestry (and the superpowers to go with it).
  • Claimed to take place in the Mass Effect universe by Matriarch Aethyta when she says that having a krogan grandfather is the reason her daughter Liara sometimes displays rather violent tendencies, being a quarter krogan and all. Liara gets annoyed and points out asari reproduction doesn't work that way.
  • The Elder Scrolls
    • Averted in individual instances. Several in-game books and backstory details state that half-breed children favor their mother's side almost completely, with a few of the father's traits potentially sprinkled in. For example, if an Altmer father and Nord mother produce a child, it wouldn't be a Magic Knight combination of each race. Instead, the child would be almost entirely Nord with the potential of having some Altmeri traits, such as slight points to his ears, higher cheekbones, or a slightly different skin tone.
    • Over the course of many generations, however, this trope can eventually occur. This is how the Breton race came to be. Their (human) ancestors were Breeding Slaves to the Direnni Altmer of High Rock. Over the course of many generations, some of the Elven traits started to come through with greater dominance. This has led the Bretons to be the most magically inclined race of Men in Tamriel at the cost of some of the Humans Are Warriors traits of the other races of Men.

    Web Comics 
  • Drowtales specifically attempts to avert this by saying that the Mixed Ancestry offspring of a Drowolath and a Drowussu are very rare and that each one is a unique blend of the parents. Most half breeds have a skin tone that is some mixture of the two and varying heights and hair colors, with some taking more after one parent than the other.
  • This is why Candi has pink hair.
  • Tedd from El Goonish Shive:
    • Tedd. Father is Caucasian and has natural blue hair. Nanase, Tedd's cousin on his mother's side, is a redhead of Asian descent and it's often extrapolated that Tedd's mother is too. Result: Tedd has purple hair and his eyes are slightly slanted (less than Nanase's). Nanase's recent magic-burnout turned her hair black, so it could be that having magics screws around with your hair-color.
    • With Grace it was pulled the other way around. She's a human-squirrel hybrid, right? Do you think Shive doesn't know how a squirrel's teeth look like or she got Cute Little Fangs just for fanservice? When the exposition caught with her, Grace turned out to have two other parents of different extraterrestrial species. And the exact make-up greatly surprised even another chimera who knew what can be expected.
  • Anti-Heroes has Aldran and Eldhin's wing color.
  • The Order of the Stick has a half-elf with one pointy ear (and presumably one rounded ear — in OotS art, human-type ears are simply not drawn in at all).

    Web Original 
  • Usually averted in Chakona Space, offspring between Chakats and "older model" taurs (wolftaurs and foxtaurs mostly) are always Chakats, usually with their other parent's coat color. Chakat matings with the newer Skunktaurs and Stellar Foxtaurs take after the mother, though Chakats sired by Skunktaurs inherit their father's Psychic Powers. Otherwise hybrids tend to fall under one of three cases:
    • Parents are related species such as arctic and red foxmorphs, offspring combine both parent's traits.
    • Parents are related but one species is dominant, such as the "new" and "old model" taurs mentioned above.
    • Parents are incompatible but one had their gametes genetically reprogrammed to match their mate's, offspring are not hybrids in the slightest.
      • Though in one case a human whose sperm automatically changed to match the species he was having sex with had a Chakat daughter who inherited his ability and mated with a cougarmorph and had a cub who inherited hir prehensile tail and reproductive anatomy. The ability was the result of the human in question being Touched by Vorlons rather then standard genetic reprogramming though.

    Real Life 
  • Sometimes, crossing the genes from two animals (in this case in the natural fashion) does give you an almost cartoonish half-way mixture. Add zebra to donkey, get donkey with stripes.
    • Also known as a Zonkey, Donbra, Zeedonk, Zebrinny, and Debra. The standard for making portmanteau hybrid names is to use the father's species first (unless there's a non-portmanteau name already in use).
    • For lions with stripes, there are Ligers and Tigons or Tiglons.
      • Ligers are an example of getting all of the strengths, because they are larger (often as large as both parents put together) and stronger than either parent and the largest big cat on Earth. This does result in a higher likelihood of joint problems due to the Square-Cube Law. Tigons are generally about the same size as their parent species, and less likely to survive birth, but they are not, by any means, small.
    • Mules as well, although they are also a bit Mary Sue-ish (sterility aside), being described as "more patient, sure-footed, hardy and long-lived than horses, and they are considered less obstinate, faster, and more intelligent than donkeys."
  • Cross a black chicken with a white chicken, and (once the babies grow feathers) you can get mottled black-and-white chickens!
    • Depending on which kind of black, and ditto white, you are working on. Different types of black/white hybrids could be 1) Black; 2) White; 3) "Blue" (which is actually grey); 4) Mottled black and white; 5) Barred, brown, pencilled, whatever, showing little resemblance to either parent colour.
      • By mottled, do you mean Splash? Splash is irregular patches of black, grey, and (few) white feathers. Mottled is an entirely different gene which makes each feather solid with a different colored tip. Not to mention dominant and recessive forms of white, or homozygous/heterozygous versions thereof. White isn't usually involved in the BBS (Black Blue Splash) continuum. But yes, there are lots of codominant genes in chickens.
  • Cross an orange cat with a brown or black based cat, and you get a tortoiseshell (black/brown and orange, though it would be called calico if it also has white markings) but because it's a trait carried on the X chromosome this only happens if it's a female or an XXY male, which is incredibly rare and frequently causes sterility.
    • Other coat colors and patterns can produce a similar effect. For instance, crossing a black and white with a grey tabby cat can result in a grey tabby and white cat, although this is due to a different set of genetic rules and not actual codominance.
  • Human skin colour is a polygenic trait, controlled by over twenty genes with darker skinned people having more melanin-producing alleles and lighter-skinned people having fewer. If one parent is lighter than the other and one is darker, the children are often some shade in between the parents. One very prominent example of this would be the US president Barack Obama; compare him to his father and his mother.
    • Though a mixed race couple can have children who is closer to one parent more than another in this regard. Most people have some combination of "dark" and "light" alleles among the many genes that control skin colour, and only half of each person's genome is passed to any given child. If, say, both parents happen to pass mostly the darker alleles to a child, the child could end up considerably darker than the average of the parents.
      • Including several known cases of mixed-race parents producing twins who appear to be of different races, with one "white" and one "black", for example.
    • This can sometimes happen a few generations later, which can get really interesting when the mixed race ancestry has been forgotten.
    • The same general principles work for those of us who aren't mixed race, though the differences in colour are often smaller.
  • Human height works in many ways similarly to the skin colour example above, but is complicated by the effect of sex. Still, a person with both tall and short people in their ancestry is likely to end up somewhere in the middle, but it's possible for a person to be taller than either parent, or shorter than either parent. It's even possible for two parents with dwarfism to have children without dwarfism, and vice versa. Mother Nature's a MAD scientist, Jerry!
  • QTLs (Quantitative trait loci) can seem to be this, but is actually a subversion. The offspring will be in between the parents in a certain trait, but this trait is controlled by many Mendelian genes that gives a "codominant" result when combined.


Example of: