All Genes Are Codominant

A Half-Human Hybrid and Heinz Hybrid trope.

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


 * The Mary Sue route: The Half-Human Hybrid inherits both species' strengths and none of the weaknesses.
 * All Genes Are Codominant: 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 to 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 the standard 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.

In real life, there are two or more alleles for any one gene. These alleles will determine which form of that gene the being will display in their phenotype. 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, blue eyes are bb, brown eyes can be either BB (homozygous) or Bb (heterozygous). The B allele is dominant, and blue eyes only appear if it is not present. From this, heterozygous people can be 'carriers' of traits without displaying them, which may appear in their children if the other parent either has the recessive trait or is also heterozygous for it.
 * 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.

Very prominent in fanworks.

Anime and Manga

 * Dragon Half: The daughter of a dragon and a human was a Cute Monster Girl with horns, wings, and fire breath. As well as inhuman physical strength and agility.
 * Inuyasha was 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 totally into a giant dog. Of course, 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 the half-horse-demon is over ten feet tall and has a literal horse-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 halfbreed 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.
 * Dragonball Z, It's noteworthy that the Hybrids end up being more powerful than the originals
 * Well, the only hybrid we see (not involving test tubes) are human/saiyan hybrids. It's specifically shown that they're not so much "more powerful" by default, but rather they inherit the Saiyan ability to constantly get stronger and the human intuition, brains and potential. On the downside, whether or not they inherit the Saiyan warrior spirit seems to be a 50/50 chance (Trunks, Goten and Pan do, Gohan and Bra not so much) and they need to constantly keep up their training, otherwise their strength and prowess starts to drop just like their human family.
 * They actually are more powerful by default. Gohan was an order of magnitude stronger than Goku at the same age, Goten...holy hell, let's not get started with Goten. Additionally, Gohan was capable of gaining much more strength much faster than either Goku or Vegeta, both of whom had spent their entire lives in intense training. Gohan's strength never dropped when he stopped training, it simply failed to progress at the same rate as the others who did not stop.
 * Dhampyrs in Durarara!! appear to work this way, if is anything to go by. Being quarter-vampire,  has more common vampiric traits and weaknesses in a diluted form (for example,  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).

Comics

 * Kryptonian Human Hybrids Follow this trope perfectly in any alt universe or future story that shows Superman with children. Superman's children with Lois half 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.
 * Of course, 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.

Literature

 * 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.
 * 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.)
 * The Drisalian Curse in Stationery Voyagers plays with this. It "flips a coin" to determine how hybrids will turn out. If it lands on edge, a weird sort of half-and-half results. But this is rare. More often, the child will end up being all of one parent's version of the curse and none of the other. Some of the afflicted are arbitrarily discriminated against by the curse and declared incompatible, meaning that cross-breeding them will not produce any children. It actually hotwires the universe to override biology if necessary.
 * Pencils and Gel Pens may interbreed, but will almost always get children that are one or the other.
 * Whiteouts are only compatible with other Whiteouts, as Ooze Pens are only compatible with other Ooze Pens.
 * Mosquatlons and Humans interbreeding will result in a child that's either a full Mosquatlon, full Human, or Half-and-Half that can live for 400 years and go an hour in the sun before burning up, rather than a few seconds. Aviatets and Humans don't even try to interbreed. Markers will only attempt to interbreed with other Stationeries, never with Humans.
 * 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., 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.
 * It should be noted that the combination of the power of the two species makes it so that it would be dangerous for any wyvern (or any Halfbreed ) 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 . . .)

Live Action TV

 * 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). This is her superpower.
 * 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.
 * 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.
 * In Star Trek: The Original Series, it is averted in the case of Spock, who is pretty much indistinguishable from full-blooded Vulcans—to humans (Vulcans seem to be able to tell the difference. Somehow). Spock was not revealed to be only half-Vulcan until they decided he was....
 * Pretty well 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. Of course, they might have done this to hide the fact that he was now being played by a different actor.

Tabletop Games

 * Dungeons and 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
 * This is also the classic method of half-human stats in Shadowrun.
 * 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.

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 of course). 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.

Web Comics

 * Drowtales specifically attempts to avert this by saying that the Halfbreed 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: 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.
 * Anti-HEROES has Aldran and Eldhin's.
 * Order of the Stick has a half-elf with one pointy ear and one round ear.

Web Original
"Fluttershy: is recessive."
 * 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.
 * Defied (for laughs) in Friendship Is Witchcraft:

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 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. Tiglons are generally about the same size as their parent species, and less likely to survive birth, but they are not, by any means, small. However, in both cases males are sterile and females are generally fertile.
 * Except Ligers have their own special weakness...they are so large, that they often have problems with their joints.
 * Mules as well, although they are also a bit mary-sueish (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'll get mottled black-and-white chickens!
 * Depending, of course, 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.
 * Cross an orange cat with a tuxedo (black and white) cat, and you get a tortoiseshell (black and orange, sometimes with cream tuxedo markings).
 * Only if it's a female, though. Or an XXY male, which is incredibly rare and produces universally sterile males.
 * Human "races" are a social construct. If you mix a "white" parent with a "black" parent, you get children some shade in between. You also get protection against hemophilia (common to Caucasians) and sickle-cell anemia (common to Africans).
 * Though in extra-rare cases you may get one black and one white baby (if said babies are twins).