"Remember, genes are NOT blueprints. This means you can't, for example, insert "the genes for an elephant's trunk" into a giraffe and get a giraffe with a trunk. There are no genes for trunks. What you can do with genes is chemistry, since DNA codes for chemicals."
Science stipulates that DNA are the building blocks of life. This trope takes it to its literal extreme.
With LEGO Genetics, you can fiddle with DNA wherever you like, intentionally or accidentally, and all the cells will change overnight (if that). Just wake up and presto! Wings! Fur! Gills! Hulking muscles! Giant brain! Stem cells! You don't even have to eat the equivalent of your entire body mass to create all those new body parts; the old cells just rearrange themselves like LEGO bricks. That part is usually Hand Waved if not lampshaded.
Instances of genetics in media tend to fit the "dumbed down" perspective on genetics taught to middle/high schoolers. Condensed to a more basic level, curriculum often has it summarized as:
"The genetic double-helix is like a ladder"
"The rungs are nucleobases Adenine, Cytosine, Thymine, and Guanine"
The logic of Cell's creation in Dragon Ball Z: he is a unique life form created from the combination of DNA from the strongest people to have been on Earth, such as Goku, Vegeta, Piccolo. Furthermore, he's born knowing all their techniques and possesses all of their strengths.
An episode of Cowboy Bebop had a villain attempt to deploy an airborne virus that rewrote DNA and turned those exposed into apes. The transition time seemed to be a couple days, tops.
Sort of justified in Naruto with Yamato's backstory: Orochimaru wanted to recreate the First Hokage's unique powers over the bijuu, so he spliced spliced Hashirama's DNA into that of sixty infants. This was kept within the same species, so it was not too far into the realm of lunacy, but ultimately Yamato was the only success. The other fifty-nine infants died.
Using Orochimaru's research, Danzo apparently spliced some of Hashirama's DNA into his right arm. While the resulting flesh was able to grant Danzo control over the Sharingans he had also implanted in his arm, the tissue was near-cancerous in behavior. The arm was either sealed with a heavy metal lock to control the tissue or fed a constant stream of chakra to keep the tissue in line.
Gundam Seed VS Astray has a bizarre application with its "Carbon Humans". These guys are psuedo-clones of other people created by splicing another person's DNA into an ordinary human, then giving them that person's memories via some form of brainwashing. They can even have multiple genomes spliced in. The series makes it very clear that these guys are not clones, though really the difference isn't too great.
A good number of modern superheroes get their power this way, including the most recent incarnations of Spider-Man. The '90s animated series tried desperately to justify and handwave this issue away by explaining that everything was caused by "Neogenics", a new genetic science that essentially used ray guns and magic radiation to create LEGO Genetics. In other words, you'd put animal DNA in one part of a ray gun and shoot the gun at a man to get The Scorpion. Really. And of course, if a spider accidentally got into the Neogenic Recombinator's beam, and soon bit someone...!
Has happened both ways when Marvel Comics mutants have lost their powers. Sometimes they transform to human; sometimes they keep physical changes such as a tail or wings. But then, most lost their powers when a powerful reality-warpingScarlet Witch said "No more mutants..." and the results varied even then: some become completely human, some retain their altered appearance but have no powers, and a few who had physical mutations that disagree with the laws of physics lost whatever made it work before. (A long-necked woman's neck snapped and killed her, Chamber's "energy furnace" disappeared, leaving him without multiple internal organs, a Giant Flyer fell from the sky.)
Longtime X-Men character Longshot is an other-dimensional Artificial Human created by the Spineless One scientist Arize. To make matters even more interesting, many of Longshot's genes were taken from his biological son Shatterstar, who due to Time Travel had been sent into the past, where Arize discovered and used him in his experiments, thus making Longshot part of a Stable Time Loop wherein he is the father of his own "father".
All over the place in Ultimate Marvel. Most ridiculously, The Wasp once injected herself with a genetic Super Serum and within seconds gained the ability to grow 60 feet tall.
Ultimate Marvel at least tries to explain its scientific flim-flam. The Wasp and her husband put years of effort into the serum. Of course, it started out with sixty feet being the limit because the human skeleton would snap apart after that. This was quickly abandoned for the Rule of Cool.
Transmetropolitan used this with the tempers, and later the transients, to graft animal traits onto themselves. The former usually "wear" animal traits for a day, such as becoming part-dolphin to swim in the ocean with dolphins, and the latter took on alien DNA to establish a new identity. People also can slap on skin patches that give them new DNA traits, such as immunity to cancer (in this case, from smoking).
The Blood (later known as Bar Sinister) from Shaman's Tears were artificially created human/animal hybrids; each designed to have specific traits from the animal side (wings on the bat hybrid, prehensile feet on the monkey hybrid, etc).
The remake of The Fly averted this with the malfunctioning transporter, while the original stepped right in it; Brundle's DNA has been changed, and he gradually becomes a sickly, deformed human-fly hybrid creature as his cells grow, instead of popping out half-fly instantly as he did in the original. In fact, the first thing that shows the beginning of his mutation is the appearance of a strange looking fleshy hair growth in a wound on his back which he got before the failed teleportation.
Done in The Relic. A retrovirus found in prehistoric plants horribly alters the victim's DNA by inserting genes from past victims. The reason why there's a dinosaur ape-thing running about in the Museum of Natural History is because these plants could only ease the constant pain and insanity of the affected, since, you know, forcibly altering someone's DNA isn't going to have pleasant results. A later character is able to use rabbit DNA to make a safe street drug, basically a mild version of crack with no downs or life-threatening brain holes.
Underdog takes this trope to its logical extreme. Shoeshine is injected with a "serum", with contains genes for the wings of an eagle and the strength of an elephant, thus giving Shoeshine his abilities with no visual changes! And later in the film, he is forced to give up his powers. They turn into blue pills. He gets them back when he eats one.
In Gremlins 2: The New Batch, the gremlins find a genetics lab in the corporate office building and inject themselves with, among other things, female DNA, spider DNA, bat DNA, and, uh, lightning DNA and gain the associated traits.
The human protagonist in District 9 is sprayed with a chemical (nanite solution) that gradually replaces his human flesh with alien anatomy over the course of a few days. At one point his body is described as being a genetic hybrid.
The Animal has the protagonist implanted with various animal parts, obtaining their instincts.
In the first Spider-Man, Peter Parker is bitten by a genetically-altered spider and develops super-strength, "Spider-Sense," and the ability to climb walls among other Superhero powers overnight. In the display room where he got bitten by the spider, we see a plasma screen showing the bits of DNA added to the spider to create the "super-spider." Later, there is a CGI sequence in which we see the rungs in the DNA double helix changing to the exact same colours as the spider's.
Though it might help to not look at the CGI shots as literal, but more a creative way to show the audience what's happening quickly.
In the Continuity Reboot of the series, The Amazing Spider-Man, Dr Connors and Peter Parker develop the serum that turns the former into The Lizard this way. There is talk of "cross-species genetics" (the correct term is "transgenesis), and someone makes a joke that a woman injected with zebra fish DNA to cure Alzheimer's would have to look past "the gills on her neck". This is combined with Extreme Graphical Representation with DNA represented by little blocks in a holographic interface.
Sharktopus: The government stuck the head of a shark onto the tentacles of a very, very large octopus.
Mission to Mars plays with this trope. One astronaut painstakingly constructs a double helix out of M&M candies in zero-G. When another astronaut asks him what it is, he replies, "the perfect woman." The second astronaut reaches in and takes one of the candies, scattering several of the rest, and asks, "what is it now?" The first astronaut looks at it, puzzled, and says, "a frog?"
And quite a few that don't, including Angel's ability to transform into either a giant bird or a girl with dark hair, Iggy's ability to "see" colors, and Nudge's ability to become magnetic at will.
In Dora Wilk Series magic is genetically inherited through recessive genes, which author handled quite resonably... except for the main character, who has only recessive genes where there was choice (highly improbable in real life), giving Dora some mary-sueishness, although narrator lampshades how improbable it is.
Spoofed in the Discworld novel The Last Continent, in which the God of Evolution explains that he was hoping to make the burnt offerings more efficient by finding the instruction that tells a cow to be soggy, and replacing it with the instruction that tells a tree to be flammable. It doesn't work, although in the way it really wouldn't work: he ends up with a bush that produces milk and makes distressed mooing sounds.
And cows that would, on hot days, in certain rare circumstances, spontaneously combust and burn down the village. But is that any excuse for ingratitude?
Speaker for the Dead reveals that the alien "Descolada" virus caused this effect on the native life forms of the planet Lusitania, resulting in plants and animals literally giving birth to each other.
And then in the next book, Xenocide, a modified version of the virus is used to cure Path of its OCD problem in two days. Handwaved by the suggestion that the descolada replaces parts of its victim's DNA with random code (in the case of the descolada on humans and Formics) or a preprogrammed code (in the case of Lusitanian life and the humans on Path) so fast that the body cannot reject the modified cells fast enough before the body sees it as its own.
The Descolada is explained as "ungluing" the DNA double-helix into two separate strands. This is fatal to humans and most other organisms but essential for the cellular reconfiguration involved in the life-cycle of Lusitanian lifeforms in their transition between plant and animal stages. Basically, it's an extremely aggressive and complicated viral-molecule that goes around trying to make Lego Genetics work on any DNA it encounters; the handful of surviving species on Lusitania have evolved such that they are resistant to the random insertion of DNA while depending on this process to enter the reproductive stage of their lives. Once this is understood, it becomes a convenient vector for quickly (as in almost overnight) removing the artificially introduced OCD-genes on the planet Path without sacrificing the super-genius genes these were attached to. A "neutered" version of the Descolada is also created in order to make it safe for Lusitanian and other life-forms to share a biosphere; the new molecule performs the necessary functions for Lusitanian life but will not infect (or mutate to be capable of infecting) other life forms.
In Orson Scott Card's Alvin Maker series, at one point young Arthur Stuart, the child of a runaway slave, is being tracked so his owner can reclaim him. The trackers are using magical ability to follow him via his genetic code: they have a lock of his hair, and this allows them to find him anywhere. Alvin's solution? He rewrites Arthur's entire DNA so it doesn't match the sample. By concentrating really hard. To get around the problem of Arthur's cells dying too quickly to be replaced, he dunks the kid in a river and then "orders" all the cells to adopt the new DNA simultaneously. The only negative effect this has on Arthur is that he loses his ability to mimic others' voices perfectly.
Mind you, in some ways, this is more acceptable than many other examples. Alvin picks a few spots in Arthur's DNA and changes them to match his own, rather than make random changes or try to give him frog DNA or something. And this doesn't magically give Arthur Alvin's not-inconsiderable range of powers, either, which is a miracle in and of itself given the rest of this page.
Octavia Butler's Lilith's Brood trilogy is centered around the Oankali, an alien race that is capable of probing and sequencing genes on an instinctual level and that flies around the galaxy engaging in "trade" of gene sequences — whether their partners want to or not in the case of humanity. Naturally, they have impressive abilities to meld the traits of wildly divergent species.
The CDF's genetically engineered Super Soldier bodies in Old Man's War are stated to blend DNA from a variety of sources, including aliens. However the mixture of genes makes them all sterile, which may have been intentional as it gives recruits an incentive to defend humanity instead of trying to replace it.
Specifically averted within The Moreau Factor with a thorough explanation of how pleiotropy makes Lego genetics impossible.
This is how the virus worked in Dan Brown's Inferno works. Just let the virus inject its DNA into the host's cell, and vola, a third of the human population is rendered infertile. More acceptable then other examples, however.
Live Action TV
The various Star Treks do this at least once a season.
It's hard to imagine that TNG episode "Genesis" will ever being outdone as it involves the entire crew undergoing this... a cat becomes an iguana, one crew member becomes a snake, a fish becomes a jellyfish, and Lt. Barclay becomes a spider. I mean, that's a jump somewhere around the superphylum level.
Red Dwarf, never one to take science seriously, has the crew discover a LEGO Genetics device in the episode "DNA". It turns Lister into a chicken, a hamster, and eventually a one-foot-tall Robocop ripoff. They resort to the last one because the crew had turned a vindaloo into a half-man half-curry hybrid.
Not to mention that it inexplicably turns Kryten - a mechanical droid - into a human being simply due to the presence of a tiny organic something-or-other.
Jekyll has The Men in Black rushing desperately to find the main character, because each time he transforms from his Jekyll persona into his Hyde persona (which he can do in less than a minute) his entire genetic structure is apparently changed. This is doing untold amounts of damage, and giving him only a few months to live.
Stargate SG-1, "Bane": Teal'c is injected with alien bug DNA and starts mutating not even into a single bug, but into several of them at once. Not that the resolution was any better than the "science"... This might be reasonable if the bugs were parasites using him as a host but as they are claiming to alter his body by changing his DNA, they fail biology forever.
The main villains of Stargate Atlantis are a result of LEGO Genetics; a life-sucking Iratus Bug mixed human and/or Ancient DNA with its own, and produced the Wraith. Our Heroes have developed a retrovirus that is capable of discriminating between the two genetics, and by separating them, "purifying" the subject to entirely human or entirely Iratus bug. And in the Season finale series of season 4, we discover that a character has been able to modify the virus to add Wraith DNA to pure humans. Truly LEGO Genetics at its purest; take a little from column A and a little from column B at will. One wonders why the humans in the Stargate universe haven't started ad-hoc mixing of their genes from Earth creatures for their own advantage; surely the military could use some Marines with some grizzly bear genes and some tiger and some lion genes... oh my, that's a powerful soldier.
Dark Angel: The protagonists (and some antagonists) were genetically modified with the traits of various animals. The earlier versions look like hybrids, the later versions look fully human but still have various animal traits.
In the third season of Sliders, the writers lampshaded the change in Colonel Rickman's appearance (caused by the change in actors from Roger Daltrey to Neil Dickman) by stating that he absorbed the DNA of his victims whenever he injected himself with their spinal fluid. This was played further in the episode This Slide of Paradise, when the only people available for Rickman to steal fluids from were the animal/human hybrids created by Dr. Moreau^h^h^h^h^h^h Vargas. Rickman, by the time the episode starts, has become somewhat feral, as a human/human/animal hybrid. Or something. Pretty bad, even by Season 3 standards.
CSI: NY featured some genetically engineered goats which produced spider silk (see Real Life below, if that sounds crazy).
Fringe featured a manticore-like creature formed of Gila monster, tiger, scorpion and a couple others. It could also infect people with its larvae through its stinger, an ability not shared by any of its composite creatures. Apparently this was all made possible by splicing in bat DNA.
One of the animals used was a wasp. Some wasp species' larvae eat their way out of hosts in a similar fashion.
Doctor Who featured the Krillitanes, a race that absorbed biological components from the species it conquered. When the Doctor last met them, they looked like humans with long necks; when he meets them again, they're bat-like creatures.
Doctor Who is quite fond of handwaving stuff by gibbering about DNA. Another good example is the Dalek that "regenerated itself" by "feeding off the DNA" of a time traveler. In the process, it absorbed said time traveler's DNA and thus became part human, giving it human emotions.
River Song, AKA Melody Pond, has Time Lord DNA despite the fact that her parents, Amy and Rory, are human. This is a result of being conceived in the TARDIS and exposed to the temporal schism.
Evolution of the Daleks. Oh my God, Evolution of the Daleks. When the Dalek-Humans are created, they all look completely human; this does, however, make them think like Daleks, except not, because the Doctor's DNA got mixed in by being too close to the freaking power source . . .
And then of course there's the fact that the Doctor can survive being extremely close to dying by COMPLETELY REWRITING HIS GENETIC STRUCTURE IN SECONDS.
In "The Lazarus Experiment" Professor Lazarus' project to turn himself young again goes horribly wrong and makes his DNA unstable, causing him to turn into a giant scorpion with a human head that feeds on the life force of others.
An early episode of Farscape had a villain making use of this trope - his plan was to stick Pilot's DNA into Aeryn, wait for her to develop Pilot's multitasking ability, and then somehow take it out of her and add it to himself. And this wasn't the first time he'd done something like this.
The Kroot, an allied species of the Tau Empire in Warhammer 40,000 use this as a means to evolve. By eating the flesh of another organism, a Kroot gains some of the traits its dinner had. For instance, a Kroot that consumes enough flying animals would eventually grow wings. Kroot chieftains, called Shapers, use their knowledge of Kroot genetics to pick out creatures with the most desirable traits for their kindred to eat, in order for their tribe to grow strong and conquer their foes. Their Tau allies see this as utterly barbaric, but value the Kroot's friendship over their habits.
Technically it's the next generation of Kroot which gets the consumed abilities, in a hint of sanity in the flood of madness in 40k.
The Tyranid are also very good at this. Most of their enhancements are homegrown, but two Tyranid hiveships meeting is a very bad thing because they will fight to the death, then whichever one wins will take the most effective enhancements of the other and incorporate them into its soldiers. Also because of the Tyranid's ridiculously efficient digestive and reproductive processes, fleet will grow to the size of both of them put together.
It's also mentioned in the 'Xenology' book that the Tyranid genome is basically a whole series of different gene sequences and types with different alleles and such all spliced together into an impossibly complex whole. The background image for the Tyranid gene sequence indeed reflects this, being made of... everything. (It's all still built from a standard genome template though, which is alluded to being the Tyranid Ripper).
In Hunter: The Vigil, the Cheiron Group gives its employees supernatural powers by cutting out bits of monsters and stitching them into the subjects. This is given a handwave of the "Nobody has a clue how this works, it just does" variety. Not too surprising given that all the 'spare parts' come from creatures that are more-or-less explicitly magical.
The "Moreaus" (animal-human hybrids) in d20 Modern. Also, the entire premise of the "Genetech" campaign setting (which extensively features the aforementioned hybrids in a battle against prejudice rather reminiscent of the X-Men films).
Averted (and lampshaded in the quote on top of this page) in Sid Meierís Alpha Centauri, where University faction leader (and resident Professor) Prokhor Zakharov is quoted more than once on the subject of the limitations of genetics.
BioShock has "plasmids", genetic upgrades that instantly give you fantastic abilities like telekinesis or the ability to shoot fire, lightning, or ice from your hands. Handwaved by advanced scientific research into creating stem cells, but even that doesn't begin to explain it. In Real Life a plasmid is a ring of DNA which can indeed be used to perform a very limited version of LEGO Genetics, but only to transfer a small number of genes into cells (and only in bacteria). The game also allows the PC to take an active plasmid out of his genome, which is a lot less plausible.
The explanation for all these abilities (and the driving force behind most of the plot points) is Rapture's form of Unobtainium, ADAM. Its exact functions are unclear, but it makes the Little Sisters (who produce ADAM via a sea slug implanted in their guts...yeah) virtually invincible, as any wound they sustain is healed almost instantly by accelerated cell division. It is also what allows gene splicing by injection, and is apparently very addictive.
Becomes a major plot point (with a twist) in Wing Commander IV. The Big Bads have a pretty neat covert operation going—mass murders, being blamed on a Conveniently Available faction. Problem is, the murders (which are originally thought to be a plague), turn out not to be—turns out the Big Bads have developed a nanotech weapon that kills people based on their genes—have the right gene set, you live, don't, and you die, in a genocide that would make Hitler green with envy. The parallelism is there and used—including the insane-general-that-thinks-humanity-is-weak-and-is-going-to-purge-it bit that Hitler used to rationalize his genocide. The results are shown in nauseating fashion—the weapon kills slowly, by dismantling the cells that have the incorrect gene sets—dissolving the person slowly and painfully.
Used slightly believably in Crusader: No Remorse, where a Mad Scientist explains to you that the "new generation" of Silencers does not have the Silencer's "fatal flaw" (that being something vaguely approximating a conscience). Depending on how much understanding of genetics human science acquires this may not be entirely implausible.
A much older game with a similar premise to Spore, E.V.O.: Search for Eden, runs on this trope. Every time you add or remove a part, the change is done instantly. This can be exploitable in boss battles by changing one's neck from short to long or vice versa whenever you get low on health, completely refilling your health. The neck is the cheapest part to change, but you can substitute any part and do the same thing.
A better tactic - and an even stranger example of this trope - was to grow a cheap horn, which would also refill your health. The horn would 'break' after attacking with it 3 times... and this would somehow count as an evolutionary change, which would refill your health again.
Admittedly, EVO isn't exactly clear on whether or not it's supposed to represent real evolution. There's substantial hints that the whole process is being hijacked by aliens, at least in the case of certain enemies, and many creatures berate you for not evolving "the proper way".
Creatures is a rare aversion (at least partially). At least several "genes" go into the functioning of each organ, and let's not even get started on the brain... The visible parts (head, limbs, etc.) can change in appearance with just one gene, but not be removed entirely or visibly duplicated (though the gene can be). (The "double tail" seen on certain C2 Norns, and the lack of tail on Ettins and some Norn breeds is a sprite thing.)
Metroid did this to Samus, not once, but twice: according to Samus' backstory, she was "infused" with Chozo DNA in order to allow her to survive on the planet Zebes, which is the reason for her superhuman strength, stamina, and agility. In the game Metroid: Fusion, Samus is injected with the DNA of a Metroid, the only known natural enemy of the X parasite, following her infection. Not only does the vaccine completely destroy all traces of X in her system, this somehow alters her DNA so that she has the Metroid's ability to safely absorb X for health and upgrades, alters the appearance of her suit (which is biologically linked to her), and gives her the Metroid's weakness to cold.
The X parasites themselves steal the DNA from host creatures and then instantly assume their forms.
Nei and Rika of the Phantasy Star series. Nei is explained to be part biomonster and was created in a genetics lab. Rika was also created in a biology lab, but she was produced over the course of a thousand years' worth of research and testing to produce a stable and functional improvement on the Nei pattern, which was drastically flawed.
Depending on whether they're viewed as an animal or a virus, this trope could be considered the very definition of StarCraft's Zerg; who swarm over alien races to absorb certain genetic traits from them, and at the start of the game, are trying to acquire beings with "psionic genes." The sequel shows more examples of such behaviour:
With Heart of the Swarm, the pseudo-Bioligy gets taken to an entirely new level, with the genetic Chef Abathur sending you out to collect Essence from different species to aquire certain traits, from immunity to cold to being able to be split in two, with both halves surviving independently.
MOTHER 3 has a multitude of battle encounters that are simply the unlikely combinations of two creatures, referred to as Chimeras. Such combinations include the Cattlesnake◊, the Batangutan◊ and the Slitherhen◊. You even have to venture into the labs where these Chimeras are created.
In Evolva, your Genohunters are able to use their enemies' DNA to transform their body and acquire their attacks.
Star Control 2 had this as a major plot point for some races. The Umgah can easily manipulate DNA with their vast scientific knowledge, and end up accidentally reactivating the latent powers of one of the most deadly psychic creatures the universe has seen. The Mycon can supposedly do even better, altering their DNA and abilities by mere thought.
The Metal Gear Series involves a subplot in the fourth game that's causing the rapid aging of Snake: he was considered a high risk of being kidnapped and cloned again, so the biologists on the team simply tacked infertility on to his genes, as well as "terminator genes" to force rapid aging and an early death.
El Goonish Shive has Uryuom eggs, which somehow combine the DNA of all of the parents and create a composite being with all of their traits. If more than one species is involved, it generally also gains the ability to change shape. Those must be some pretty advanced eggs.
Later justified as it being a type of magic (well, sort of; it's complicated and hasn't been fully explained yet) inherent in the species.
Played to the hilt by Narbonic. Not only will an infusion of a computer geek's DNA turn you into a computer geek, it'll even give you his cigarette habit, and magically reappearing cigarettes!
The device that altered Spinnerette worked basically this way, as well.
In Sluggy Freelance, Santa Claus believed that genetics could be the toy craze of the future, with kids playing with DNA like it was Lego bricks. As part of his research initiative, one of his elf scientists designed Rockem' Sockem' Robot gas, which makes two strands of DNA bash together and battle each other for survival. Considering that the actual Santa Claus is involved, you can guess that Sluggy Freelance doesn't take genetics too seriously.
In The Dragon Doctors, Mori points out that DNA can't be treated like a bunch of building blocks ordinarily, but the use of magic allows one to treat traits as conceptual objects that may be swapped out at will.
In The Greening Wars "The Greening" is an organization that basically has this a policy
The protagonist in Wildlife is a Cosmic Horror that does this. She can absorb biomass from (almost) any living thing and use it to create or modify her own creatures in any way she desires.
A Grey World Manages to achieve this with plausible, yet awesome, outcomes. Alexis is implied to be the child of a Mad Scientist's For Science! experiments. But reasonable genetic material sources and sensible results give it a realistic feeling.
Orion's Arm, for all its much vaunted scientific accuracy, still features this trope in the form of splices (animals augmented with human DNA) and rianths (humans who added animal genes to themselves).
SCP-040 has this ability. The creatures she creates are designated SCP-040-1 and essentially act as her pets.
Batman Beyond features a gang called the Splicers who have had their DNA spliced with different animal DNA turning them into Petting Zoo People. Terry is turned into a literal Bat-Man (ala the Man-Bat) at one point, but it is easily reversible. Splicing is made illegal, but at first was perfectly legal and akin to body piercing and tattooing.
And, of course, Man-Bat is also an example. He was trying to isolate the bat's sonar-genes in order to cure deafness.
Maybe in the comics. On the Batman series, his father-in-law was into the whole 'bats will survive the next cataclysm' deal. Apparently Langstrom tested the human-to-bat formula on himself.
Teenage Mutant Ninja Turtles had a character called Bugman, whose origin had him accidentally combine his genes with those of various arthropods. For some reason, his acquired traits are only expressed when he gets angry.
The Darker and Edgier second season of Legion of Super Heroes features a clone of Superman provisionally called Superman X, who had Kryptonite planted in his genes! Instead of making his embryo self nonviable due to both Kryptonite and the loss of whatever the Legos that got visibly removed to make room for green K, Supes X has scary-looking eyes, immunity to Kryptonite, and shoots blasts of green ice in addition to the usual Kryptonian power set. (His extra powers, and entire existence, came from a bit of Executive Meddling about "beefing up" Superman.)
Danny Phantom, full stop. He got ghost powers implanted into his DNA!
An episode of Totally Spies! involved a scientist who used a... laser-gun-machine-thingy to "inject human DNA" into animals, making them walk upright and consume human food and think and speak perfect English and oh my god.
And in another episode, a fashion designer uses a serum of some sort to give humans animal features, so she can make seamless coats out of them. Of course, it is perfectly reversible.
The Sushi Pack episode "Fish Tales," Oleander teams up with a scientist who specializes in "DNA stuff that can alter human beings." By combining his DNA stuff with her "special" seafood bisque (and Kani's recently shed shell), Oleander is instantly endowed with a crab shell on her back, and her hands turn into pincers. The effect only lasts for two hours, though.
In Street Sharks, Dr. Paradigm literally stacks segments of DNA onto each other like Lego. He explains: "A little bit of this, a little bit of that, add some splicing agent, radiation and done."
In the 1980's Jonny Quest episode "Peril of the Reptilian," not only does Dr. Phorbus create his lizardman by combining human DNA with DNA from dinosaur bones he also has an entire island full of mix-n-matched dinosaurs such as a tyrannosaurus with pterodactyl wings, and a pterodactyl with a brontosaurus head and neck. (Since Phorbus touts the lizardman as his ultimate creation, these latter creatures seem to serve no purpose.)
In The Whywhy Family episode about genetics, Micro and Scopo shrink down and go inside Zygo to show Victor how DNA works. Victor physically moves Zygo's genes around like building blocks, causing him to turn into a multitude of other animals.
Played with in "Family Gay" when Peter is injected with the squirrel gene and gay gene. He also gains the Seth Rogen gene which gives him the appearance of being funny even though he hasn't done anything funny.
Spider-Man: The Animated Series: In this universe, there are in many scientists who specialize in it; it's called "Neogenics." Most of the Rogues Gallery were created by neogenics either Gone Horribly Wrong or Gone Horribly Right. Despite this, most characters' comic book origins are maintained. Connors trying to regrow his arm, Stillwell creating the Scorpion for Jameson, and of course a certain Unlucky Everydude who was bitten by a spider that had passed through the ray (a "neogenic recombinator" uses radiation rather than radiation just being magical) all happened by way of this new science that's all the rage in this universe. However, it was Peter's neogenics experiment (trying to cure himself during the six-arms saga) that Michael Morbius borrowed. A bat got into it, Michael got bitten, and you know the rest.
This is basically everything Dr. Sevarius does in Gargoyles. He even made his own Gargoyles by using bat DNA for wings, big cat DNA for sharp claws and teeth, & electric eel DNA to power the wing muscles, which also has the side-effect of allowing them to shoot electricity blasts. All he needs after that is a live human to serve as a base. As if you didn't already see that coming.
D.A.V.E. from The Batman is this with the brainwaves of Batman's Rogues Gallery like Joker's acrobatic feats and Penguin's mastery of martial arts. He also has the brainwaves of Hugo Strange and Riddler, making him intelligent.
Jenny and Zap from Dex Hamilton: Alien Entomologist are both the product of genetic engineering which grants them special abilities Including fly wings in Zap's case).
In the year 2772, a meteor strikes Acmetropolis, releasing bizarre radiation that gives the Loonatics Unleashed their superpowers. Not to be outdone, The Ringmaster and Otto the Odd bombard the Loonatics with their "sonic DNA scrambler," which mutates them in seconds into Mix-and-Match Critters with superpowers.
In recent years, genetic engineering has become incredibly simple and there are large libraries of open source genes which one can order to splice into the genetic code of a sample. The best known term for this technology is BioBricks.
As revealed in the February 2012 coverstory of National Geographic Magazine, the distinctive traits of domestic dog breeds work like this.
A small mutation with a large effect is unlikely to benefit the animal in the wild, but humans tend to take such surprises and breed them. Thus, such changes have become the building blocks of dog breeding, allowing humans to radically alter Canis lupus at a relatively rapid speed.
Therefore, an odd physical trait in a domestic dog tends to result from one mutation with a large effect, rather than the accumulation of many genes with small effects seen in the wild.
This is made all the more remarkable when we remember that the genetic engineering techniques used here were of the oldest variety known to humanity.
Modifications that work on the chemical level do have Lego characteristics. Most prominent would be the coding of fluorescent proteins derived from jellyfish, inserted into the DNA of various animals as advanced as mammals and actually working—the mice in question do produce the chemical, which can then be tracked down for interesting insights. There are also bio-engineered crops, "injected" with traits that strengthen crops (e.g. protection against the cold, built-in anti-parasite genes, et cetera) taken from fish and bees. It sent hippies into a fury. Note that this is always done to fertilized eggs; there currently is no way to specifically alter the DNA in every cell in a fully grown animal (or a still growing animal). Of course, once you have said animal, breeding becomes an option, although the viability of that with either modified or non-modified animals varies considerably.
However, note also that this is a logistical problem, not a genetics or physics issue. There's nothing physically impossible about taking an adult organism, and going through every one of its cells one by one, making the same change in them all. We "just" don't have the technology to do so while not killing them, not missing any, and doing it all fast enough to outrun and overtake the continual introduction of new cells. Of course, even if you did manage to do so, there's no telling what would result from applying the "new" chemistry to the "old" existing structures, the end result may still be very different from having made that same genetic change at the single-cell stage, or even a few years, days, or even hours earlier or later in the creature's life cycle.
However, it could possibly be done with a virus, as they work by altering DNA. Of course, it's still difficult to get it right, but this is being considered as a way to treat genetic illnesses
This is also only applicable in a very, very limited fashion. Producing glow-cats means just one or two extra molecules manufactured by the body, but making them grow scales or tentacles would be something completely different.
It's more of a question of fully understanding all the interactions between genes. There are no "tentacle" genes, but the growing of tentacles is coded as a complex interaction of genes. Most likely the entire skin and ossature, as well as some fundamental structural changes, would have to be made. Those changes would however be so fundamental the cat would really no longer a be a cat in any way we define the term "cat".
The sequence for GFP (green fluorescent protein) tends to work because it provides instructions to generate a protein that glows when stimulated, rather than a piece of the instructions for forming specialized "glow tissue/gland/organ cells" which most splicing subjects would likely be unable to use. Generally speaking, any cell with genetic material is capable of generating a protein so long as it has sufficient molecular material to build it; it just won't always know what to do with that protein. Cells don't need to do anything with GFP; it just sorta' sits there and glows. Because of this, the sequence for GFP is basically "compatible" with almost every type of cell, allowing scientists to make things glow when they get bored highlight cells that they want to observe in studies and experiments.
Goats can be genetically modified to produce spider silk (in an example of Reality Is Unrealistic, someone actually gave an episode of CSI: NY that showed this an entry on this page, claiming it was a particularly bad example of unrealistic science). How that actually works - the genetic alteration causes female goats to produce the protein spider silk is made of in their milk. To get spider silk from that, the protein has to be separated from the milk (which is a process), and then somehow pushed through a ridiculously small aperture to make the molecules snap into place. The silk made through this process still isn't as thin or as strong as natural spider silk. See the other wiki.Spidergoat, Spidergoat...
There's also the hox genes, discovered so far in a number of creatures (particularly fruit flies) which appear to control the physical structure of the body. Messing with them can produce major changes in the body of the target, such as the aforementioned eyeless (or legless, or legs-instead-of-eyes...) fruit flies, but is also often fatal. This is subject to the usual proviso of not affecting developed organisms.
Hox genes, short for "Homeobox" genes, are in fact named as such because they are highly conserved, being found in everything from plants to fungi to fruit flies to vertebrates. Think about that for a minute or two. Yes, that means you have them too.
Not only that, these genes are expressed in different parts of the body, some in the head, others in the thorax, others in the abdomen, etc, and the order in which they are expressed from head to tail correlates exactly with the order in which they are written on the chromosome. This suggests that we could reorder the genes to reorder the body.
This also depends on studies from the fields of epigenetics and developmental biology. As the same gene may be expressed differently in different parts of the body based on how they are transcribed and what the protein produced from translation does.
There is also this interesting note: If you splice some genes known to generally control the development of human eyes into a fruit fly genome at a point on the Hox gene controlling development of the abdomen, an eye will grow on the fly's abdomen—but it will be a perfectly ordinary fruit fly eye (well, ordinary except for the fact that it's on the abdomen).
It is also possible to swap the gene that controls the formation of eyes from, for instance, a mouse into a fruit fly and have it control the production of normal fruit fly eyes (even in the normal place, if you insert it in the correct location). Mice are deuterostomes while fruit flies are protostomes (this has to do with very basic details of how the embryo forms the gut of the organism, and is one of the oldest divisions in animals), which means that hox genes from the most distantly related branches of the animal kingdom are so intensively conserved that they remain interchangeable.
And let's not forget genetically modified foods. The few successful ones so far have generally involved adding DNA, usually bacterial, that causes a plant to produce a protein that it normally doesn't. For example, corn that produces bT toxin, effectively making its own pesticide. They also managed to make soybeans resistant to pesticides. These receive testing that compares toxins, nutrients, and allergens of the modified crop to the normal one.
Every year, hundreds of teams of university students participate in the International Genetically Engineered Machine (iGEM) competition, which is built on the premise that life can be broken down into a series of off-the-shelf, interchangeable parts and reassembled into creatures that have never existed. These parts are called "BioBricks." The competition's grand prize? A solid-silver Lego brick. It's a way to introduce a new field of science called synthetic biology.
"...Two things set synthetic biology apart [from genetic engineering]: The DNA building blocks don't have to come from nature; they can be designed and created in a lab, a process that's becoming faster and cheaper. And there's the idea that life, like cars or computers, can be designed and built from standardized parts that behave predictably." 
Completely synthetic DNA has now been made, it just needs to be injected in a microbe to kick-start it. It's more complicated than that of course, but it's awesome.
Bacteria, being basically protein-producing sacks of cytoplasm, don't have to deal with the complexities of large-scale structures in their bodies. Not only can they be artificially injected with entirely foreign DNA to produce new proteins and behave in new ways (like producing human insulin or eating oil slicks), but they regularly drop bits of their own DNA and pick up bits that were left behind by other bacteria as part of their natural life cycle. That includes bits dropped by other species - in fact, the typical definition of "species" doesn't really apply to bacteria at all, since they all have the ability to take bits and pieces of each other, the entire bacterial ecosystem is more of a loosely-bound genetic "marketplace" with LEGO Genetics pieces up for grabs.
This "horizontal gene transfer" ability can cause significant problems in the battle of humans against dangerous bacteria - for instance, if you use too many antibiotic medicines when you don't need them, you could put pressure on the harmless, symbiotic bacteria living in your body to become immune to them. Not too much of a problem on its own - but if a dangerous bacterium ends up in there later, the dangerous bacterial strain will sometimes grab the "immunity gene" from the harmless bacteria already living in your body and become a drug-resistant pathogen from the start.