Only 0.2% Different
Here's normal human DNA, and here's the DNA of the monsters we're fighting. My God! it's only 0.2% different! Despite the fact that the monsters have unexplainable powers, and look, act, and think almost nothing like us, there's no real difference between us and them! Really makes you think who the real monsters are, doesn't it? A subtrope/sister trope of What Measure Is a Non-Human?. There are a few problems with this. The first is that humans aren't their DNA, so even if the DNA is very similar to that of some other being, it doesn't always tell a lot about what that being is actually like. Among humans with very similar DNA types, the variation is huge, and humans are not even a genetically diverse species compared with the norm among animals. Even twins with identical DNA can turn out very different if some environmental input in the womb or after birth changes one of them, like an accident or brain trauma. The second is that the seemingly small difference in percentage could still be referring to a huge number of genes of varying importance (or even all of them, as in theory 0.2% could mean every gene is different by 0.2%, making its resulting peptide very different chemically). The third is that, between the gene and the human, there are millions of complex connections, causal chains, and effects that make even one small difference in genes potentially magnify at larger levels (as in some genetic disorders). The fourth is that the most often used examples, humans and chimpanzees, are astonishingly different in many respects despite the DNA similarity, making it a little awkward. In Real Life, it is true that we share some portion of our genome with many other species. Hell, humans have specific genes that are pretty much the exact same as plants. According to most scientific theories, this is because all organisms are descended from the same progenitor organism species, which also used DNA, the universal genetic code. Some experts theorize that the first species may have used RNA, which was later used to code for DNA. The genes that were present in that original organism are still around, since they generally serve very important purposes in controlling different aspects of development, but those aspects that they control have been changed and co-opted into other pathways over time. So genes that function to pattern flower growth in some plants (determining which parts of a bud turn into petals, stamens, etc) are necessary for directing heart development in humans. Further, DNA codes for chemicals, specifically proteins. Given that, and given that there are only so many ways to, say, break down a fat molecule into energy, it's hardly surprising that two wildly different animals, such as a mosquito and an elephant, would have large quantities of DNA (or rather, code for various chemicals needed for life) in common. Keep in mind that when people who know something about genetics talk about differences or similarities within the same species, they may be ignoring the large percentage of genetic material they would by definition already have in common by being the same species.
Examples:Anime and Manga
- The Angels in Neon Genesis Evangelion (though the actual reason involves panspermia). Though the reasoning for this is a bit sketchy. According to Ritsuko, the Angels are made of Hard Light, and she found a strange sequence of Hard Light in the Angel's tissues. Which for some reason she decided to compare to the human genome and came up with this trope.
- The androids from Armitage III are ranked according to the order they were created. "Firsts" are non-human robots, "Seconds" are androids, and the "Thirds" are so close to human they can get pregnant. And yes, they can reproduce with a human. The "Fourth" were modeled after plants and are arguable improved versions of natural plant life (such as trees that are capable of moving around.)
- The Abh from Crest of the Stars. Enemy propaganda says they are living robots. Their own propaganda says they are a superior race of living art. In reality, other than blue hair, Unusual Ears and a different organ in place of their sinuses, they're still human. Despite this the Abh themselves are far more advanced than the humans from Earth though and are quite arrogant about throwing their technology around in fight where they have overwhelming technological advantages.
- The Zentraedi from Super Dimension Fortress Macross and Robotech, justified at least in the former case in that they and humans have a common ancestor (the Protoculture).
- Cowboy Bebop has the episode Gateway Shuffle, in which an Eco-terrorist by the name of "Twinkle" Maria Murdock develops a retrovirus that transforms humans into apes. It's explained that there's only a 0.2% difference between human and monkey DNA, and the virus is designed to act specifically on the 0.2%. In a forced example of Honor Before Reason, the crew of the Bebop find themselves having to save the Europa government from being annihilated by a missile containing the virus at the expense of losing their bounty. The reasoning being that they can't collect the bounty if the ones who issued it in the first place are apes themselves.
- The Catians in Asobi ni Iku yo! reveal that the genetic difference between their race and humans is not even as large as 0.2%. There's only about a 0.00001% (One One-Hundred-Thousandth) genetic difference between the two species, and this plays a plot point later on in the anime.
- Ultimate Spider-Man has a rather inaccurate inverted example when Spidey encounters the new Scorpion...who looks just like him. He brings Scorpion to the Fantastic Four, and Reed finds that Scorpion's DNA is 94% similar to Spidey's. For comparison, that's how much humans and chimpanzees have in common DNA-wise. (Although he might be ignoring homo sapiens standard DNA, and only counting DNA that could be expected to vary from individual to individual.)
- The Doom movie had an additional chromosome that caused people to become demons.
- Mission to Mars had a puzzle in the face on Mars involving human DNA. This movie has several DNA failures. In that scene, a character says, "That DNA looks human!" while looking at a few base pairs. Out of a few billion. Another scene in the movie has a character displaying his "dream woman" who, apparently, consists of only a few base pairs again. In this scene it was pretty obvious that Phil was just cracking wise when asked what the short segment in front of him represented. The scene was to set up the idea that Phil was very well-versed in biology so he would be believable when he identified the passcode gene.
- The clones from Star Wars: Attack of the Clones. This is lampshaded in one of the animated series when three clones are alone with Yoda. Yoda explains that though their faces may be alike (being clones of Jango Fett and all), through the Force he can see their minds are as diverse as other human groups.
- In Prometheus, it is revealed that the Engineers are a 100% match for human DNA, despite being ten-foot tall hairless albinos. Considering that we know very little about their environmental development, this is probably one of the most plausible bits of "science" in the entire film.
- Inverted on The Event. The people from the Inostranka crash have 2% different DNA from humans, and the president interprets this fact to mean that they're just like humans. His adviser informs him that they have less in common with humans than apes do (despite them looking exactly like us and eventually turning out to be our  ancestors). This is used to argue that they don't deserve human rights, an idea which the president doesn't approve of.
- Referenced on House:
House: Oxygen saturation is 94%, check her heart.
Foreman: Her oxygen saturation is normal.
House: It’s off by one percentage point.
Foreman: It’s within range. It’s normal.
House: If her DNA was off by one percentage point she’d be a dolphin.
- The entire premise of ABC's one-season-wonder Prey was based on a killer strain of humanity with the same genetic variance from homo sapiens as exists between us and chimpanzees. Also, they were caused by global warming. The idea being that a localized acceleration of global warming accelerated human adaptation to that phenomenon among a particular population, who then spread out in secret among the rest of humanity. The show named them Homo dominus. For some reason, this new species not only has a higher level of intelligence but also completely lacks emotions beyond an irrational need to breaks necks of helpless monkeys to show their superiority.
- In a rare non-human example of this trope, Mass Effect 2 has, during the Collector ship mission, the discovery that the Collectors you've been fighting are actually mutant Protheans, fair enough. Then you realize that the statues of Protheans you see in Mass Effect 1 look absolutely nothing like Collectors, for one thing, Protheans look as if they're mammals, while Collectors are, as said before, Insectoid Aliens. However, in Mass Effect 3 DLC, you meet a Prothean, who looks a lot more like the Collectors, and it's implied that the aliens depicted in Mass Effect 1 weren't Protheans, but rather the precursors to the Protheans, called the Inusannon.
- Solid Snake and Liquid Snake are retconned into being 0.2% different in Metal Gear Solid 4, to explain why Snake's FOX-DIE infection isn't killing him. This leads to plot holes seeing as 0.2% difference is actually significantly larger in genetic terms than the practical difference between Solid and Liquid (who are identical twins).
- Lan and MegaMan have a 0.1% difference in their DNA, due to MegaMan being a program designed after Lan's dead twin brother, Hub. However, this is changed to a 0% difference between the two at the end of the first game, making the two have exactly the same DNA. This is all despite MegaMan being a computer program.
- In reality, there are fairly common cases of an additional chromosome. "Down's Syndrome" is actually one of the more benign duplicate chromosomes: in fact, only duplicate sex chromosomes are less harmful. A duplicate of any chromosome other than 21 or 23 drastically shortens your lifespan. Full trisomies of any chromosome other than 13, 18, 21, X or Y are incompatible with life, and cause miscarriage. Partial trisomies can be survivable however.
- We share 75% of our known human disease genes with the fruit fly. Which is awesome, as we can experiment on them and not on us. A particularly weird bit of experimentation showed that injecting the gene product of the PAX6 gene (which is found in humans and other animals and turns on eye development) into fruit flies caused eyes to develop (functioning like the insect's version, appropriately called eyeless). The eye that developed was still an insect compound eye though, as the proteins that are expressed as a result of the gene being turned on are still proteins specific to forming an insect eye, not a mammal's eye. Which goes to show how important that other 25% is in terms of genetics.
- Genetically, mushrooms are closer relations to us than to plants. Also like us, fungi are heterotrophs, which means that they must 'eat' and can't make their own food like plants can. They also store energy as glycogen, as do animals and humans, as opposed to plants, which store it as starch. They also make their tissue out of chitin (the stuff in insect exoskeletons) rather than cellulose (the stuff in plants). In fact, fungi have more in common with sessile animals like sponges than they do with plants.
- Dogs, Canis lupus familiaris, are a single subspecies of the gray wolf, Canis lupus, but have far greater physical differences from each other than any two species of the Canis genus.
- Jonathan Marks mocked this whole view in his book What It Means to Be 98% Chimpanzee (it was written before the chimpanzee genetic code was fully sequenced, and revealed to be closer to 99% identical to humans). He pointed out that, first of all, since there are only 4 base pairs, a random string of base pairs would be 25% identical to humans. Furthermore, simply because of common ancestry, almost all species on Earth are closer to us than that. He estimates that daffodils share 1/3 of our genetic material, but that saying a human being is "one third daffodil" would be ludicrous (and, by extension, saying we're "98% chimp" is ludicrous as well).
- DNA does not work the way most people think it does. A lot of it has to do with things like the basic functioning of cells (breaking down complex molecules for energy, reproducing, and so on), which is pretty much the same for any cell, so it's hardly surprising that the DNA coding for the proteins involved in those processes is quite similar across species.