A set of twins who look and act for all the world like they're identical, except for the miiinor detail that one's male and the other's female.
It's common, especially in drawn or animated media where the creator has complete control over the appearance of the characters, to use brother-sister twins as each other's Distaff Counterpart and Spear Counterpart. They often display identical twin tropes, such as Twin Telepathy or Synchronization, especially if they are Creepy Twins. It's a way to make sure that twins, even if they're not identical, are immediately identifiable as such if it's relevant to their characters, and there are no other visual cues to show that siblings are fraternal twins instead of just regular brothers and sisters.
This is, of course, vanishingly rare in Real Life - any set of twins that isn't identical won't be any more likely to resemble each other than siblings born years apart, and if the twins have different sexes then that means they're not identical (or one has an extremely rare genetic disorder). Of course, some siblings do have a Strong Family Resemblance. Also, this could happen if one identical twin is Transsexual and the other isn't.
Compare to the rare real-life phenomenon of semi-identical twins, where the twins have identical genes from their mother and different genes from their father, and thus can be the same sex or opposite sexes. This could lead to an Uncanny Family Resemblance if the sperms are similar enough.
This trope is perhaps the most glaring example of Always Identical Twins.
Compare Opposite-Sex Clone.
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Anime & Manga
In Samurai High School, Tsukiko and Kou don't look similar at all, but nobody in school ever notices when they swap places, so we guess this counts.
Possibly Hansel and Gretel from Black Lagoon. They appear to be brother and sister, and are nicknamed "Vampire Twins", but it's never revealed whether they are different sexes or not. Confusing things further, they have switched roles on-screen at least once, by having "Gretel" remove "her" long-haired wig (revealing a haircut to identical to the one "Hansel", down to the color and style) and hand it to "him" to put on. As their highly stylized school uniforms are effectively unisex, the switch is flawless. It gets weirder in the end when (Hansel?) Gretel talks with both Hansel and Gretel's personality by (him?)herself. It's highly possible that they are same-gender full-identical twins who are just acting out separate gender roles. Though it's strongly implied that they are either transgender or were mutilated and forced into it, judging by the look on Rock's face when Gretel shows him. Squick in any way.
This is lampshaded in a manga omake where it's stated the Animated Actors who "played" the twins are actually the same gender (female).
In the omake where everyone's genders are switched, they look exactly the same.
Kozue and Miki in Revolutionary Girl Utena, though unlike most examples, they have different hair colors. The Movie uses a cameo that simplifies the twinness by giving them the same hair color.
Johan and Anna/Nina from Monster, who were dressed identically when children and were almost indistinguishable. It remains when they grow up, to the point that Johan disguises himself as a woman to hide, and when Nina arrives to the same town she's confused for her brother's female identity.
In Shugo Chara!, Nadeshiko's identical twin brother Nagihiko shows up when Amu visits her house. Immediately subverted in the exact same chapter, where it's revealed after she leaves that they're the exact same person.
Yuki and Jun Kanzato from Persona -trinity soul- would fit this trope to the letter if Yuki's soul wasn't in Jun's body after her death. Jun also manages to keep a transplanted piece of his sister's brain in his head without any need for anti-rejection medications.
Terriermon and Lopmon in the Digimon franchise. In Digimon Tamers, they aren't related (Lopmon's a Heel Face Turned Deva who's introduced right before things heat up), but they still manage to get an accidental Twin Switch in the second movie.
Suou and Shion from the second season of Darker Than Black. Explained by the fact that Suou is actually a copy that Shion made of himself, but because the copies he makes with his power always have one major difference from the original, Suou turned out female. Except it's not justified because even though the current Suou is a copy of Shion the original is still shown to be a half identical twin in the flashbacks she appears in. The fact that the current Suou looks like her younger self despite being a copy of her brother is evident of this.
Daisy and Violetta, children in Princess Knight and its sequel, Twin Knight. Not only do they have the same beauty spot on their left hands, but Violetta has to, like her nearly-homonym Viola from Twelfth Night, impersonate her brother. And it works.
Sora and Haruka from Yosuga no Sora. That they both have unisex names doesn't help.
Lampshaded in Boku Ni Natta Watashi: "Is it even possible for fraternal twins to be completely identical?"
In Bizenghast, Edaniel and his sister Elala look very similar, as to a small degree Edrear and Eniri, despite their different skin tones.(they were made from bits and pieces of leftover souls so they don't entirely resemble each other)
Atelier Lana's Star Trekker is about the adventures of Captain Aya Nakajima, with her First Officer and identicle twin Brother Commander Homare Nakajima. From the explanation given in the manga:
These two each received half of their parents' genes and are identical twins thanks to biochemical composition.
Android #17 and #18 from Dragon Ball Z, who would look identical if not for their different clothes and hair color.
Vassalord has two sets of these: Rayflo and Rayfell are actual Half-Identical Twins while Charley and Cheryl are unrelated (as far as we know) but look uncannily similar.
Tatara and Sarasa in Basara: when Sarasa decides to impersonate her twin brother, she just cuts her hair and almost everybody believe she is Tatara - even Tatara's fiancée. However, it may be because people deeply want to believe it.
Kanba and Masako in Mawaru-Penguindrum. It eventually gets lampshaded when Sanetoshi points out how physically similar they are as a part of a Breaking Lecture that he delivers to Kanba as he's getting ready to use a Survival Strategy to revive Masako, who has recently gone through an Heroic Sacrifice for Kanba.
Subverted: In Case Closed there's a pair of twins who look rather different, but have almost identical genes, to the point where the female twin's suicide is blamed on her brother/fiancé (she'd only just found out that they were related, and he didn't find out until after she was dead) because of skin that is stuck under a false nail, and he has to be bailed out by Conan-as-Kogoro. It's explained as the girl having been born with Turner Syndrome, where she was born with only one X chromosome. Presumably, this explains why they look so different.
Subverted in Axis Powers Hetalia. Switzerland and Liechtenstein look almost exactly alike save for their height and eyes, but they're actually adopted siblings who met each other right after World War One, when they were the nation-tan equivalent of older teenagers. It's safe to assume they are at least distantly related since they both represent Germanic nations, but it's not specified what they are to each other in this sense.
Kurando and Mafuyu from Popcorn Avatar are this, to the point when the latter comes to visit Kurando's classmates mistake her for him crossdressing.
Andrea & Andreas von Strucker (a.k.a. Fenris), and Jeanne-Marie (Aurora) & Jean-Paul (Northstar) Beaubier. Both have superpowers that are activated by touching each other, though the latter pair also have powers on their own.
Brian and Betsy Braddock used to be Half-Identical Twins (Betsy's Caucasian body was naturally blonde and resembled her brother a bit). They aren't any more, they aren't even the same ethnicity anymore, hard to imagine anyone would think they are related at this point, let alone twins.
The rare aversion to this trope in Marvel are Vanguard/Red Guardian and Darkstar of the Soviet Super Soldiers/Winterguard and X-Corporation. He's a redhead Husky Russkie and she's an icy blonde nor do their powers resemble at all. He fires concussion blasts that he must focus through his hammer and sickle while she's a darkforce manipulator.
In the original continuity of Legion of Super-Heroes, Lightning Lass easily posed as her dead brother for one story, but was revealed because she didn't have an Adam's Apple. Much later, it was shown that her planet is almost all twins, but the twins were generally depicted as identical.
An interesting case was a story arc in the Buffy the Vampire Slayer comic (the "Season Zero" between "The Origin" and "Welcome to the Hellmouth", published in 2003) where she goes to Las Vegas. The villains are a pair of conjoined twins: a male vampire and a female mortal. They didn't particularly look alike, but the fact remains that conjoined twins are identical by definition and always the same sex, so a pair that are opposite sexes is a case of Artistic License - Biology.
In Marvel Star Wars, Vila and Denin aren't exactly the same. You can see from the second link that Denin is taller and a little broader across the shoulders. Still, after Denin died Vila was able to impersonate him.
Sidney and Toots the twins in the Bash Street Kids (a comic strip in The Beano).
Zipi y Zape: The twins look exactly the same... except for their hair color.
The Official Fanfiction University of Redwallhas an excuse for this; in one of the original printings of the book Redwall, the character Killconey swapped between male and female pronouns thanks to a typographical error. (According to later printings he's meant to be male, in case you're wondering.) In the OFUR, this has resulted in there now being two of him, one male and one female. The female one is generally referred to as Konnie to avoid confusion. She's technically more of an Opposite-Sex Clone, but they think of each other as twins.
Ash and Caiden from VOCALOID Forever, along with Rin and Len Kagamine listed below.
In A Series of Unfortunate Events the Quagmire triplets are "absolutely identical," so how the Baudelaires tell whether they're talking to male Duncan or female Isadora is a mystery — although Isadora is illustrated with subtly longer hair. But at least the two brothers Duncan and Quigley never share a scene. Jacques and Kit are an aversion, as the book does not mention any similarity. At all. If anything, there's more similarity between Jacques and Olaf.
As a result, once Jaime starts to grow a beard in the third book, it becomes a very important symbol of their growing distance.
In Kathryn Lasky's Starbuck Twins Mystery series of books for children, fraternal twins Liberty and July Starbuck are described as indistinguishable from one another save for the fact that Liberty's hair is long, and her brother July's is short. They also display Twin Telepathy.
As children, Thom and Alanna, of Tamora Pierce's Song of the Lioness series, were so much alike in face and body shape as to be mistaken for each other, if dressed alike, which they exploit so Alanna can train for a knight and Thom can be a mage. The only distinguishing feature of the twins at that time was the length of their hair. It stops applying once they're grown, as Alanna comments that Thom has grown taller than her when they meet as older teenagers.
Continuing from the example of Star Wars above, Leia and Han's first two children, Jaina and Jacen Solo. Timothy Zahn, who wrote the novels in which they first appear, deliberately modeled them after the Luke/Leia duo. In this case though, it makes a little bit more sense, since they are raised together.
In Donna Tartt's The Secret History the twins start out similar, but in the end turn out to be two completely different people because one of them has a breakdown and the other turns into a Shrinking Violet.
Heinlein's Time Enough for Love involves a couple of female genetic scientists creating a pair of Half-Identical Twins for Lazarus Long: two redheaded women named Lazuli and Lorelei, or Laz-Lor. Lazarus calls them his "identicals" even though they're not exactly, being girls. And then they engage in a threesome that is either Twincest, Brother-Sister Incest, or father-daughters incest, depending on how you interpret the relationship. Please keep in mind that Lazarus himself is a pretty blatant Author Avatar.
Lazarus also bought a pair of slaves he bough to free that had been produced by careful genetic engineering that ensured that while they were brother and sister had no genetic relationship (having been created from perfectly complimentary gametes, each bearing different halves of each parent's genes). They were sold as a novelty, with complete documentation included in the package. Since they know they're full siblings, they had to be taught why it wouldn't be a good idea for their children to produce children with each other.
P.C. Hodgell's Chronicles of the Kencyrath has heroine Jame and her Half Identical Twin brother Torisen. Jame is frequently mistaken for her brother, and once she's better known, he is mistaken for her — but only once, because that's when he stops finding it amusing and decides to grow a beard.
The legendary immortals known only as the Twins in Trudi Canavan's Age Of The Five trilogy appear to be this to begin with - they were originally not only identical, but conjoined. They separated themselves with magic to avoid detection, and some time afterwards, one of the twins changed from female to male using similar magic. It was really confusing how one could be female and the other male given their history, and the reference to 'the change' is really easy to miss...
In Sheri S. Tepper's Sideshow, two of the main characters are conjoined twins of different genders. This is justified in that they were both born intersexed, with ambiguous genitalia. The doctors asked their parents for their opinion on what to do, and while the father was certain that the first one was male (the Virgin Mary had told him so), the mother thought it would be nice to have a little girl. Naturally, the two run into some problems at puberty, since they share a circulatory system, but they each identify as the gender they were assigned and raised as.
Poppy Z. Brite's novel The Lazarus Heart features identical twins, one of whom is a transgender woman.
In Maggody and the Moonbeams, Dahlia insists that her twins are "identical", because they look a lot alike to her and she's convinced it'll get them successful Hollywood careers. The contrary fact they're different sexes is ignored, as she doesn't actually know what "identical" means in this context.
In Orlando Furioso, Ricardetto takes advantage of being always mistaken for Brademante to woo a princess who fell in love with his twin sister. He claims to have been turned into a man by a grateful water nymph.
Jobeth and Tommy in Clive Barker's The Great and Secret Show.
Averted in Lev AC Rosen’s All Men of Genius: Based on William Shakespeare’s Twelfth Night, Violet does disguise herself as her twin brother Ashton, but only to people that don’t know them. People have no problem telling Violet/“Ashton” and Ashton/“Ashton’s cousin Ashton” apart.
Lawrence Sander's The Tomorrow File references a pair of opposite-sex "identical (and incestuous) twins" named Francis and Frances.
Power Rangers RPM has the peppy twin geniuses Gem and Gemma. They have several superficial differences if you look close enough, but you have to look very close- not because the differences are minor, becuase they aren't, but because the twins in question are just that single minded.
Justified in a Law & Order: SVU episode, where what appear to be Half-Identical Twins turn out to have been born as identical twin males. One twin's genitals were badly injured by a botched circumscision, and a crackpot doctor convinced the parents he'd grow up happier if given a sex change to female (s/he wasn't).
Tragically Truth in Television there was apparently a real rash of this kind of thing back in the 70s, when cutting edge thinkers had decided that gender was just a matter of socialization. Turns out it's a little more complicated than that. See The Other Wiki: David Reimer. It happened more than once because the first doctor, John Money, who performed it wrote medical articles stating that the procedure went well without aftereffects, because he didn't like the results he got — they conflicted with his theory.
The short lived FOX series Mental had the part about a botched circumcision resulting in a sex change. It was all fine, until the "girl" ended up lighting him/herself on fire for no apparent reason and kept seeing him/herself in the mirror without a face. The protagonist (a rebellious psychiatrist) realized what was going on. The "girl"'s true identity as a male was trying to push its way to the surface. "She" didn't even have proper female genitalia, just a prosthetic designed to look like one. The crackpot who convinced her father to do this planned to give "her" a vagina when "she" was 18. (Because every girl waits until she's 18 before having sex or having a Date With Rosie Palms, and wouldn't notice the difference because No Periods, Period is normal.) Naturally, when "her" boyfriend finds out the truth, he runs away in disgust. At the end, "she" gets a boyish haircut and decides to try to explore the male lifestyle, even though, without certain body parts, the experience would be far from complete.
Butch Lesbian "Walter" from German series Hinter Gittern Der Frauenknast ("Behind Bars - the Women's Prison") has a twin brother who's apparently indistinguishable from her without his beard. Which they used in one episode to change roles, allowing her to escape and him to get closer to several female prisoners.
The Big Bang Theory averts this by giving Sheldon a twin sister, Missy, whose face does not resemble his. Sheldon explains the scientific errors behind this trope in his typical Sesquipedalian Loquaciousness. Howard replies to this, "Hallelujah!" (Missy is rather attractive.)
Also averted on The Edison Twins: Annie is a redhead, while her twin Tom is blonde. (There was an even an episode in which they explained the concept of fraternal vs. identical twins.)
Meanwhile, their younger brother Paul is a brunette!
Pictured above, Vocaloid's Rin and Len Kagamine are often portrayed this way; for example, in "The Daughter of Evil" series. Officially, they look suspiciously similar (same eye color, same hair color, same height) and are opposite genders, but they're technically "mirror images," not twins. Various fanon incarnations make them twins, lovers, both, or something else entirely.
Similarly, Raul and Fiona Gureden, who were male and female versions of the same character in Super Robot Wars Reversal, were made into twins for Original Generations.
On a similiar note, Viletta Badim and Ingram Plisken have the same deal in Super Hero Operations, and were later made into clones.
Palom and Porom from Final Fantasy IV may qualify for this: While the artwork, particularly the sprites, lack sufficient detail to be certain whether they were meant to be identical or simply share a number of similar traits and style of dress, there is a distinct theme of similarity coupled with reversal; not only the sexes, but elements of personality (Palom is boastful and rude, Porom reserved and polite [when not berating her brother]), their style of dress (both are pictured wearing red and green outfits, but one's is red where the other's is green, and vice versa), their use of magic (Palom uses the destructive Black Magic, Porom uses the healing/supportive White Magic), and hairstyle (the sprites show a right/left reversal between the two, otherwise being identical; the portraits show Porom's hair looking well-groomed and Palom's hair looking rather messy). However, by Final Fantasy IV: The After Years, puberty and years of different lifestyles have taken their toll, making them look very noticeably different from one another. Also Porom dyed her hair pink for no particular reason. That might make them seem a bit dissimilar.
In The Last Blade 2, a woman impersonates her dead twin brother, Kojiroh, a member of the Shinsengumi, in order to bring his killer to justice. She ends up living the rest of her life under the charade, taking on her brother's identity as a tribute to him.
Alfred and Alexia in Resident Evil: Code: Veronica. In fact, the resemblance is so good that Alfred is able to convincingly pretend to be Alexia for several years after she was frozen in order to make people think she was still alive. Kind of explained in that they are really clones.
Nox and Matin Catorce are identical brother and sister twins from Blind Justice ~Torn souls, Hurt Faiths~ by Zektbach in the beatmania series. Both twins also have special swords that are identical, although one represents Hope and the other Despair.
Eagle Eye Mysteries: The title protagonists, Jake and Jennifer Eagle. Besides the obvious gender differences, he's a red-head and she's brown-haired.
It's not normally obvious due to their normally different outfitsnote Rania wears a dress, Rahal wears armor and hairstyles, but Rahal and his sister Rania Suikoden V are near-identical. When Rahal is required to disguise himself as a woman, the player is likely to not realize what happened at first, and instead wonder "How did Rania get here, and what happened to her glasses?"
Subverted in BioShock Infinite. Robert and Rosalind Lutece seem to be this at first, though this is the least abnormal trait they display. They actually turn out to be different versions of the same person from parallel universes.
In the Bleach videogame The Third Phantom, we have the main prtagonists Matsuri and Fujimaru Kudo.
Averted and lampshaded in Ever17. You is confused as to why Sara and the Kid don't have the same abilities or look similar despite being twins, at which point it's explained with some amusement that biology doesn't work like that.
In the Murder MysteryVisual NovelJisei, one of the main characters has an identical twin sister who you don't see until the end of the game. It turns out that she and her brother were private investigators, and she deliberately sent her brother in as a mole to check out the murder scene. She was the telepathic voice helping the protagonist to solve the crime all along.
John and Jade from Homestuck. Even before The Reveal, they are very similar in appearance and behavior (much more so than Rose and Dave).
Though this could be partly due to the fact that their biological parents looked pretty similar as well.
Litter mates Bear and Bryony from Catena don't act alike, but except for their hair and Bear's chin they're pretty nearly identical. Their other sister Patches, on describing Bryony, even comments to Bear that "She looks like you, only, you know, pretty."
Sam and April of Dragon City look identical aside from gender and April's glasses, but according to the Fourth Wall Mail Slot they actually are identical, gender in dragons is influenced by incubation temperature (as in crocodiles) rather than genes. Every other set of twins in the comic are obviously fraternal, even having differently colored scales.
Averted in Princess Chroma with April and Sonny, who look no more alike than any other brother and sister.
In Questionable Content, people think that Claire and Clinton are this, but she's a couple years older than him. It also doesn't work because Claire's actually trans, so she was presumably raised as a male.
Red vs. Blue subverts this with North and South's faces—while both have very light blond/white hair, they look quite a bit different—but it's played fairly straight with their armor, which is subtly different colors of purple and green. When only one is on screen, it's hard to tell which it is unless they're standing next to someone (South is shorter than North) or you've gotten really good at picking out the different colors of purple (South is lighter and North is darker).
Phil and Lil on Rugrats. (Although it could be argued that most babies look alike anyway. However, in the spinoff, where the kids are 10-13, Phil and Lil can still pass for each other with a simple change of clothes and a wig). One episode has the twins managing to distract a robber just by removing Lil's bow to make her look exactly like Phil.
In Justice League Unlimited, the Ultimen are based on the Super Friends team members who were created for the show instead of the comics. Their versions of the Wonder Twins, Downpour and Shifter, are so similar it's very difficult to tell them apart. Shifter doesn't have much in the way of the Most Common Superpower, so even examination of the chest area only works if they're standing side by side to compare. They even share a voice actor.
Justified since they are not actually twins, but somewhere between clones and genetically engineered super beings.
She-Ra and He-Man. Both have super-special-sword powers, and blond hair, and extensive muscling in super-form. Though to be fair, it's hard to mistake Adam for Adora, or the other way 'round.
Downplayed on Gravity Falls:Dipper and Mabel are pretty much identical in terms of face and body type, but their resemblance is harder to notice because of their different outfits, hair styles and personalities. Also, Mabel has braces while Dipper has a (rarely seen) birthmark on his forehead.
Desna and Eska, Avatar Korra's fraternal twin cousins, are described by The Legend of Korra writers as "androgynous, creepy twins."
While extremely rare (only three to five known cases), there are cases in which otherwise identical twins can be of the opposite sex. More info here.
One real-life occurrence that can produce boy-girl twins who are genetically identical (but who do not bear a strong visual resemblance), is when a zygote that is 46,XY (a normal healthy boy), undergoes incomplete mitosis in the womb. One of the resulting zygotes is 46,XY (still a healthy boy), and the other has a missing Y chromosome, and so is 45,X (a girl with Turner Syndrome). Genetically identical apart from the sex chromosomes, although Turner syndrome produces some fairly noticeable differences (physical and otherwise).
When two sperm fertilize one egg, which is rare, the resulting embryo almost never survives. However, there is a unique (in recorded history) case of "semi-identical twins": a pair that inherited exactly the same set of genes from their mother, but share only half from their father.
There's also the less rare instance where fraternal twins of opposite sexes simply have such a strong family resemblance that they can pass for each other... at least til puberty, anyway.
Can happen when one identical twin is transgender and transitions later in life, or if a genetically male individual grows up female due to one of many medical situations where that could happen, whereas the other twin grows up male without the condition. As of 2011 there are now news stories about a pair of teenage identical twins where one is trans.
One example of this is with Laverne Cox of Orange Is The New Black — when a scene called for a representation of her character Sophia before her transition (as the character is transgender as well), Laverne had said that she wouldn't mind dressing as a man for the role, but the producers were horrified at the thought of putting her through that kind of discomfort. The solution? They got her identical twin brother to play the role, and he was more than happy to.