We all know the basic Mendelian model of inheritance; traits are usually either dominant or recessive. Having one copy of a dominant allele means that the person will express the dominant trait, and the only way for someone to express a recessive trait requires having two copies of a recessive allele. Even outside of scientific terms, it's common reasoning that certain traits tend to run in families and we expect relatives to "match" in some way.
These facts are sometimes ignored in Hollywood casting, which can end up having (biological) families looking like just a random group of people. As a result, you'll see blondes having black-haired children, biracial people who look more black/Asian/etc. than their black/Asian/etc. parent, inexplicably large discrepancies in height between parents and children, and so on.
Sometimes families can look so unrelated that it can take focus from the story. "Where did Alice get black hair and brown eyes if Bob and Carol are green-eyed redheads?"
This can even interfere with the plot if a physical resemblance is a plot point (especially in an adaptation when characters get an Adaptation Dye Job or otherwise don't match the physical description of the character in the original work).
There are certain justifications; the mechanics behind real life genetic inheritance are more complicated than the Mendelian model, which doesn't factor in mutations, malfunctions, and polygenetic traits. Also, casting around getting the the most plausible genetics is not always practical—after all, you're not going to turn down Harrison Ford for a part because his eyes are the wrong color.
On the other hand, you don't have that excuse in non-visual media or in animation where the powers that be have complete control over how characters look. And you could always give Harrison Ford contacts.
Compare Identical Grandson, Uncanny Family Resemblance, and Patchwork Kids, which can veer too far in the opposite direction. When done intentionally, the dissimilar child is a Chocolate Baby. Also see LEGO Genetics, for the Science Fiction take.
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Anime & Manga
General note before adding examples:note As mentioned in the You Gotta Have Blue Hair article, hair and eye colors in Japanese media are normally chosen to tell characters apart and to match their personalities (e.g. Shy Blue-Haired Girl, Red Eyes, Take Warning)—combine that with Only Six Faces and Mukokuseki, and anime characters having anything notable in common with their families is the exception rather than the rule (although there will be an occasional Hereditary Hairstyle). For that reason only aversions or egregious examples should be listed.
Bulma has her mother's blue eyes (not obvious since her mother's eyes are normally closed) and her father's purple hair, both of which she passed on to her children Trunks and Bra. Trunks has Vegeta's skin color and eyebrows, while Bra is more or less a clone of her mother.note In the anime, both Bulma and Bra have blue-green hair, while her father and son have purplish tones.
As a little girl, Krillin and Android 18's daughter Marron looks just like Krillin with 18's hair color, including having no nose. By Dragonball GT, she looks just like her mother, and bears little resemblance to her father.
Gohan looks like both of his parents, while Goten looks exactly like Goku did in the original Dragonball, including his hairstyle. Gohan's daughter Pan in turn inherited the shape of her mother Videl's eyes but his eye color.
Naruto averts this in general; if a character has an unusual hair color or facial feature, it's normally accounted for in their family— in the title character's case, his Implausible Hair Color and eye color were inherited from his father, while the shape of his face was inherited from his mother. Sasuke and Itachi Uchiha look exactly like their mother Mitoko except for the creases under their father Fugaku's eyes that Itachi inherited. Sakura's pink hair was inherited from her father (albeit his is a darker shade), while based on her mother's rather extreme bangs it's obvious who Sakura got her Forehead of Doom from.
Invoked for the merfolk in One Piece: due to their unusual biolgy, merfolk carry the genes of all their immediate ancestors, so family members are not even guaranteed to be of the same species. These three◊ are full-brothers. Compare them in both appearance and size to their sister.◊
Chibi-Usa from Sailor Moon generated a ton of Wild Mass Guessing and Fan Wank back in the day because her coloration (pink hair, red eyes) was so different from that of her parents (blonde, blue-eyed Usagi and black haired, blue-eyed Mamoru). According to Word Of God, her coloration was meant to make her resemble a baby rabbit to match her Punny Name. Also, hair colors in the manga weren't static—sometimes her mother Usagi was depicted with silver hair and even pink hair in her original design. Usagi herself is the daughter of a purple-haired mother and a black-haired, dark-eyed father and has a brother with light brown hair and dark blue eyes.
Tenchi Muyo! is a strong aversion, even regarding facial features. In fact, the only character whose look isn't readily accounted for in their family is Tenchi's mother Kiyone (silver hair and gray eyes while her parents have black hair/brown eyes and green hair/yellow eyes), and since she and her parents are two different kinds of Human Aliens, we have no idea how their genetics exactly work anyway.
Actually invoked in the main continuity with Aeka. She was born with curly aqua-green hair like her mother and sister, but later underwent gene manipulation to activate her father's genes for straight purple hair to look more like her black-haired stepmother Funaho in order to win the affections of her half brother and betrothed, Yosho.
Spider-Man's Mary Jane Watson inherited her red hair from her father. Her mother is blonde... which means her mother either dyes her hair or there is a huge elephant in the room, as the older daughter Gayle is a dark brunette. You know, Gayle being the product of an extramarital affair could partially explain why their father was such an Abusive Parent...
Despite being a clone of Wolverine, Laura Kinney, a.k.a X-23 of X-Men is portrayed as having similar features to her mother Sarah, who while not genetically related to her, carried her to term. Some stories claim that Dr. Kinney used some of her DNA to stabilize the sample they had from Wolverine since it was damaged, which would partially justify it.
Also from X-Men, supervillian Vulcan is the black-haired son of brown-haired Corsair and blonde Katherine Summers. The black hair could be part of his mutation, though.
Harry Potter inherited his mother Lily's Green Eyes, which is a plot point. In the film adaptations, blue-eyed Daniel Radcliffe was cast. This is fine in and of itself, since they also cast blue-eyed Geraldine Somerville as his mother, preserving the similarity. However in the film version of Harry Potter and the Deathly Hallows, during a flashback which detailed her childhood, the actress cast had deep brown eyes.
Terminator 2 has brown-haired, brown-eyed Edward Furlong play the offspring of blonde, gray-eyed Linda Hamilton and blond, blue-eyed Michael Biehn. It's also a retcon because Kyle mentions that John has Sarah's eyes.
Many musicals, especially onstage, will completely drop any attempts at realism and go for color-blind casting, often intentionally casting different races in the role of family members. One of the most memorable examples of this in film is Rodgers And Hammerstein's Cinderella starring Brandi. The king is white, the queen is black, and their son is Asian. And it was completely intentional. You're not supposed to care.
In the same film, the stepmother and one of the stepsisters are white, while the other stepsister is black.
Similar to the examples in the film series, in Harry Potter JK Rowling really didn't pay attention in high school bio. Though magical abilities seem to act recessively (e.g. two non-magical parents can have a magical child), Rowling ''says'' that it's actually dominant and can "become dormant" in certain lineages and resurface later. (This would explain squibs— muggles from magical families).
Believe it or not, one fan explained a method, and wrote a page biological paper on it. It utilizes the mechanism of single nucleotide repeats, the same mechanism for Huntington's disease.
A plot point in A Song of Ice and Fire, King Robert Baratheon had black hair and Queen Cersei Lannister is blond and all three of their children are blond as well. But all of Robert's known bastards are black haired and every other time their houses have intermarried the children have had black hair so Ned Stark figures out that Cersei's kids were actually sired by her brother. It's plausible to put the Baratheon children's paternity in doubt, since having three children in a row with the same recessive trait only expressed by one parent is unlikely, but having a trait stick so tenaciously to one family after centuries of mixed breeding is extremely unlikely. A child has to inherit one allele or another from each parent, even if it isn't expressed.
In many of the Chivalric Romances, Percival has a half-brother with a white father and a black mother. The brother is piebald (that is to say, he has light and dark patches of skin like a checkerboard).
A common joke in 7th Heaven 's Hatedom was Little Miss Snarker Ruthie Camden either being adopted or the product of an affair with the gardener, because Mackenzie Rosman had a noticeably darker complexion and less WASPy features than the rest of her TV family.
Dexter has Rita and ex-husband Paul, both blond. Their kids Astor and Cody have medium to dark brown hair.
Dumb Blonde Kim from The Parkers is significantly shorter than both her parents (Countess Vaughn is 4'11, Monique is 5'9, Tommy Ford is 6'4). Lampshaded by an Enfante Terrible in one episode who, when told by Kim that her father was also tall, responded with Mama's Baby, Papa's Maybe. Kim is also much lighter in complexion than both parents, but due to a great deal of African-Americans having recent white ancestry, this is not as implausible.
Similar to the above, Sondra and Denise Huxtable, the oldest two daughters from The Cosby Show are much lighter than both parents (both of their actresses were biracial). Cliff's father is also very light, which could justify it, but then Cliff himself is darker than both of his parents. Two children in a row with the same trait that neither parent expresses... makes one wonder what Clair was up to at her law firm.
In Dirty Sexy Money, Juliet has brown eyes while her parents are both blue-eyed.
Lampshaded on the "Hackidu" episode of Everybody Loves Raymond. Ray and Debra have dark hair, while the kids have blond hair. Ray trades Ally's $65 "Hackidu" card to Bill Parker, a blond guy. After getting teased for being scammed, Ray decided to get the card back.
Ray: (to Debra) I'm going to go see your boyfriend. (leaves) Marie: Boyfriend? Robert: You know, this could explain why all the kids are blond. Debra: And smart.
This was an early concern during casting of Family Ties: Michael J. Fox almost didn't get the role of Alex P. Keaton because the producers at NBC didn't think it was realistic that the 5'4" Fox could be the son of 6'4" Michael Gross and 5'7" Meredith Baxter.
While it's not really viewed this way by viewers, Meredith Baxter said in an interview that she was surprised to see someone as beautiful, dark and kind of exotic as Justine Bateman being cast with such a white-bread looking family.
Seemingly averted, but actually played straight in an episode of House. A patient in an early episode found out that he was adopted because, as he claims, parents without cleft chins can't have a child with one, and the doctors agree with this claim. In fact, such a parent-child scenario is possible— just unlikely. Based solely on that trait, the patient shouldn't have been able to tell either way if he was adopted, and Foreman (correctly) explains that while it's unlikely for him to be his parent's child, it's not impossible.
Orphan Black: In the third episode, Cosima tells Sarah that clones' fingerprints may be close enough to all match when in reality not even identical twins have the same fingerprints. However this may be a case of Artistic License since it's still early in the series.
Even accepting this premise leaves a Plot Hole, unfortunately. Beth's finger prints would have been put into the system when she became a police officer. If Sarah's arrest happened first, Beth would have had quite some difficulty getting into the police at all. If it happened second, the whole mess should have exploded then. Since only Sarah's fingerprints came up when Katja's were run, it must be that Beth's aren't in the system - presumably the showmakers don't know that police employees' fingerprints go into the system.
In Reba, the redheaded title character and her blond ex-husband Brock have blonde Cheyenne, redheaded Kyra, and... brown-haired Jake? In one episode it's stated that Brock dyes his hair, but it's not made clear if blond was his original color or not.
Roots's Chicken George is the son is Kunta Kinte's daughter Kizzy and her white slave master; he is one of the darkest characters in the series, significantly more so than his mother (and looks about the same age, though that's due to a different trope).
On Smallville, half-Dutch, half-Chinese Kristen Kreuk was cast as Lana Lang. Oddly, unlike their version of Pete Ross, there was no Race Lift involved—both her mother and father were portrayed by Caucasian actors. Potentially played with after the reveal that Lewis Lang isn't her biological father but then when she finds her bio-dad, he turns out to be white, too. Many viewers at first sight assume the What The Hell, Casting Agency? was involved.
Stefan and Damon sort of (if you squint) resemble their father, but the two brothers don't resemble each other at all.
Elena looks very different from her brother and aunt, but then it's revealed that she was adopted. Though it's later played straight when her birth parents were her father's brother John Gilbert and Isobel Flemming. While you could say she shares some of their facial features, there's no way two pale-skinned light-eyed people could have produced an olive-skinned child with dark brown eyes.
Elijah bears no resemblance to his family either, having much darker features overall. Ironic since Klaus, who does match the rest of the family, is supposed to be the illegitimate child.
Lampshaded in a sadistic Halloween prank in an episode of Roseanne, where the dark-haired Roseanne and Dan had the dark-haired Darlene and D.J., but a blonde Becky (which isn't as unbelievable as some versions of this.) When their snobby neighbor Kathy Bowman came over, she found Dan lying on the kitchen table, "gutted open" with assorted pieces of raw meat and fake blood sitting on his stomach. Roseanne walks in, covered in blood and holding a dripping knife, and she casually complains that she was so much better at this the last time. When Kathy asks what "last time," Roseanne casually says, "Becky's dad, he was blond, too."
Averted strongly with the casting of D.J. While neither Sara Gilbert (Darlene), Lecy Goranson, or Sarah Chalke (Becky) bore a notable resemblance to their TV family, the main reason Roseanne Barr pushed for Michael Fishman to be cast was because he looked like he could be in her family, and even as an adult he looks like her more than her real children. He even played a younger Roseanne in flashbacks.
Gaia Online, otherwise pretty good about NPC hair colors, retconned Ian into being a vampire in '07, around the time his father Vladimir was introduced (and killed). Ian's hair has since been recolored black, which he says is his natural color. The problem? Vlad has been confirmed as having dark brown, possibly even auburn, hair, and Ian's late mother Rosalie was a blonde.
Xenogears plays with this—because both of her parents are blond (as is everyone else on the Floating Continent Solaris) and she's a redhead, Elly feels like a literal Red-Headed Stepchild to her mother, thinking that her real mother was her red-haired nanny. However, this is both plausible (the way red hair works genetically allows for blonds to be recessive for it) and a moot point anyway, since the reason she looks like she does is because she is a Reincarnation of The Anti-Type.
Played oh-so-straight in Solatorobo, where not only can vastly different races within the species breed freely (foxes with wolves and housecats with lions, for example), but Caninu and Felineko can have children with no problems as well. To top off the Hollywood Genetics, their offspring will be either Caninu or Felineko, not some sort of dog-cat hybrid.
Invoked via the musical example in Kevin & Kell, where in a production of West Habitat Story Fiona, a fox, is cast as Maria, while Rudy, a wolf, is cast as Maria's brother. Granted, Rudy does have some fox ancestry in him but he looks and identifies as a wolf. And it becomes moot when he's recast as Maria's would-be fiancÚ anyway.
In the American Dad episode "The Kidney Stays In The Picture", it is revealed that Francine slept with another man right before her and Stan's wedding, meaning Haley may not be his biological daughter. Francine is a brunette who dyes her hair blonde. Stan and Haley both have black hair. The man Francine slept with had brown hair. The one thing Haley and him have in common? A bandana. The funny thing is that many viewers would have thought their son Steve would have a different father, due to him having brown hair, but even if Francine was a natural blonde, such an outcome is far from unlikely.
In Batman Beyond, Terry's mother was a redhead and his dad was blond, but Terry and his little brother had black hair. In the Distant Finale which was also a Fully Absorbed Finale it was revealed that Bruce Wayne is Terry's biological father, fixing the whole mess. Apparently an Author's Saving Throw as a solution to people noting its impossibility (It also helps that Terry's parents already had marital issues that suspected infidelity could have contributed to). Note that it never occurred to anyone to simply suggest that his mother dyes her hair...
From Danny Phantom Sam Manson has black hair while her father and mother have blond hair and red hair respectively. Someone in the family probably dyes their hair. Given the fact that Sam's a Goth, it's most likely her.
Family Guy has brown-haired Fat IdiotBumbling Dad Peter and red-haired Lois. Their children are brown-haired Butt Monkey Meg and Brainy Baby Stewie (Stewie Griffin: the Untold Story confirms this) and blond Chris. In and of itself, this is reasonable; Peter could have a recessive blond gene (which will dominate the red). However then we get Bertram, the kids' half-brother on their father's side. He has red hair, even as a sperm cell. Peter's biological father is red-headed, so he is definitely is carrying the genes for red hair for Bertram to inherit, but it's very unlikely that he's masking blond hair, too. The fact that Lois is redheaded means that she most likely only has genes for red hair, so barring a mutation, it can't come from her, either.
In The Incredibles, Mr. Incredible is blond and Elastigirl is a redhead. Dash has blond hair, Violet has jet-black hair, and Jack-Jack's is strawberry blond/red. Dash's and Jack-Jack's hair colors are plausible. Violet's hair color is not, unless she has some sort of science-defying mutant superhero hair gene (perhaps the gene was not dominant because it's invisible?) or (more likely) hair dye was involved.
On The Jetsons, George and Jane Jetson are both redheads, yet their children Judy and Elroy are both blonde (Judy's being platinum, to be specific).
In the prequel to Disney's The Little Mermaid, both King Triton and his wife have red hair; this means their children can only have red hair, but Ariel is the only redhead among the seven princesses. Either merpeople's dominant and recessive genes are different from humans', or...
It makes even less sense in the Broadway stage production. Where Triton is black, and his daughters are all Asian, black, and white with a rainbow of various hair colors.
What really drives the trope home: Mrs. Cake's pegasus ancestor was her great-aunt's second cousin twice-removed. Mrs. Cake does not actually share any DNA with this pegasus, yet she somehow passed it on to her daughter.
In South Park, Kenny is blond, his parents have brown and red hair. The reason could be some people have blond hair as children and light brown hair as adults, but Kenny's little sister Karen already has brown hair.