Yeah, what’s the deal with that?! Where’s the beef?!
Two mixed-race parents (like, say, the significant majority of African-Americans) can have children of almost any color, ranging from much darker to either parent to much lighter than either. So there wasn’t necessarily anything odd about the Huxtable family.
In my elementary school’s production of Pinocchio, I played the “Goody-Goody Fairy”, who was the daughter of the Good Fairy. My character appeared first, making many references to how my mother was coming and wouldn’t be pleased with Pinocchio’s actions.
When the Good Fairy did show up, there was hysterical laughter from the audience, which I did not understand at the time. She was played by an African-American, whereas I am Caucasian.
I remember reading in Reader’s Digest about another school play, Goldilocks and the 3 Bears, in which the adult bears were wearing brown costumes, while Baby Bear was wearing a white one. When Papa Bear said, “Who’s been sleeping in my bed?” an audience member said, “Well might he ask.”
Various Dorians who showed up on Scrubs were played by actors who looked enough like Zach Braff for TV purposes.
In “The Great Muppet Caper,” Fozzie Bear and Kermit the Frog played twin brothers. Made sense when you saw a framed photo of their “parents”. 
On a more serious note…yes, this varies from show to show, but certainly there was a time when, on average, film directors (somewhat less so TV ones) started paying more attention to rather more plausible family resemblances. I’d put the shift around 1975-1980.
Actors playing the same role at different ages is a related but different topic. For obvious reasons, on average more effort has gone into making these plausible. But this, too, has seen a shift in recent decades, to the point where any lack of attempt to do this starts to be bothersome (unfortunately, I’d say – no need to confuse acting with documentaries.)
Shawn Spencer looked like two different kids entirely! Indeed, Gus looked like three!
Well in Doctor Who, the younger Amy Pond was played by her actresses cousin. Ralph Finnes nephew played the younger Lord Voldemort. But, this is one area I agree that the casting directors misfire, persons can quite plausibly look different at different ages for reasons other than simple aging. I remember listening to a commentary of some movie and the actor says that the photos of his character as a child are of his nephew, since actual pictures of the actor at that age showed him looking too different to have plausibly grown into what his character looked.
Well, heck, doesn’t just about every teen movie feature the awkward bespectacled girl growing into a supermodel? Sometimes the roles are even played by the same actress!
They lampshaded that one; Shawn has a dream where he meets his younger self, and makes some comment that the kid looks nothing like him. The kid responds that he often doesn’t look the same from week to week (or words to that effect). He also looks nothng like Corbin Bernsen (dad) or Cybill Shepherd (mom, who they named Maddie).
As for the Huxtables, she could have had the eldest first and then married Cliff. In the first episode, he asks her why they had 4 children and she replies, “because we did not want 5.” Voila, #5 shows up later.
The Jeffersons: white guy marries black woman. They have a completely white daughter and completely black son. I don’t think that’s how it works.
Also, I think the baby Harry Potter in film 1 grows up to play Harry Potter’s son eventually.
Just for the heck of it, two examples of doing it right:
Long Riders: stars three Carradine brothers as the Younger brothers; brothers Christopher and Nicolas Guest as the Ford brothers; Stacy and James Keach as the James brothers; Randy and Dennis Quaid as the Miller brothers.
Aliens: Ripley looks at a photo of her daughter (who died of old age while Ripley was suspended for 57 years); the photo is actually Sigourney Weaver’s mother. A neat little trick.
I had the opposite (same?) experience. During Six Feet Under, I didn’t think they looked at all alike, but the first time I saw a promo for Dexter (played by Michael C. Hall), I thought it was Peter Krause.
Yeah, WTF. That used to bug me.
Honestly, I’m all for casting agents just going “Ah, screw it” and casting whoever they feel is the best actor.
Louis C.K. casted a black woman as the mother of the very white actresses playing his daughters. You just get over it.
Some mixed people come out looking entirely white. One example is actress Jennifer Beals, who some folks STILL can’t believe is mixed black and white. My criteria for “white looking”, btw, is that if this was the 19th century Deep South, and this person tried to pass for white, could they do so.
I pointed out the implausibility of that casting in a previous similar threat. Tracy was 57 and looked 70 while Wagner was 26 and looked 19. It would’ve been more believable if he played his grandson.
I’m not sure if they ever addressed the age gap between the two in the movie and said something like Wagner’s character was the Tracy character’s half-brother.
They are, however, both enormous.
Well, Lucia di Lammermoor is supposed to be set in Scotland. That’s not at all unusual in opera* (singing in a different language than the setting implies, not being set in Scotland). It’s not particularly uncommon in other dramatic forms either. How many American movies set in foreign countries actually use the native language?
*(The most amusing example of this is Madama Butterfly in which the American soldier and Japanese geisha converse in fluent Italian.)
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The Sons Of Katie Elder has John Wayne and Dean Martin as brothers, and another brother who was played by Michael Anderson Jr, who was over 35 years younger than Wayne. And looked nothing like him or Martin!
I remember being amused at Joey’s herd of nearly identical sisters in Friends. So that casting agent, at least, paid some attention.
Another impressive child / adult combo was the young Snow White in the recent Once Upon A Time. The young actress nailed her performace, which helped.
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Real-life families don’t necessarily resemble each other that strongly. In Donnie Darko I always thought Donnie (Jake Gyllenhaal) looked a lot more like the actress playing his mother (Mary McDonnell) than the actress playing his sister (Maggie Gyllenhaal).