Too Many People Living Under One Roof syndrome. To provide fodder for plotlines, a sitcom family must all live in the same house and be huge. Pretty much all family sitcoms suffer from this syndrome.
I Only Have a Handful of Students and I Teach Them All Subjects. Head of the Class was notorious for this, as was Welcome Back Kotter. For instance, what subject did Gabe Kotter teach? And did he teach anyone else besides the Sweat Hogs?
We Refuse to Grow Up!. Sorta related to the first syndrome, in that kids do not grow up and leave the nest. Facts of Life is the biggest victim of this syndrome, as is A Different World to a lesser degree.
Predominately women casts suffer from their own set of syndromes, as typified in Golden Girls, Designing Women, Living Single, and Girlfriends. See if you can find the characters fitting the below descriptions:
One of Us is Narcissitic, Petty, and/or Slutty syndrome. Has a revolving door of boyfriends, always talking about finding “Mr. Right”, always ready to criticize the other character’s looks. But deep down has a good heart.
*One of Us is Sweet but Stupid ** syndrome. This character usually always has a boyfriend, often someone as “slow” as she is. The other characters are protective of them, even though they are often patronizing. (Addendum: a related syndrome is “One of Us is Sweet but Not Cool” syndrome, whereby a character is weird but not really dumb. Freddie on A Different World and Lynne on [/iGirlfriends both fit this bill.)
One of Us is Too Head-strong and Independent for Her Own Good syndrome. Storylines usually revolve around them as they try to get their acts together.
**One of us Always Has the Funny One-Liners ** syndrome. They typically have their shit together and serve only to make fun of the others. Rarely do they have a storyline all of their own. (I believe Anthony fills this position on Designing Women.)
Back to general syndromes…
I Am Totally Different Now That I Have My Own Show syndrome. This happens when a character gets their own spin-off sitcom and is only vagualy like they were when they were on the original show. I can think of two examples. When Kim Parker was on Moesha, she was of average intelligence (mostly she was just a humorous sidekick of Neecy’s) but once she got her own show, she became a full-fledged idiot. When George Jefferson was on All in the Family, he was serious, prim-and-proper man and Louise Jefferson was demure and sugary-sweet. But once they moved up to the eastside, George became more of a buffoon character and Louise became more domineering.
Whitley Gilbert’s character on A Different World sorta fits this, although not quite. When Denise Huxtable was the focus of the show, Whitley was basically a shallow cartoon character serving as the show’s antagonist. Once Denise left, Whitley became the focus of the show and the writers turned her into a more 3-dimensional character.