IT probably looked like a good idea. The Big Bang Theory, the long-running sit-com about a group of geeky friends, has been a big success, so let’s have more success with a spin-off about one of the characters as a young boy. But take one look at the history of television spin-offs and the advice is clear: don’t do it.

The plan for Young Sheldon, as the Big Bang spin-off will be called, is that it will follow Sheldon Cooper as he grows up in Texas, a brainy boy interested in physics with a mother who’s a fundamental Christian who believes physics is the work of the Devil. There’s certainly comedy potential there and it will be narrated by Jim Parsons, who plays the adult Sheldon.

However, by the looks of the early outlines of the show, it has fallen into the old, old trap of spin-offs and has done what many failed spin-offs have done in the past which is to take the original and just add a little water. The same original idea is still there but it has been diluted just a bit to make it go further while pretending to viewers that the taste is still the same. But if a spin-off is to succeed, it must made a big break from the original show, it must divorce its parent.

Look at the very few television spin-offs that have worked and you can see what I mean. For example, Mork and Mindy, the 1970s sitcom about an alien living on earth which was the launch pad for the extraordinary, eccentric talents of Robin Williams. It came from Happy Days, a show about some high school kids set in the 1950s.

Frasier is the same. The long-running sitcom was about a psychiatrist and aesthete and his pernickety, obsessive brother, both of whom loved German opera and fine wine. It came from a show about some drop-outs who meet in a low-brow bar in Boston.

There are a few other examples – most recently, Better Call Saul, which followed a low-rent lawyer and told a very different story from the show that inspired it, Breaking Bad. There are also shows which have kept spinning off over and over again. Z Cars, you may remember, led to Softly, Softly which led to Softly, Softly: Taskforce which led to Jack the Ripper. These weren’t just spin-offs, they were spin-spin-offs, or spin-off-offs: shows that seemed to defy the old principle that you can have too much of a good cop show.

Almost every other spin-off you care to think of has wobbled and crashed. Just look at the list. Going Straight, the spin-off from Porridge. No thanks. Joey from Friends. Horrible. The New Avengers from The Avengers, Grace and Favour from Are You Being Served?, Class from Doctor Who, Holby City from Casualty, the Girl from UNCLE from The Man. Make it stop.

All of those shows failed because they were afraid of swimming too far away from the mother ship. Let’s add "New" to Avengers, or "Girl" to UNCLE and we’ll have another success. In fact, you have to be different to keep the success going. You also have to hide the fact that a spin-off is usually the sign that the original show is starting to flounder. Young Sheldon will go ahead come what may, but it can’t hide the sound of ticking on The Big Bang Theory. For any television show, a spin-off is the first sign of a shut-down.