Sherlock writer Steven Moffat has joked that star Benedict Cumberbatch's real-life parents were cast in the show to continue the drama's tradition of "nepotism".
Viewers saw Timothy Carlton and Wanda Ventham make guest appearances in the New Year's Day episode, The Empty Hearse, playing Sherlock Holmes's family.
And show boss Moffat - who writes the show with Mark Gatiss - laughed off the casting by saying that the production team did not like to go "outside our tiny circle".
In an interview on Radio 2's breakfast show, he told presenter Chris Evans: " Well Sherlock largely runs on nepotism. I'm married to the producer, one of the execs is my mother in law, Mark and I are old, old friends, Martin (Freeman, who plays Watson) is working with his partner Amanda Abbington.
"So generally speaking we just don't like to go outside our tiny circle very much."
He went on: "The absolute fact is that once we came up with the idea, wouldn't it be funny for the first time ever in any Sherlock Holmes film if you could see mummy and daddy Sherlock, the fact is his parents are both actors, really good actors, so it was irresistible.
"And that's the only gene combination that could results in anything as extraordinary as Benedict Cumberbatch - you wouldn't believe anyone else as his parents, so we had to cast them."
He confirmed he hoped to make further stories for the popular BBC1 show but said it was a case of fitting in with everyone's diaries.
"Well, apparently Benedict and Martin are quite popular in the movies these days so it's quite difficult to schedule around them. And obviously Mark and I have our other commitments too, but it's just a matter of scheduling. We're all keen to continue."
Moffat was dismissive of critics who carped about apparent holes in the programmes' plots, pointing out the shows were written for sophisticated audiences who had to fill in the pieces themselves.
And he added: "It is of course made up - it's not real. That isn't even really Baker Street . And you know Benedict Cumberbatch, he doesn't solve crimes at all. I've checked repeatedly with his agent."