In the early 20th century, Francis Crookshank, a Fellow of the Royal College of Physicians, achieved a degree of prominence through his written work on medicine, psychology, philosophy and history. His first book bore the surreal title, Flatulence and Shock.

But he was best known for a book called The Mongol in our Midst, published in 1924, in which he wrote: “Mongolian imbeciles are usually the offspring of feeble, immature or exhausted parents… Sometimes there is a clear history of maternal ill-health, debility or privation. Sometimes there is parental syphilis.”

Crookshank’s words are quoted in a new book, A Major Adjustment, by biographer and comedy writer Andy Merriman, to demonstrate how harmful the ignorant or malicious misuse of language can be — especially by “experts”. Which is just one of the many enlightening points Merriman makes in this follow-up to his 1999 publication, A Minor Adjustment.

That was written to reflect the first few years in the life of Andy’s and his wife Alison’s daughter Sarah, who has Down’s Syndrome, the term that has come to be used over the past half-century in place of the thoughtlessly insulting “Mongol” or “Mongolism”. (Happily, the appalling word “imbeciles” has long gone with the wind, along with the writings of flatulence guru, Crookshank.)

The Merrimans had no idea that the baby they were joyously awaiting in the new year of 1992 would have Down’s Syndrome. And so it came as a great shock. Alison, who works for Norwood, later recalled that, when she was first left alone with her newborn daughter, she “looked at her, peacefully sleeping, and felt nothing. No rush of love, of protectiveness or even pity. I picked her up mechanically and held her, to try and evoke some emotion. I felt dead inside and returned Sarah to her cot.”

The distress and helplessness that Alison and Andy experienced in those early weeks, and that threatened to affect their loving relationships with their young son Daniel, their parents and other family members, were described with complete candour in A Minor Adjustment. Andy confessed that he found himself, “looking at my daughter, not even a day old, wishing her dead.” But, seven years on from those dark days, Andy was able to describe how proud he was of his happy and lively young daughter. The book, he recalls, “caused quite a stir. Nothing had been written as honestly, certainly by a father, about a child with Down’s Syndrome.”

A Minor Adjustment is also the title of a radio series written three years earlier for the BBC by Andy with his father Eric Merriman, a well-known and successful comedy writer for the likes of Tommy Cooper, Dave Allen, Morecambe and Wise, and, as Andy wittily puts it in the new book, “others too humorous to mention”.

A comedy drama about a family adjusting to life with a child who has Down’s Syndrome, it starred Samantha Bond, Peter Davison, Clare Russell… and Sarah Merriman, as Amy, the child in question.

Since the first book came out, Andy Merriman has, he says, often been asked to write a sequel, “but it never seemed to be the right time. Sarah was at school doing much the same as everyone else and there wasn’t a great deal to tell.” But then came milestone after milestone.

After gaining the equivalent of four GCSEs at her local secondary school, Sarah moved on to Haringey Sixth-Form College, where she studied for a City & Guilds Entry-Level Certificate in Food Studies. There followed a residential stint at Foxes Academy in Minehead, Somerset, based in a seafront hotel where students learn catering and hospitality skills, from which Sarah graduated with a Level 1 NVQ.

And then, in 2015, the bright lights of fame beckoned. Sarah landed a place on a Channel 4 reality show called Kitchen Impossible, in which the eminent chef Michel Roux Jr would mentor a group of adults with learning disabilities with a view to their being found jobs in catering and hospitality. By now, Sarah was — and still is —living in the Edgware Langdon Community for Jewish people with a learning disability, where she met a young man called Leon.

So, Andy reasoned, “she had moved out of home, been in a TV series with Michel Roux and she’s got a boyfriend. Also, new tests were being brought in to predict Down’s Syndrome. So it seemed like a good time to write a sequel to A Minor Adjustment.

“I realised it wouldn’t have the same effect as the first book but so much had changed with the advent of the internet and the mutual support there among parents. And Sarah has achieved so much. Her elder brother Daniel describes her as the heartbeat of the family.”

As well as Daniel, Sarah has a younger brother, Joel. The two boys have both worked with Sarah’s friends, and, says their father, “they have volunteered wherever they’ve been, Daniel volunteered with a [Down’s Syndrome] football team. Joel, when studying in San Francisco, volunteered at the local centre. They’ve never been embarrassed by Sarah — even though it is normal to be embarrassed by your sister! Daniel is about to qualify as a human-rights lawyer and Joel wants to be a special-needs teacher. I’m sure that’s not just a coincidence.

“We still, when asked, sometimes say that we have three grown-up children, one with Down’s Syndrome. Some people go, ‘ah, shame,’ and I say, ‘no, she’s great, she’s brilliant.’ And you do often have to dispel the stereotypes — that ‘they’re all musical’ (if they heard Sarah sing, they’d quickly realise they are not all musical), or affectionate, or stubborn, and so on.

“Nowadays, people with Down’s Syndrome are much more out in the community, not shut away and institutionalised as they were in generations before Sarah.

“It is often said of people with learning disabilities that they have ‘the mental age’ of someone of nine or whatever. But Sarah’s emotional intelligence is as high as that of anyone you could meet. She spots when things aren’t quite right.

“At school, she’d always be the first to volunteer. Her raison d’être is not letting people down. When she was little and we’d be in a restaurant, say, and needed an item of cutlery, it was always Sarah who would say ‘I’ll get it’ while the boys would be shy. She wants to make people feel good. She gets it if someone isn’t quite at their best. She understands.”

Sarah’s involvement in Kitchen Impossible led to her getting a job at a smart central London hotel, where she is still employed and loving it. Andy and Alison are particularly pleased that she is able to travel on her own from the immediate vicinity of Edgware and into town.

“Her reading is good,” Andy says, “but not maths, and this does seem to be a trait of people with Down’s Syndrome. She can’t do the simplest of sums, but she can find her way around the Tube system. She’s good on the phone —she does WhatsApp and texting — but is not so good on email. She can be very funny. She says what she thinks. You know exactly where you stand. She wouldn’t pretend.”

Shortly before Sarah’s stint on Channel 4, her old college, Foxes Academy, called Andy to ask if she would agree to be a guest speaker at their graduation ceremony.

“I was able to say, ‘I’m really sorry, but she’s making a TV series’,” says Andy. “If only I could have imagined when she was born that I would one day be in a position to say such a thing. At the time, it was such an apparent disaster (a ‘friend’ asked Alison if she would be hiding Sarah’s face when they went out) but now we couldn’t imagine life without her.

“It’s ironic that, at a time when Sarah is doing so many more things and being included, that a new neo-natal test which is about to become available on the NHS will mean that fewer babies will be born with Down’s Syndrome. In Iceland, no such babies are being born.

“The expectation is that, if you have a test, you are more likely to terminate. I’m pro-choice, not anti-abortion by any means, but there is no doubt that this test has a kind of eugenic agenda.

“In 1939, Sarah’s grandmother, Ursula Wellemin, Ursula’s parents and her sister, left their home in Germany to seek refuge in the UK. They were living within sight of Sonnenstein, near Dresden, where nearly 14,000 ‘mentally retarded’ people were gassed and cremated.

“I cannot help but think what would have happened to Sarah if she had been born in Germany during that period. Would she have been part of the children’s euthanasia programme or would she have survived this only to become another Jewish victim of the ‘Final Solution’?”

‘A Major Adjustment’ is published by the Down’s Syndrome Association, to whom all royalties from sales of the book will go (www.downs-syndrome.org.uk) The radio series, ‘A Minor Adjustment’, is being repeated on BBC4Extra from May 4