So what if William's first child has some kind of mental disability?

So what happens if William’s first born child has some sort of mental disability- let’s say Down’s Syndrome, and isn’t able to rule?

Are they King/Queen, and there would always be a Regent to do the Royal stuff, with succession going down the line of a later child or cousin after death, or is there some provision to disqualify mentally disabled heirs?

How does being mentally disabled prevent one from being royalty?

Serious question from an American. Don’t royals mostly just be royal?
Seems like an ideal- and endearing- gig for a guy with Downs, they tend to be cheery and outgoing.

A good and tricky matter really, probably only resolveable when it actually happens as it would depend on social values of the time.

At present the Regency Acts allow for the reigning monarch’s prerogatives to be exercised by a Regent or Council of State, so the actual machinery of government would still function broadly as before. There’s nothing to stop the monarch themselves still undertaking public functions if they so chose.

I have a downs syndrome sister, she’s lovely.

I ignore the Windsors; but I believe there was a prince in their family in the 20th century who had a disability and died young. Had he been in succession, there would have been a regency; just as with Otto I of Bavaria who was the brother of Ludwig II, who was mentally unbalanced — charmingly called ‘Mad’ by the gibbering loonies who run the press — was secluded, did no harm, and lived a quarter century as King with a cousin as Regent. After then he was deposed on the ground that after 10 years of lunacy with no hope of remission, the next in line might as well be King. I disagree with that, considering there would have been no harm in letting him continue for another 3 years, but it’s doubtful he knew anything about it.

A couple of the 19th century Hapsburgs were mentally ill ( sigh not due to interbreeding ) and had they succeeded much the same thing would have happened. Franz Josef had no mental issues; and really, nearly every other family on the planet has unfortunates.
Nowadays blame only attaches to royalty, which is due to envy: in the old days quite ordinary families felt shame for being related to the mentally ill, locked them up, and never spoke of them again.

So does the current Queen take part in the actual governing?
Does she have veto power? Tiebreaking vote? Or is it all symbolic in nature?
(I know she’s part of the required ceremony of government.)

IOW, what would a regent do that Malden Capell’s sister wouldn’t?

Generally the Regent would do everything else; appointing PM and ministers, judges, ambassadors, signing Bills into law, C-in-C of the Armed Forces, dispense honours and peerages, and exercise of the Royal Prerogative.

Of course, this is all done anyway on the advice (read: instruction, most of the time) of the government, but there still has to be a signature on the page.

For Elizabeth, “Queen” is not merely her title; it’s her job description.

It would lead to overseas possessions declaring independence.

But would they keep using Loonies as currency?

Why would they? They don’t now, having thrown off the yoke of a mad king’s oppression in the 18th century.

The queen signs bills into law, appoints the Prime Minister and cabinet, etc - much like a governor or president. The difference is the executive is really the prime minister, who is elected. The Queen is much of a figurehead. She basically has a nuclear option - she can refuse to do what the government tells her to do, and then public opinion may back her (bye bye Prime Minister at the next election) or may back the elected government, and bye bye queenie. So it’s a massive game of chicken, don’t piss off the queen unless you are sure you have massive backing or you alienate a huge block of voters. In return, the queen does not stick her nose into petty politics, she won’t opine (publicly) on the advisability of a tax hike or new law unless it is abhorrent to a large segment of the public.

Perhaps you are thinking of Prince John, 1905-1919 Prince John of the United Kingdom - Wikipedia - this article says he had epilepsy, but IIRC some other commentaries I read suggested he had other brain problems too, he was a bit simple.

The article says he was a normal boy, but Edward says in private the kid was closer to an animal - harsh or realistic?

T’will be a grand day when an honest, well-beloved by all, respected, courteous, un-yoblike, soft-spoken, Australian politician who’s not a slithery little conformist suit-wearing wowser can unite and represent all the Australian people and parties.
What a pity Sir Joh’s no longer available.

Prince John

The British Government has proved itself decidedly pragmatic over the past few hundred years: Edward VIII, the Regency, and the Glorious Revolution are the three most prominent examples.

Two first cousins of the current queen, Katherine and Nerissa Bowes-Lyon, were sent off to an “Asylum for Mental Defectives”.

So, the Queen ‘can’ exercise control over all of that?
Appoint her own PM/judges/etc/ without limit or review?
The Prime Minister isn’t whoever the people elect?

And what is the Royal Prerogative that gets exercised?

In practice the Queen always appoints a government capable of commanding a majority of the Commons, which means the leader of the largest party - in other words, the people’s choice. And judges, ambassadors and so on are appointed on the advice of the Government.

So while she generally does these without protest, she is still the one whose signature gives the actions legal enactment, and she is expected to be fully informed of the decisions and appointments the Government are proposing to do in her name. She will be asking questions, inquiring and making her views known.

Royal Prerogative in the United Kingdom

Isn’t that why you have towers in your castles?

When has the Queen opined publicly about an unpopular law? Wouldn’t that have the danger of causing a constitutional crisis?

Exactly. It is in fact the “nuclear option”. She’d better be really really right and the cause better be really really serious, or the government woul remove he right to object - either pass the crown to someone else, or remove the royal role.

There’s no productive reason for the queen to insinuate herself into public affairs - say like the abortion debate or Trayvon case or Obamacare in the USA. She has no authority to do anything, she does not control any ministries or introduce laws, so her only weapon is public opinion. If the counry is fairly divided, all she will do is get the one half truly mad at her.

Still, the monarchy, like the president of Israel or Italy or similar parliamentary democracies, has a veto power over laws and selection of the government, calling elections, etc. She just better not use it for anything less than a major emergency, so she is one last backstop against a totally out of control government. Since they know this, most governments heed the small input she does have on some issues. (She apparently meets with the prime minister reularly to go over what’s happening in government.)