Mum and dad yellow they have 5 yellow pups and 1 black one. Is this unusual?

A yellow x yellow should not produce a black.  Is there any way another 'Daddy' has managed to get to the Mum??

Or perhaps mum and dad are not as described or not pure themselves, but agree that it is likely there are two studs used here wonder if they will be registered with the KC and if they are if both studs will be registered. Will need to do a DNA test to tell which pups belong to which sire.

Just wondering, do the KC notice things like this, I mean if the litter is registered as from two yellows will it be refused because it is not possible?

Purdey's last litter :  she was a black lab, all her line were black.   She was mated to another black lab - result : 11 puppies, 10 yellow, one black (with white locket!) !

Both pedigrees absolutely genuine, with absolutely no chance of another suitor - just where down the lines the yellow came from, heavens only knows!

Would seem there was yellow on both sides to achieve a litter like that. Think this one is for those who study colour genetics.

This is a helpful page. It seems that two yellows cannot produce a black, although two blacks can produce yellows.

That can happen with blacks, who can carry other colours. Yellows carry others two, but when put to another yellow only yellows should be produced because they only have 'yellow copies' of a particular gene, which overrides all other colors

Just wondering, do the KC notice things like this, I mean if the litter is registered as from two yellows will it be refused because it is not possible?

Good question. With cats, the GCCF's computer would instantly say it's impossible and so registration would be refused.

It is all down to dominant (black) and recessive (yellow). This website helps you understand BUT it does say that yellow and yellow producing black can only be due to a mismate.
http://www.oakhillkennel.com/library/color.html

I have a feeling the KC a few years ago have put into place picking up on impossible results.

Now if we had an open registration database with real time results, people could point such things out to KC in plenty of time, ditto false sires.

On the breeds page under breeding restrictions it mentions not registering impossible colours with yellow to yellow only yellow, and choc to choc only choc and yellow. So they should have something that checks it.
http://www.the-kennel-club.org.uk/services/public/breed/restrictions.aspx?id=2048

Wonder if they would insist on dna testing the whole litter as surely the black pup has a diffrent farther and if he carried yellow some or even all of the yellow pups could also be from him rather than the yellow male. Or would they just register the yellows from the yellow and refuse the black pup if sire unknown.

I think the whole litter would have to be DNA'd

May be the OP will came back and explain the situation of this litter - sure I have read that this poster was interested in GSDs so may be this is just an advert they have seen and wondered about and not a personal experience.

Mum and dad yellow they have 5 yellow pups and 1 black one. Is this unusual?

Not unusual.... impossible!
The KC will not register such a litter.  I have heard of a couple of cases, but on further investigation, it turned out that a different stud dog was used than the one thought.

Yes I was looking for a GSD but after taking advice to widen my choice, for example rescue, I have been trawling the internet.  I was shocked and amazed at what I have found. This black lab was returned to breeder as 'buyer was ill'. All yellow pups except the one returned. 
Also there was a GSD puppy, with another seller who bought it on impossible.  She has 6 children and another dog and can't cope.  Breeder won't take it back.  Not innoculated but is 13 wks. I would have taken a chance had it been a girl.

Thanks for the explanation, seems that it is a good example why shopping on the internet for a puppy is not a good idea and should only be used for searching out a possible breeder.

I was shocked and amazed at what I have found. This black lab was returned to breeder as 'buyer was ill'. All yellow pups except the one returned. 
Also there was a GSD puppy, with another seller who bought it on impossible.  She has 6 children and another dog and can't cope.  Breeder won't take it back.  Not innoculated but is 13 wks. I would have taken a chance had it been a girl.

Walk away and go to a reputable breeder. What breeder would sell to someone with 6 children and another dog. The mind boggles