Here is a photo of a sheltie/cocker cross 
How many breedings would it take for this to look like todays Cocker? 

NOT MANY..

Dear Professor Strain, 
 I have been reading your letter concerning the merle gene in Chihuahuas.It was your opinion that the merle gene was introduced by another breed. In my research, I have read every available resource on color genetics in canines. I am a professional breeder of American Cocker Spaniels and my breed seems  to be undergoing the same situation- the appearance of a merle Cocker in  1981. I have gone over many merle 
Cocker pedigrees and they all go back to a dog named Rusty Butch, registered as a buff. Both sire and dam are buffs, as are the 5 preceding generations (as far back as I went but could  go further). It is the opinion of the merle breeders that this is a gene mutation.  I opine a dapple Dachshund in the mix a much more likely scenario than a  gene mutation. After all, there is no record of any merle in AKC history  since the first one registered in 1857.  My concern has nothing to do with color because we already have a firestorm with the sables, but a health issue since AKC does not have a pattern listing for merle, only roan. 
I have run across several breeders that can't tell the difference between a merle and a roan and do merle to merle breedings. There have been 
instances of sudden death at 2 to 3 years of age in several merles. Perhaps related to the merle gene but perhaps not, I'm not sure in addition to the usual double merle issues. 
 One thing I AM sure of, our breed is already is a serious health crisis without adding merle into the gene pool. No other Spaniel breed carries a merle gene. Could there be any other explanation other than a cross with another breed that does carry merle? I would like to have all the facts 
available to make a proposal to ASC, our National breed club, at least to have a "Z" list for the merle pattern for the future of our breed. 
Any insight you could give me would be greatly appreciated by the entire Cocker fancy that is dedicated to the health and well-being of the 
breed we love dearly. 

HIS ANSWER:
I don't know of any way for merle to show up except for breeding in from some other breed that carries this gene. 
We don't yet know the location of the gene for merle in the canine genome. 
It is very remotely possible that a gene mutation would produce the same color pattern gene responsible for merle in other dog breeds, but I would be highly suspicious of this until proven otherwise.
 George M Strain
_________________________________
 George M. Strain
 Professor of Neuroscience
 Comparative Biomedical Sciences
 Louisiana State University School 
of Veterinary Medicine
Baton Rouge, LA 70803
Voice 225-578-9758  Fax 225-578-9895
 Research: www.lsu.edu/deafness/deaf.htm

The reason I got "involved" was to find out the "background" on the merle cocker. Believe me I think the color is pretty (although the blue eyes are kind of spooky). 
However, I am one that believes that for me to support the gene that it must be proven that it is a valid gene in spaniels. 
That is why I was asking for all the pedigrees. To see if we could trace it back, knowing all the resources we had. 

The sable supporters went back years and had a genetics guru on our side and ASC would still not accept the sable. Right now we have no proof that the gene is any older than the 1980's. And I still don't have a valid reason to breed a color with a gene that has problems. 

Yes, YOU, might be a responsible breeder. But honestly, tell me, how many people breeding merles out there are? 
How many of them test their dogs. 
How many sell with limited registration. 
How many of them care if the puppy they sell is a double dilute?  How many of them will care about anything except the extra money they are going to get for the color. 
 And believe me, you cannot TEACH these people to be responsible breeders....it doesn't happen. 
 Why should they when they can get $1000 for a merle puppy, and a black puppy will maybe bring them $250? 
 25% problems plus or minus is a high number of puppies when dealing with people who don't care. 

And it's not like AKC is accepting the merle color. They are accepting the fact that the dogs are being registered as roans. 
I don't think they know or care about the difference. 
 Only that they are getting more money for more registrations. 

I just recently found out that a dog I sold to a good show/breeder person was bred to a very nice bitch. 
They produced a very nice litter. One of the pups of that litter was sold to a "supposedly" good show/breeder couple. 
That male dog is now the father of a "merle" litter. 
So my bloodline will now be in Merle pedigrees. 
This is not something I, nor the breeder I originally sold the dog to would ever want. 
She sold the pup in good conscience to a show/breeder that was going to show him...then they got "bit" by the "color" bug. 
It shows that you never know where you bloodlines can end up. 

After much research..every merle dog pedigree I have traced back to one dog. Rusty Butch..a buff. 
No matter what bitch he is bred to he produces a blue merle. Now in reading genetics..this is what I find.. 

In order for an offspring to be a merle, even if hidden, ONE OF ITS PARENTS MUST BE A MERLE. 
The merle gene cannot sit dormant for multiple generations and then express itself. It is dominant. ONLY RECESSIVES can remain "unseen" for 
multiple generations and then be expressed suddenly. 

I've also included the pedigrees of dogs behind "Rusty Butch", they show a fairly dominant buff gene pool, no merles, or even parti's or roans. 

Most of the bitches that were bred to Rusty Butch were also buffs. 

So I think someone might need to talk to the breeder of Rusty Butch...I think an Australian Shepard might be in the works somewhere..it would only take two generations of breeding to have a dog that looked like a cocker again. 

Now I agree that after 5 generations they are considered pure bred ...however...this to me is proof that merle is not a gene that has been with the 
American cocker spaniel for a long time, unlike the sable gene which was present long before the split of the two breeds.  This gene looks like it showed up in the 80's. 

Another term I have to say I dislike is rare color. This color in my opinion is not rare. Does rare mean we don't see it often or does it mean we chose not to breed this color? It is definitely different in any way we look at it but not rare. I have been to many kennels over many years from the state of the art facilities to serious puppy mill horrors. And my view is that many people who chose to bred these colors due so financial gain. There are some that are curious about the color and want to do some of what I call investigative breeding. But the majority want something that is not main stream to make a profit off the backs of the animals. If we continue to call them rare they will continue to have a higher price tag for a less quality dog as I have seen many times. If they were any other commonly seen color such as buff or black with the same conformation you could not give them away! 

I am a hobby breeder of Parti and ASCOB cockers mostly show.. I dont just breed for pets. I dont breed to simply have a pup for sale. I don't use my boys at public stud and if I did  let someone twist my arm...the bitch must be CERF'd OFA'd and I must see the pedigree and have knowledge of some of the dogs in the first few generations or FORGET IT. We are responsible for what WE do.....why breed your dog to another dog just to get dogs..arent there enough dogs out there? 
I am against the merle color even being attempted to be bred in our breed. I am and will be very opinionated on this forever. As another person questioned why it is so bad if other breeds have the color... it's because we never have and it CAN be the downfall of ANY breed if not registered correctly and then bred correctly. 
What a lot of people dont seem to recognize is ... there is a standard to our breed and it was set by those who actually DID the Breedings that eventually became the type and colors we see today.... I have respect for these breeders and their breedings and for MY breed. 
I know a few geneticists and I have worked for vets off and on for years, and currently do work at a veterinarian hospital. 
This oh well they are here attitude concerns me greatly... and those who have it... In reality I wish didnt own this breed!!!!!!!!! 
The American Cocker will eventually will be affected by this there is not doubt about it.... 
I'm just one person trying to do the part ...that a whole lot of others themselves should do. 
.. Lets all just realize... pedigrees are basically as good as the breeders integrity..... I think that will explain  why there is not much info on the internet about merles.... there was NO such thing.... it was created by (my opinion and many others)... a herding dog being in the HITCH.... 
My Honest opinion