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August 2015

harlequin merle cardigan corgi puppy
Dog Health, genetics, Responsible Breeding

Merle genetics for Cardigan Welsh Corgi breeders, 2015

It’s obvious that there is a lot of bad information out there – and a real lack of GOOD information – about breeding merles. What I am going to write is applicable to almost all merle breeds, but I am going to write here as a Cardigan breeder, knowing what I know about Cardigan pedigrees.

I want to get one thing out of the way immediately: I am, personally, very much pro-merle (as a color throughout dogdom) and I am not automatically against double-merle breedings. I think the best breeding rules are the ones that tell clubs to get the heck out of the way and let breeders do the best breedings they can.

Having said that, I believe that every breeder deserves to understand what’s going on so she can make her own decisions, not follow mine. And if genetic information is going to be shared, it has to be accurate. 

I am marking this with the year in its title so that, if it is dug up in the years to come, somebody reminds me to come refresh it with the newest information. It is, however, current as of this day, this month, and this year.

What the heck IS merle?

Merle is a mutation in the SILV gene, called PMEL more accurately. PMEL is an important gene area for all kinds of colors in many animals; different mutations in PMEL create white chickens, silver horses, and silver dun cattle, among others.

Merle works by disrupting a certain stage in the formation of melanin. That’s how it changes pigment from its solid state to either white (if the mutation is present in its homozygous state, also called “double merle”), or to an intermediate color somewhere between white and the solid color on the dog. This intermediate state creates the “blue” of a blue merle or the creamy tan of a red/chocolate merle, and also lightens brindle and red/sable. 

Every merle mutation carries with it a “tail,” a stretch of about 100 base pairs (a base pair is the linkage between T and A, or between C and G, that you learned about in junior high). That tail is fragile and often breaks. If it breaks shorter than about 65 base pairs, the cell it’s in doesn’t become merle; it stays fully colored.

Because this tail is so long, and it’s quite fragile, it breaks a LOT, and everywhere it breaks becomes a black patch. So when you see those black patches on your merle dog, they’re where, when the cell was first developing, way back when the puppy was an embryo, that tail broke off and the cell looks and acts non-merle.

What about double merle?

In a double merle dog, the action of the mutation is not affected by the non-merle gene, so you don’t see the intermediate state (the silver/grey in a black-based merle). You see a much more complete interruption of melanin formation = a mostly white dog. But the causative factor is the same; it’s an interruption of the way melanin is formed. It’s not, and never would be, a “bleaching” of existing melanin, a white “spot,” a destruction of cells, etc. 

What’s the deal with deafness? Do white ears or white skin inside the ears on a merle or double merle mean the dog is deaf?

What creates deafness in merle and double merle dogs is NOT white skin! That’s like saying that spaghetti causes sauce. You have to go back into the “recipe” to understand where BOTH are coming from. 

Melanin-producing cells don’t just create skin color. They also create certain aspects of the ear and the eye. In the ear in particular, they keep the cochlear hairs healthy. When they are malfunctioning, these hairs die and the dog is deaf.

From LSU:

“The deafness, which usually develops in the first few weeks after birth while the ear canal is still closed, usually results from the degeneration of part of the blood supply to the cochlea (the stria vascularis). The nerve cells of the cochlea subsequently die and permanent deafness results. The cause of the vascular degeneration is not known, but appears to be associated with the absence of pigment producing cells (melanocytes) in the blood vessels. All of the function of these cells are not known, but one role is to maintain high potassium concentrations in the fluid (endolymph) surrounding the hair cells of the cochlea; these pigment cells are critical for survival of the stria. “

You can have COMPLETELY WHITE SKIN, including surrounding and inside the ear, and entirely normal hearing – because hearing does not come from skin cells. It comes from the cells that feed the cochlear hairs, and those are not skin cells. 

Conversely, you can have absolutely black ears, black hair and skin, and no hearing, because (again) hearing does not come from skin cells.

The fact that skin color is not the same as functional hearing is also why a small proportion of NORMAL merles (single merles, not double) have pigment-related hearing loss. If the merle gene affects enough of the cells that eventually become the ones that feed cochlear hairs, those hairs can be starved and die. Thankfully, it’s pretty rare. Single-merle-related deafness is pretty much invisible in a merle breed, because it’s almost always unilateral and so the dog functions normally. But we shouldn’t pretend that we never have deaf single merles; the evidence is entirely against us.

What creates heavily marked merles versus lightly marked ones?

This is not something that is known for sure, but the working hypothesis is that lightly marked merles have longer and/or more stable tails, and heavily marked merles have shorter and/or more unstable tails. The more often the tail breaks off below that magical 65 base pair length, the more spots the dog will have.

I would also hypothesize  – and this is my personal belief, not backed up by research – that the timing of the tail breakage has something to do with it. The cells that will create the skin and hair of the dog start off smaller in number and then multiply, of course, as do all embryonic cells. If the tail breaks when there are relatively few cells, meaning that the broken-tail cell goes on to make many billions of eventual adult cells, that might create the very large patches we see. Tails that break later in the process would create small patches. But, again, that’s my personal guess.

What about cryptic merles?

Well, that depends on what cryptic means to you.

If cryptic means that the dog IS merle but it’s very heavily patched with black, so heavily that the merle is visible only on, say, a cheek or on a bit of one leg – that’s just an extremely heavily marked merle. So you’d go back to the above explanation, where a lot of those tails broke off during the development of the puppy.

There are two other, less commonly used by breeders, definitions of cryptic merle, which deserve to be explained. But you should not take my explaining them as a reason to think that they’re common or they’re going to happen to you. They are phenomena that you are very unlikely to see in your lifetime with Cardigans.

The genuine, real cryptic merle is called Mc (for merle, cryptic) is a normal merle gene minus much of its tail. It occurs spontaneously in multiple breeds when the tail breaks off the merle gene very, very early, even in the sperm or egg cell that eventually goes on to make the puppy. These dogs will be entirely solid, without a hint of merle, but still have the mutated PMEL that means they are merle.

Is Mc the “bogeyman” we’ve all been so afraid of, where using one will result in unforseen double merles? No. An Mc dog will produce like a non-merle in every way, including when bred to a regular merle. A M/Mc puppy may have some additional white, or its color may be a little funky, but often it’s completely normal looking. And M/Mc is not associated with deafness or other issues.

Can Mc revert to a long tail and become merle again? I’ve seen it mentioned in the literature as a possibility, but I think it is a guess by researchers. I don’t believe a reversion has ever been recorded in the literature (if you know of a case study please show me so I can fix this).

Why are some merles so brown and others are light powder blue?

There are a few things going on that can explain this well-known phenomenon.

First, a dog who would have been quite red if it was a non-merle – the tris that have extremely red undercoat and red shafts to their black hairs, or the brindle-pointed blacks that have a lot of brindle visible in the hair – will have very red-tinged merle. The red/yellow pigment – the phaemelanin – is not affected by merle as much as the black pigment. So red will survive the merleing process and look very obvious on the resulting merle dog.

Second, the merle gene itself – and here I am not talking about the tail alone – is an odd kind of mutation called a transposable element, or retrotransposon. It’s not a stolid, predictable, old mutation like “dominant black,” which always does the same thing. It’s a young, rebellious, unpredictable mutation that changes, mutating within itself, creating new and different versions all the time. Those different versions create various shades and effects of merle, from very heavy and muddy all the way to so light that the color between the black patches is actually gone and the dog is white and black instead of silver and black. Some secondary mutations create dogs that are simply grey, no black at all. Others place a patchwork of colors and white that is so striking that it gets its own name, tweed. So when you see a particularly odd merle, especially if it is visibly passed along from parent to puppy, you can often blame one of those secondary mutations.

(By the way, I have seen all of the above – the no-spot, white, and tweed merles – in Cardigans.)

What about “harlequins”?

There are two mechanisms that create harlequin, which is a merle with very little or no color between the black patches. Harlequins in Great Danes are created by the combination of a merle mutation and a completely separate mutation, called H for harlequin. When H is present in a non-merle, it’s invisible, but when it occurs in a merle it erases the color between the patches and creates a harlequin.

The second, and much more common (not in numbers of dogs, but in “how often it has happened in the history of merles”), mechanism is found in non-Dane breeds. In these breeds a mutation within the merle gene itself erases the intermediate form of the color and creates a white and black dog. There are multiple known mutations that do this, and there will likely be many more discovered.

In non-Danes, the mutated merle is passed along by the parent. So an oddly colored merle – whether harlequin or tweed or muddy or whatever – will generally create more of itself. These mutations can therefore be traced through pedigrees… at least until the merle gene mutates again!

Can merle be “carried”? Can it be hidden in a pedigree?

Most breeders use “carry” to mean that the dog is not a color, but has the ability to produce that color. We often say that our reds and brindles carry black, for example, or that our brindles carry red.

If that’s what you mean by carry, then no. Merle can never be carried. Every dog who has a merle gene IS merle. There is never any such thing as a dog who passes along merle to its children but is not merle itself.

The only way merle can hide in a pedigree is if the dog is both merle and ee red. Because merle affects black pigment but spares red/yellow pigment, and ee red dogs have no black hairs and only red/yellow hairs, they can BE merle without LOOKING merle. They are not carrying it; they are still very much merle. But they don’t have the big visible patches.

For this reason, if you have en ee red Cardigan who had one or more merle parents (including ee red merles), it would be smart to gene test it before breeding if you are considering breeding it to a merle. You want to know if your ee red is also a merle, in that case. If your ee red had non-merle, non-ee-red parents, then there is no way it can possibly be merle and you do not have to worry.

What does all that mean? The short story is that if you have bred two known non-merles, the likelihood that a puppy will be merle is INCREDIBLY SMALL. It approaches zero for most breeders.

My dog doesn’t look merle, but he’s got mottled ears. Are these spots on my Cardigan’s white ears, or these spots in his collar or on his face and feet, evidence of merle?

Most likely, no. If your puppy was born without those spots, ABSOLUTELY NO. Cardigans also have a type of marking called ticking, which is spotting that appears on white areas of the hair in the weeks and months after a puppy is born. Sometimes that ticking can be so heavy that it joins together and looks like a merle patch, but merle patches are there when the puppy is born. And, of course, if neither parent is merle, you don’t have to worry even if the spots look a little weird.

Was the Cardigan originally merle or did someone cross a sheltie or collie in?

The original color of the Cardigan was known to encompass “brindle, brown, gold, tri-color, merle,” according to Mrs. Bole, and “yellow,” “blue and grey merle,” and “most frequently a … golden brown merle” (a brindle or sable merle) according to Mr. Lloyd-Thomas. So merle is very definitely an original color of the breed, and was bred frequently in its sable and brindle forms as well as in its black-based “blue” forms.

As a note, isn’t it interesting to see “brown” and “grey” mentioned? From those narratives and from the existence of dogs like Farlsdale’s Silver Smoke, we know that chocolate and slate are also original colors and not evidence of crossbreeding either historic or modern.

Why do people say that merle disappeared and was rediscovered in Cardigans?

Merle never disappeared in the Cardigan, but the black-based blue merle did indeed go away, for about twenty years, from the 1930s to the 1950s.

Since by the 1930s the breed was only ten or twenty years out of the hills from whence it had come, it was very vulnerable to the preferences of the handful of people who had them. There were very few breeders at that time, and they were shouldering the task of keeping the breed going both in the UK and in the US. There was little or no market for show puppies, so generally if the breeders themselves did not keep the dogs for breeding they were never bred.

Mrs. Wylie, in the UK, had been committed to the merle color, and had many lovely dogs. But with her death, especially since she had not spread her dogs around, the black-based “blue” merle disappeared from view.

About twenty years later, the color “reappeared” from reds and brindles.

The key to understanding how this happened is to realize that lumped in with “red” were (and still are) what some breeders called “pale red” and Lloyd-Thomas had called “gold.” In other words, ee reds, which we colloquially call pink. As I said above, dogs who are ee red can also be merle, and still remain (visibly) red. And for those two decades they had indeed stayed red, and thus progressed through pedigrees for several generations.

But, interestingly, the breeders still knew what they were. Those interested in producing black-based merles again were told that these “reds with blue eyes” (ee red merles), when bred to other colors, would make blue merles. So even with the limited knowledge of color genetics of the time, there was no mystery about the fact that ee red and merle could coexist and be used to produce black-based blue merles. And, in fact, that’s exactly what happened.

I’ve heard that “pinks” are a terrible threat to the breed because they hide merle.

You may have a clue about what I am going to say based on the last paragraph – but to make it clear, NO. Pink (ee red) can make merle less visible, but knowledge is all that is needed. Whether you call it yellow, as Lloyd-Thomas did, gold as Mrs. Boles did, light red as is often recorded in pedigrees, or our current pink, ee red is a common and original color in the breed. And it was handled (and, I might add, valued), in concurrence with merle, by breeders without our current gift of easy color testing. Surely we can expect as much of ourselves, especially since we have gene testing at our fingertips.

If you have a pink you think might be merle, test it. If both parents are non-merle, you don’t have to worry.

I still have more questions. How do I get them answered?

Please put them in the comments. If I can answer them, I will. If I need to send you elsewhere, I’ll be happy to do that too.