What is a Merle Coat?

Written by AZ Animals Staff
Published: December 3, 2021
Image Credit Maria Ulzutueva/Shutterstock.com


Merle is perhaps the most fascinating and misunderstood coat pattern in the pet world. The striking markings are only found in certain dog breeds; no other animal has this trait. Technically, the merle coat pattern can be bred into just about any line, but in practice, merle breeding is highly restricted to only certain breeds. This article will cover some fascinating details about genetics, life expectancy, and the health problems associated with the merle coat.

What is a Merle Coat?

Dogs with merle coats are more likely to have blue eyes over any other color.

Merle can be defined as a mottled coat pattern that combines lighter and darker splotches of the same color. The random patterns are usually unique to the dog, so no two coats may be exactly alike. The merle coat typically comes in either red merle (which is more like liver) or blue merle (which is black or gray); this is surrounded by solid areas of white. Dogs with the merle gene but little to no actual merle patterns are called phantom merle or cryptic merle. The patches are so small it barely looks like a merle at all, but it can still pass on the merle trait to their offspring.

Over the years, two other types of merle variations have gained recognition beyond just the “standard” and cryptic merle types: these are called the diluted merle and the harlequin. The diluted merle has light red or black coat colors with no patches at all, whereas the harlequin has a white background with a few large patches of dark pigmentation. It might be difficult to tell just from looking at them that these are merles, because they don’t exhibit the usual mottled pattern, but they do carry the modified merle gene that can be passed on to their descendants.

In addition to the coat colors, the merle gene also changes the color of other parts of the body. The eyes are much more likely to be colored blue, while the nose and paw pads may have a more mottled pink appearance.

What is the Correct Pronunciation of Merle?

The pronunciation of merle is just as simple as it looks: it rhymes with “girl” or “hurl.” The modern word and its pronunciation probably come to us from the Latin name for the blackbird. It may have entered English usage through the French language in the Middle Ages.

How is the Merle Coat Created?

The merle coat pattern arises from a modification in the gene that produces black and brown pigments in the hair. Merle puppies will always inherit this modified DNA strand, whereas non-merle puppies inherit DNA with the merle trait absent. This section will get a little technical about genetics. Essentially, all animals inherit two copies or variants of the same gene from both parents. These variants can be either dominant or recessive, the former usually being expressed over the latter. If the offspring inherits two dominant copies or one dominant copy and another recessive, then the dominant trait will be expressed. If the offspring inherits two recessive copies, then only the recessive trait will be expressed.

The merle trait works a little differently from this. It is what’s known as incomplete dominance. This means the recessive copy still affects the trait, even in the presence of another dominant copy. If the offspring inherits two recessive copies of the merle gene, then it will have little to no merle at all. But if it inherits a dominant copy and a recessive copy, then it will have a normal merle coat. If it inherits two dominant copies, then it will have a double merle coat. Despite the name, this double merle coat barely looks like a merle at all; it resembles the harlequin, in that it has a lot of solid white fur and relatively few patches. The double merle is generally avoided by responsible breeders for health reasons (which will be covered in the section below).

You might be wondering, then, what is the genetic difference between a cryptic merle, a diluted merle, a regular merle, and a harlequin? The short answer to the question is that there are additional genetic factors that determine the amount of pigmentation, but that’s a bit beyond the scope of the article. Suffice to say, it has something to do with the length of the DNA segment that produces the pigmentation. Shorter or longer segments have different effects on the patterns.

Based on all this information, it’s fairly easy to predict what the puppies will look like just by knowing the genetics of the parents. By crossing two regular merle dogs together (meaning dogs with one dominant and one recessive merle trait), you will have, in an average litter, about half regular merle puppies like the parents, a quarter non-merle puppies with two recessive copies, and a quarter double merle puppies (meaning two dominant copies).

Because of the health problems associated with the double merle coat, some breed clubs will even forbid a merle-merle cross to prevent any double merle puppies from emerging in the litter. They will instead recommend crossing a merle with a non-merle dog, which dramatically reduces the chances of health problems ever arising.

If you’re unsure about the genetic composition of your dog, then you should consider doing a genetic test. Regular merles are pretty easy to identify, but cryptic merles and harlequins are particularly easy to mistake for other types of dogs. This might not matter so much unless you plan to breed your dogs, in which case they may accidentally pass down merle traits without you realizing it.

Is the Merle Coat Rare?

Blue merle is one of the rarest colors of a Border Collier.

That depends entirely on the breed in question. For some breeds, red or blue merle is one of the most common coat patterns. For other breeds, it’s kind of a novelty coat pattern but not among the most common coats. For most breeds, however, merle is completely disallowed by legitimate breeders. This makes it difficult to estimate your likelihood of locating one. While most breeders may try to avoid merles altogether, some do specialize in merle breeding, so you may need to do some research first.

To help you through this difficult process, the best way to track down a new puppy is to get in contact with a trusted breeder through the American Kennel Club or other breed-specific clubs. Breeders should be very knowledgeable and forthright about the benefits and downsides of purchasing a merle. If you’re more interested in adoption instead, then the likelihood of finding a merle dog will depend heavily on the breed. Merle Aussies can often be found in kennels or rescues, while some breeds are much rarer.

Which Dog Breeds Have a Merle Coat?

The merle coat is unique to certain types of herding dogs and other closely related breeds. This is because the merle coat arose from a unique genetic change in a common ancestor of many modern herding breeds. The Australian Shepherd is probably the one breed that immediately comes to mind when someone mentions merle. Both red merle and blue merle are among the few accepted Aussie coat patterns. Other common merle breeds include Border Collies, Shetland Sheepdogs, Great Danes, Dachshunds, Cardigan Welsh Corgis, and Catahoula Leopard Dogs.

While it is possible to breed a red and blue merle coat into other types of dog (like, say, a Pitbull), that may disqualify them from purebred status in the eyes of many dog organizations. This can lead to some serious problems with finding a trustworthy breeder, because if the merle trait isn’t part of the breed standard, then reputable breeders usually won’t bother with it, opening the door to all kinds of untrustworthy sources. In these cases, it’s even more imperative that you do your homework and ask the right questions (especially regarding the dog’s health and any tests they’ve done). Pitbull breeding in particular can sometimes fall prey to these low standards. Be extra careful about buying Pitbulls.

Do Merle Coat Dogs Have Any Health Problems?

Merle dogs are thought to be at low risk of developing health problems, and their life expectancy is about the same as any other type of dog, but there is one exception: double merle dogs are much more likely to suffer from vision and hearing problems. Various studies have been conducted to understand the risks of a merle coat. One study found that a standard merle dog had a 2.7% chance of deafness in one ear and a 0.9% chance of deafness in both ears. The double merle dog (meaning a dog that inherits two dominant copies of the merle gene) had an astonishing 10% chance of deafness in one ear and 15% deafness in two ears. Double merle dogs also run the risk of developing microphthalmia, in which the two eyes are unusually small and sometimes even non-functional. These risks also vary by breed. The rate of hearing problems was particularly high in double merle Australian Shepherds but a lot lower in double merle Catahoulas.

There is some debate about how serious the double merle trait is. It’s generally true that most double merle dogs will go on to live mostly healthy lives with a normal life expectancy. It’s not a life-threatening problem at all. Still, there’s no reason to put a dog at risk of vision and hearing problems by breeding a double merle dog (especially since it doesn’t even produce the full merle coat colors).

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