The Double Merle Dog and the Dangers of Merle-to-Merle Breeding

Updated on July 11, 2019
Sophie Jackson profile image

Sophie has been a freelance writer since 2003. She is passionate about history, the natural world, and her three dogs.

Red Merle Australian Shepherd puppy
Red Merle Australian Shepherd puppy | Source

In certain breeds of dog, there is a coat pattern known as 'merle'. It is sometimes referred to as a colour, but is in fact due to a gene that alters the way pigment appears in the dog's coat. The merle coat pattern is popular because it is unusual and very unique, with each merle dog having a different coat pattern.

Unfortunately, there are health problems associated with the merle mutation and the risk of these problems occurring increases when two merle coated dogs are mated together. The resulting litter of puppies has a high chance of containing 'double merles' or 'lethal whites', dogs that have very little or no colour to their coat at all. Double merles are highly likely to suffer from eye or ear deformities (in some cases both), which at the most extreme can result in complete blindness and deafness. Needless to say, the deliberate breeding of double merles causes a lot of controversy in the canine world—certain official bodies (such as the Kennel Club in the UK) have clamped down on the practice, refusing to register dogs that are a result of merle-to-merle breeding. This article explores the world of double merle genetics, the breeds affected, the health issues and the wide range of ongoing debates concerning the breeding of double merles.

Blue merle Shetland Sheepdog (sheltie). Commonly merle dogs will have at least one blue eye, but in this example, due to the tan markings on the face, both eyes are brown.
Blue merle Shetland Sheepdog (sheltie). Commonly merle dogs will have at least one blue eye, but in this example, due to the tan markings on the face, both eyes are brown. | Source

What Is a Merle?

A dog with a merle coat is characterised by having patches of hair with diluted pigment (colour). Though a variety of merle colours are referred to by breeders and dog owners, the two most commonly seen types of merle are blue merles and red merles. Blue merles are, in fact, grey. They appear like a tri-colour dog (black, white and tan), but with patches of the black appearing 'faded' or grey. Similarly, a red merle will have faded patches of red and will often look more mottled than the blue merle. While all of the breeds with the merle coat pattern produce blue merles, only certain breeds produce red merles. The strength of the other colours in the dog's coat (tan and black, or red and tan) can vary as well, with some merles appearing to have extremely pale colouring all over, while others can have quite strong patches of colour. Blue merles with no tan markings at all are known as bi-blues, but a red merle does not necessarily have to have tan markings.

Merles commonly have blue eyes. Sometimes they have one blue and one brown eye. They can also, on occasion, have two brown eyes. Sometimes dogs may appear to have normal coat colouring but are in fact merles and will produce puppies with the merle colouration. These are known as 'cryptic merles', but the exact reason why such dogs do not display the merle pattern remains unknown.

The merle gene is usually dominant, so a merle dog will have inherited the gene from one of its parents. While a non-merle dog (unless a cryptic) will have inherited no merle gene. In the diagrams and examples that follow (M) refers to the merle gene, while (m) refers to a non-merle gene.

For example, in a litter of mixed colour puppies the non-merles will be (mm), while a merle will be (Mm), meaning it has inherited one merle gene and one non-merle gene. This is deemed the 'safe' or responsible way of producing merle puppies.

A mixed litter of puppies with non-merles (mm) and merles (Mm)
A mixed litter of puppies with non-merles (mm) and merles (Mm) | Source

Health Problems Associated With the Merle Gene

There is scientific evidence to suggest that the merle gene may be linked to a higher rate of ocular (eye) or auditory (ear) problems. A 2006 paper on the merle gene first published by the National Academy of Sciences of the United States of America attempted to identify the gene in dogs that caused the merle pattern. Among their findings they recorded research on deafness in Dachshunds with the merle gene. One study reported 36.8% of Dachshunds with the merle coat pattern (Mm) suffered hearing problems ranging from mild to complete deafness. While none of the control group of non-merles (mm) had any hearing issues. [Audiometric findings in dachshunds (merle gene carriers)]. Another study [Light microscopy studies of the cornea of Merle dachshunds] found that merles had a "significantly greater" frequency of eye abnormalities than non-merles. Other studies cited by the article found that the merle gene was associated with skeletal, cardiac and reproductive abnormalities, but there is far less conclusive evidence for this.

The same study found that in Shetland Sheepdogs a mutation of the pigmentation gene known as Silver (or Silv) is probably responsible for the merle pattern. The exact function of Silv and how it affects pigment is unknown and remains controversial. Small studies of other breeds that can have the merle pattern found that they all had the mutated Silv gene.

Another study published in the Journal of Veterinary Internal Medicine in 2009 set out to discover if there was a link between increased deafness and the Silv gene. The study examined 153 merle dogs, and found that over 8% of the study group had some form of deafness. The study concluded that merle dogs had a higher risk of deafness compared to many dog breeds, but not compared to dalmatians and white bull terriers (which have high rates of deafness thought to be linked to their white pigmentation).

Limited research has been done into the links between the merle gene and eye problems, though a great deal of anecdotal evidence is presented on the subject. There may be a link between pale coloured eyes and eye problems, but so far the scientific research to back this claim has not emerged in the public domain.

Blue merle border collie with mismatched eyes
Blue merle border collie with mismatched eyes | Source

What Is a Double Merle?

When a dog with the merle colouration (Mm) is bred to another merle dog then there is a chance that a double merle (MM) will be produced. The way genetics work is that a puppy inherits one colour gene from each parent, this means that puppies in a merle x merle litter could be normal merles (Mm), non-merles (mm) or double merles (MM). There is a one in four chance that each puppy will inherit two merle (M) genes, thus making them a double merle. The odds do not increase or decease depending on litter size.

Puppies with (MM) genes tend to have health issues, particularly with their eyes and ears. The exact reasons why the two merle genes together cause these problems remains unclear. It is known that merle coated dogs have a slightly increased risk of hearing problems, possibly due to the Silv gene. So when a dog inherits two copies of the gene, the risks of hearing problems would seem likely to double. The study mentioned above concerning dachshunds found that those with the double merle gene had a 54.6% chance of having a hearing problem. The figures suggest that more than half of the double merles born will have some form of auditory impairment. Equally, though fewer studies have been done on eye conditions, double merles are very prone to various forms of eye deformities, right to the point of having very deformed eyeballs.

Double merles are often partially or completely white in coat colour (sometimes termed albino, though this is not entirely accurate). The merle gene causes the coat hair to produce a faded, or shaded colour (hence the merle pattern), two merle genes together often cause the coat to be white or with limited merle shading. However, some dogs will appear with the coat pattern of a normal merle (Mm), making it harder to determine if they are actually double merles. Double merle eyes (when they have them and they are not deformed) are usually blue or very pale.

Unscrupulous breeders will sometimes sell double merle puppies as 'rare albino' versions of the breed. In breeds such as the Shetland Sheepdog there is also a form of coat colour known as 'colour-headed white' (seen more often in the US than the UK, where it is an undesirable pattern for showing). Breeders may therefore try to pass off their double merle pups as a different coat colour that has not resulted from merle-to-merle breeding. There are also those that will cull obvious double merle puppies at birth, whether they have a health problem or not.

Diagram of coat colours produced from normal and merle x merle breeding
Diagram of coat colours produced from normal and merle x merle breeding | Source

Health Problems Associated With Double Merles

In the US double merles of any breed are sometimes referred to as 'lethal whites', though many consider the term derogatory. Though double merles do not usually have fatal disabilities due to their genetics (negating the term 'lethal') the implication that double merles are unhealthy compared to normal coloured dogs in their breed has some basis.

Lucky double merles are born without problems, but many do suffer severe auditory or visual impairments. It is thought this may be due to the way the Silv gene (the gene that causes the merle pattern) affects the pigment of the skin around or within the ear, and the colour of the eyes. However, as yet there is no firm scientific evidence to imply that a dog with white ears will be deaf, or a dog with white patches around the eyes will be blind. In fact the opposite can occur. However, this should not detract from the fact that many double merles suffer from eye or ear abnormalities due to their breeding.

Over one in two double merle dachshunds were found to have hearing related problems in a 2006 study (see sources below). Other studies have shown that double merles regularly have ear problems, ranging from slight to complete deafness. This is genetic and not related to age or other health issues. It can not be corrected.

As with hearing, double merles can have eye problems that range from slight vision loss or unusual eyes, to complete blindness. Some double merles have a 'starburst' pupil, where the pupil appears to have spiky projections. Though the dog is not technically blind, it can suffer from light sensitivity as the eye does not react as well as it should to light. This can lead to problems with vision when going from light to dark areas. Other double merles suffer from microphthalmia, where the eye is smaller than normal. In some cases, it appears as if there is no eye at all (anophthalmia). While a slightly smaller eye may not impede vision, many double merles have significantly smaller eyes which leads to varying degrees of sight loss.

While some argue other health problems for double merles there is no evidence that, aside from ear and eye deformities, they have any higher risk of other conditions. However, being deaf, blind or both is surely bad enough?

A double merle Great Dane with distinct eye problems
A double merle Great Dane with distinct eye problems | Source

Breeds That Can Have the Merle Coat Pattern

Type of Merle
Recognised by
Border Collie
Rough Collie
Shetland Sheepdog
Australian Shepherd
Miniature American Shepherd
Koolie or German Coolie
Cardigan Welsh Corgi
Pyrenean Shepherd/Sheepdog
Bergamasco Shepherd
Old English Sheepdog
Catahoula Leopard Dog
American Cocker Spaniel
Great Dane
American Pit Bull
The final column indicates the breed is recognised by a certain organisation, but does not mean that organisation recognises certain merle colours.
Blue merle Cardigan Welsh Corgi
Blue merle Cardigan Welsh Corgi | Source

Blue Merle Controversies

The question many ask is, if breeding merle to merle has such a high chance of producing a disabled puppy, why would anyone do it?

There are several answers to this question, the first being pure ignorance. Not everyone knows the risks of breeding two merles together. Ideally anyone selling a merle puppy should explain to the new owner the risks associated with breeding merle to merle, especially if that person already has a merle dog of the opposite sex to the puppy. But as this is unlikely to happen, accidental creations of double merles will continue to occur.

While ignorance is no real excuse, what's far worse is those who know the risks and still choose to breed merle to merle. A breeder who deliberately and knowingly creates double merles is after one thing—a dog carrying the MM coat pattern genes. This dog, when mated to a non-merle, will produce an entire litter of normal merles, because it carries two versions of the merle (M) gene and all the puppies will inherit one copy from it, thus making them all (Mm).

As merles tend to be the rarest colour pattern in most breeds which carry the (M) gene, some unscrupulous breeders see breeding an entire litter of them in terms of money. They will charge more for a merle pup, and guaranteeing every pup is a merle makes the litter more lucrative. Other breeders believe a mating involving a double merle will produce the best merle pattern on all the puppies, so they choose to produce or use a double merle in the hopes of getting the best 'show quality' dog.

Whatever the exact reason behind choosing to create and then breed from a double merle, the truth is that along the line it is highly likely a significantly disabled pup has been created. If the breeder continues to create double merles in their breeding program, then the risks increase that at some point they will breed a deaf and blind dog.

What becomes of these puppies? The lucky ones end up in a good rescue that specialises in dogs with such disabilities and will find them appropriate homes. Less lucky ones are sold to unsuspecting puppy buyers who only later discover the dog has a serious problem. They may then be passed to a rescue or euthanised if the owner does not think they can cope with the pup's problems. The truly unfortunate pups are destroyed days after they are born when the breeder realises they are disabled and doesn't want to have to raise such pups knowing they are hard to home. It is also a means for covering the breeder's tracks and making it appear as if they had a merle-to-merle litter completely free of double merles.

The breeding and possible destruction of disabled pups has led to heavy criticism of merle-to-merle matings. In a wider context, such breeding brings the ethics of all dog breeders into question and casts a pall over the show dog world. There is a perception that show dog breeders are the worst offenders for producing double merles because they are striving for the perfect merle coat pattern. There are those who perceive the breeding of dogs as a negative practice—no matter the reason— and believe dog owners should only pick a potential pet from rescues. Their dim view of breeders is only worsened by seeing double merle matings often quite blatantly advertised. Equally, there are those who mistakenly think that the issues with double merles indicate that the normal merle (Mm) dog is somehow less healthy than a standard colour. They consider the merle pattern an indication of weakness, casting aspersions on otherwise healthy dogs. This is often used as a reason why the merle coat pattern should not be recognised in a breed, even when it occasionally crops up naturally.

Can Double Merles Be Prevented?

As long as there are merle coated dogs, double merles will be produced, usually by accident or through ignorance on the part of the breeder. However, a great deal can be done to discourage the breeding of double merles and to educate the public on the dangers.

In the UK, double merles of any breed are not recognised by the Kennel Club, the UK's official governing body for dog breeding. Additionally, puppies who have a parent who is a double merle cannot be officially registered. This cuts out one reason for producing double merles—perfect merle patterning in second generation litters—as these puppies cannot be recognised and thus not shown. In the US, where double merles can be registered (though they are not termed double merles), top show kennels are deliberately producing and breeding from (MM) dogs to get the merle pattern they want. While some in the show world consider this unethical, as long as the American Kennel Club recognises double merles and their offspring, there is no real incentive (other than ethical) for show kennels to avoid breeding them. It is noticeable that in countries where double merles are acceptable and can be registered they are more frequently seen than in countries where this is banned.

Official bodies can lead the forefront in discouraging the deliberate breeding of double merles, but educating puppy buyers would also help. Unwary puppy buyers are sometimes sold double merles as 'rare white' or 'albino' versions of a certain breed, not knowing that the pup may prove deaf or blind. Equally, pet owners with two merles may breed their dogs without realising the consequences. Spreading the word about what a double merle is, is just part of the answer. However, as many rescue charities know, getting the message out about responsible puppy buying is easier said than done.

The one place neither of these routes is much use is with the puppy farmer or backyard breeder. They have no reason other than money in mind when they breed pups and have no concern for the health of their dogs. Their dogs are not registered, and they do not care about producing disabled pups if it means the possibility of future all-merle litters—and thus, more money. The only way these people can be stopped is by bringing in legislation to stop puppy farming, but that is another issue entirely.


Retrotransposon insertion in Silv is responsible for merle patterning of the domestic dog. Proceedings of the National Academy of Sciences of the United States of America 2006

Prevalence of Deafness in Dogs Heterozygous or Homozygous for the Merle Allele. Journal of Veterinary Internal Medicine 2009

This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.


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    • Sophie Jackson profile imageAUTHOR

      Sophie Jackson 

      8 months ago from England

      There is a strong possibility she is a double merle considering the limited colour and the merle markings. Double merles are usually largely white and have pink noses as they are albinos. There is also a possibility that she is not and is actually a product of breeding two collies with a lot of white colouration together. This can produce a pup with a large amount of white (or all white coat). There are problems with this form of mating too and there can be risks of deafness due to the heavy amount of white (like with dalmatians, the white coat gene carries problems). Depending on the honesty of the breeder, you could ask about the parents, question the possibility of her being a double merle and then consider if you would be prepared to take on a dog with potential hearing and eyesight issues. Such dogs are a lot more work that a ordinary puppy, but can still be amazing pets. It absolutely depends on your future plans and what you want from a dog. If you are prepared to take on challenges, you might still considered buying this pup. I would say, however, that if you have never owned a BC before (I don't know if you have or not) then taking on one that is a double merle is going to be hard work, as they are quite an intense breed without additional complications, and at six months there could be behavioural issues if she has not been properly socialised. Its a lot to think about, I know, but taking on a double merle is a big step.

    • profile image

      Ollie Miller 

      8 months ago

      Hello! I have recently been looking into getting a Border Collie puppy, and this is how I got here. I was looking through a few sites that are for selling puppies and i recently came across a almost purely white puppy. She is a blue merle, but she only has it small patches on one of her shoulders her back and a small bit of the base of her tail. She is a pretty low price compared to a lot of other puppies, only $350. She is almost six months old and still hasn’t been bought. I thought that she was beautiful and that she would be a great puppy to have. But I am now concerned, I was wondering if you could help me out, since I’m still not sure if she really is a double merle or not. Thanks!

    • profile image

      Emily Morrison 

      21 months ago

      Very useful article. I recently rescued a blue merle mini aussie that had been purchased by the previous owner from a breeder. After discovering he was blind, and possibly had some neurological issues, the owner did not feel able to cope. I offered to take him, and she accepted. I'm very thankful now that I did, because I imagine if the breeder had him returned, he probably would have been euthanized. Poor baby! He's a good boy overall. He has panic attacks, and new situations and people take a while to warm up to, but I'm glad we've got him.

    • profile image

      Dog Lover 

      2 years ago

      Very imformative article. Unfortunately, Any didn't understand a single word of it. No, Amy it is not safe to breed a merle dog with another merle dog. Especially when one of the merle dogs in question is obviously the result of merle to merle breeding.

    • profile image


      2 years ago

      Great article! I have a question regarding double merles. We purchased a puppy about 2 years ago, a beautiful blue merle male. In the litter, there was a puppy that was predominantly white. Only a couple years later, did I realize this was the result of double Merle breeding. Our male can hear perfectly, however, our vet told us he may have slight sensitivity to the light. However, it has never bothered him as far as we can tell. We had planned to breed our blue Merle male to our red tri female. Is the light sensitivity a genetic trait that may be passed on the to the pups? Is it safe to breed them? Thanks in advance!


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