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

Updated on September 1, 2017
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

Introduction

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. But 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 and 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 debates currently ongoing 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 1 in 4 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 1 in 2 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 lead 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

Breed
Type of Merle
Recognised by
Border Collie
Blue/Red/Sable
AKC/KC
Rough Collie
Blue
AKC/KC
Shetland Sheepdog
Blue/Sable
AKC/KC
Australian Shepherd
Blue/Red
AKC/KC
Miniature American Shepherd
Blue/Red
AKC
Koolie or German Coolie
Blue/Red
unrecognised
Cardigan Welsh Corgi
Blue/Red/Brindle/Sable
AKC/KC
Pyrenean Shepherd/Sheepdog
Blue/Brindle/Fawn
AKC/KC
Bergamasco Shepherd
Blue
AKC/KC
Old English Sheepdog
Blue
AKC/KC
Catahoula Leopard Dog
Blue/Red/Black/Grey
unrecognised
Dachshund
Black/Chocolate
AKC/KC
Pomeranian
Blue/Chocolate
AKC/KC
Chihuahua
Blue/Fawn/Chocolate
AKC/KC
American Cocker Spaniel
Blue/Red
AKC/KC
Great Dane
Harlequin
AKC/KC
American Pit Bull
Blue
unrecognised
 
 
 
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, by 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 and so choose to produce or use a double merle in the aim to get 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 because 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, as there is a perception that show dog breeders are the worst offenders for producing double merles as 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. Equally, 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.

Sources

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


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