The Double Merle Dog: Dangers of Merle-to-Merle Breeding

Potential Merle Coat-Linked Health Issues

In certain breeds of dogs, there is a coat pattern known as "merle." It is sometimes referred to as a colour, but this colouration is actually due to a gene that alters the way pigment appears in the dog's coat. The merle coat 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 increases when two merle-coated dogs are mated together. The resulting litter of puppies has a high chance of containing "double merles," also called "lethal whites." These are dogs with very little or no colour to their coat at all.

The Main Controversy

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.

In This Article

• What Is a Merle?
• Health Problems Associated With the Merle Gene
• What Is a Double Merle?
• Health Problems Associated With Double Merles
• Breeds That Can Have Merle Coats
• Merle-to-Merle Breeding Controversies
• Can Double Merles Be Prevented?

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.

Color Variations

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.

Cryptic Merles

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.

Merle Eye Colors

Merles commonly have blue eyes. Sometimes they have one blue and one brown eye. They can also, on occasion, have two brown eyes.

Genetics

The merle gene is usually dominant, so a merle dog will have inherited the gene from one of its parents. A non-merle dog (unless a cryptic) will not inherit the 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.

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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.

• Potential hearing problems: 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)].
• Potential eye abnormalities: 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 abnormalities: 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.

What Causes the Merle Pattern?

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.

Link Between Deafness and SILV

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).

Link Between Merle Gene and Eye Problems

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, little scientific research to back this claim has emerged in the public domain.

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.

Health Concerns

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 remain unclear.

• Hearing problems: 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.
• Eye abnormalities: 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 atypical eyeballs.

More detail on the health concerns for double merles is provided in the next section.

Double Merle Coloration

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.

Breeder Controversies

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.

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.

Can Double Merles Be Born Without Health Problems?

Lucky double merles are born without problems, but many do suffer severe auditory or visual impairments. 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.

More Than 50% May Have Hearing Problems

Over half of double merle Dachshunds were found to have hearing-related problems in a 2006 study (see studies linked above and 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 cannot be corrected.

A Variety of Eye Problems

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 vision loss.

Are Double Merles at Higher Risk for Other Health Concerns?

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. The risk associated with a dog being either deaf, blind, or both should be taken into account, nonetheless.

Breeds That Can Have Merle Coats

Merle-to-Merle Breeding 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.