The Merle Gene
The M locus is the home of the merle allele. Merle is dominant, and so denoted by the capital letter M. Non-merle is recessive, and denoted by m.
Merle is pretty unique because all normal merles are heterozygous (Mm or mM). A homozygous merle is actually a double merle, similar to the lethal white gene found in horses and other mammals (although not itself lethal). Click here to go to the page on double merle.
The merle gene dilutes random sections of the coat to a lighter colour (usually grey in a black-pigmented dog), leaving patches of the original colour remaining. The patches can be any size and can be located anywhere, unlike the patches on a piebald dog (which are generally confined to the body and head). The edges of the patches may appear jagged and torn.
Merle affects only eumelanin. That means that any black, liver, blue or isabella in the coat, eyes or nose will be merled, whether it's the whole of the body, a mask on a sable, shading, brindle stripes, or even a saddle. Phaeomelanin (red) is not affected at all and will appear as normal.
The merle gene is known to cause a number of health problems, mostly deafness and blindness. These problems are unusual in heterozygous merles (Mm) but unfortunately common in double merles (MM). For this reason two merles should never be bred together, as this will result in some double merle puppies.
The reason merle can be problematic is it can cause lack of pigment in certain vital areas - the eyes and inner ears. Most heterozygous merles have plenty of pigment because they still have an m (non-merle) allele to help them to make it, but double merles often have large white areas where there is no pigment produced at all. More information is available on the Double Merle page.
Breeds With the Merle Allele
The merle allele only occurs in a small selection of dog breeds. These are:
Catahoula Leopard Dog
Old English Sheepdog
Cardigan Welsh Corgi
Collie (Rough and Smooth)
Dachshund (known as dapple)
It has also recently been bred into the Chihuahua, American Staffordshire Terrier, American Cocker Spaniel and Pomeranian.
The pictures above show the range of markings found in merles. The first dog has very few black patches, and they're mainly quite small. The second dog shows the normal merle pattern - a mixture of larger and smaller patches, covering roughly 50% of the body. This pattern is generally the most preferred in breed standards. The third dog has very large black patches, sometimes referred to as blanketing. The last dog is known as a minimal merle. It is almost completely black with just a very small amount of merling. This pattern is rare and generally discouraged because it can "hide" the merle gene if the black covers up all the merle in the coat. Dogs with little or no visible merling are sometimes called cryptic merles.
The dogs above are called "blue merles" because of the bluish colour between the patches in their coat. This is a widely-used term but is actually misleading. Technically they should be "black merles". Their nose pigment is black and their eyes are brown or blue. They are able to make normal eumelanin in their coat, so their patches are black. If they didn't have the merle gene, they would be solid black. "Blue merle" is misleading because it seems to say that these dogs have blue pigment (dd acting on black - see Dilution page), when in fact they have black.
An example of the genotype of one of the above dogs would be: BBDDEEggMmSSKK (most of these aren't necessarily homozygous, but I will assume they are for ease, otherwise I'd have to keep saying, "KK, kK or Kk" etc). The genotype translates as: no liver colour (BB), no dilution (DD), no mask or recessive red (EE), no greying (gg), merle (Mm), no white spotting (SS), and solid black (KK).
Two solid blue merles (the Border Collie also has white in the irish spotting pattern and bronzing on its side).
This mixed breed could be classed as a cryptic merle as one side of the dog is almost completely solid black. The merling is only visible on the right side (first photo). This dog also has tan points (atat) and white spotting in the irish pattern.
Examples of the variation in base colour of merles. In some cases the black may be diluted almost to white, but it is mostly somewhere between light "powder" grey and dark "grizzled" grey. It is not certain whether this variation has a genetic basis.
Eyes and Noses
The random coat dilution caused by merle also affects the eyes and nose. The eyes may be all or partly blue, and the nose may be all or partly pink. Above are examples of "butterfly" noses on merles, which are partly pink.
The harlequin Great Dane here also shows a butterfly nose and wall eyes (one blue, one brown). Harlequin is a modified type of merle where the areas between the patches are diluted completely to white. See the Harlequin page for more information.
Sometimes merles have patches that are only partially diluted, and are between the base and the patch colour. These are known as dilute spots, and they may sometimes appear brownish. The Australian Shepherd above has a large dilute spot on its knee.
A normal "blue" merle becomes a "red" merle when it has bb on the B locus, i.e. when it has the liver gene. "Red merle" is also a misleading term because "red" is usually used to refer to phaeomelanin (tan, gold, cream etc) rather than eumelanin. A red merle should correctly be called a liver merle.
Liver turns all of the patches on a blue merle into brown and the colour between the patches becomes pale brown. As with all livers, liver merles have liver noses and amber eyes.
Here we have two long-haired Dachshunds. The dog at the back is a red (liver) merle with tan points, and the dog in the front is a normal blue (black) merle with tan points.
More examples of liver merles with tan markings and with or without white.
As the examples in the sections above have shown, merles can come with or without any of the tan patterns (A locus). The most common is normal tan points, but creeping tan and saddle patterns also occur.
All dogs which show tan markings must be kk on the K locus (K is dominant black, so if a dog has even one copy of it then they will be solid black, or solid merle if they are also Mm, regardless of what they have on the A locus). A tan pointed dog will be atat on the A locus (tan points are recessive and only dominant over recessive black, a, so a dog must have two at genes to display tan points).
The Norwegian Dunker is one of the only breeds where the saddle pattern and the merle gene occur together. This dog's pattern is between creeping tan and saddle. You can see that the merling covers the back, the tail, the back of the neck and the top of the head. The tan is very light and so is probably affected by the Intensity, which lightens red. This dog also has white in the irish spotting pattern. The Dunker photo was taken by June.
A dog which is Eme, EmE or EmEm on the E locus will have a mask (Em, the mask gene, is dominant). The mask can be black, liver, blue or isabella depending on the dog's pigment. If the dog also has the merle gene, the mask will be merled. Because of this, masks are not be visible on solid merle dogs - they're just merled like the rest of the coat. However, tan-pointed merles with little to no face white sometimes display masks (recognisable because the areas which should be tan on the muzzle are merled instead). Sable (ayay) dogs have masks which are much easier to see. A dog which is ayayEmEmMm (homozygous for sable, homozygous for mask, heterozygous for merle) will appear solid red with a merled mask.
A dog with AyAy on the A locus and kk on the K locus (allowing for the A locus markings to be visible) will
be a sable. Sables are solid red with or without black tipping. Sometimes, especially if sable is combined with a mask, the black tipping can be quite
extensive and can cause large black areas on the back, head and tail. In a sable merle, all of these black areas are merled. As you can imagine,
how obvious it is that a dog is a sable merle depends heavily on how much black tipping they have. A sable merle with no black tipping (a "clear sable") will just appear
to be a solid sable, possibly with one or two blue eyes (the only indication that they are a merle).
In some breeds, such as Shetland Sheepdogs and Rough Collies, a slightly different version of sable exists. This type consists of brownish hairs on the back and head (even though these dogs have black pigment), and is often called "shaded sable". On this type of sable coat, merling can be quite visible (if there is a lot of dark brown shading) or very hard to see (if the shading is lighter and not so extensive). However, the merling is usually visible at birth, so breeders will generally know if their dogs are sable merles or just sables. A sable merle will have some faint, darker brown/tan patches on a lighter base, and the merling will usually be confined to the back and head. It is often most visible on the ears, where the fur is shorter.
This is a sable merle Border Collie with black tipping and a mask. His most obvious merle trait is his blue eyes, but a small light grey area is also visible on his cheek. If you look closely, he has some silver hairs on his back where the black tipping is merled, and another hint is that the black tipping isn't spread evenly - it's patchy and seems to be more down the side of the shoulder than on the back, where it would normally appear, suggesting that the black hairs which should be present on the top of the shoulders have been turned to grey (which is not very visible against the red coat).
Dexter the Catahoula Leopard Dog cross, photos by Olivia Frost
Dexter is a shaded sable merle. Unusually for a sable merle, he has very distinct dark patches on his body. Compare to the tipped sable merle Border Collie above and you'll see that the overall effect is very different, even though both dogs are AyAy.
Simon the Shetland Sheepdog, photos by Brandee Massey
Simon is also a shaded sable merle. These pictures show him from five months to 2 years old. His shorter puppy coat displays the merle much more clearly than his longer adult coat.
Grizzle / Agouti
Haiku the Husky mix, photos by Dariana Kamenova
Haiku above shows beautifully the effect of merle on agouti (aw). The agouti pattern consists of banded hairs scattered over the dog (and mostly concentrated on the head and back). As only the black (eumelanin) parts are affected by the merle gene, the overall effect of merle on grizzle is of scattered small black spots and flecks, with some patterns forming where the black would usually be on the grizzle (as can easily be seen in the photo of Haiku to the front, where the spots on his head form lines). These two alleles do not generally occur within the same breeds, so grizzle/agouti merle is very rare and is usually seen on Husky crosses.
If a dog is kbrkbr (or kbrk, because kbr is dominant over k) on the K locus then any phaeomelanin (red) in their coat will be brindled. This applies to merle dogs too - there is no reason why a merle cannot also have brindle. If a merle dog is kbrkbr and has atat on the A locus then it will be merle with brindled tan points. If a merle dog is kbrkbr and AyAy (sable), it will be solid red with brindling all over ( and possibly with larger areas of black than is normal for a brindle on the back and/or head, if the sable has dark shading). The brindle in both the brindle-pointed merle and the solid brindle dog will be broken up into uneven spots rather than being complete stripes. This is because the stripes on a brindle are eumelanin (affected by merle, so random parts are diluted) and the base is phaeomelanin (not affected by merle).
A solid brindle merle is relatively easy to identify because of its broken-up stripes. However, when a merle dog has just brindle points it can be harder to recognise because the stripes are even less visible. The points will generally have a few black spots on them and will appear a darkish, muddy brown.
The Cardigan Welsh Corgi is one of the few breeds where brindle and merle regularly occur together (the other main one being the Catahoula Leopard Dog). The dog on the left has tan points, but they are very dull and brownish compared to normal tan points (see the dog on the right, which does not have the brindle allele). This points to this dog being a brindle-pointed blue merle, which is very common in the breed. The brindle appears only on the points, where it is broken up by the merle gene. In this case, the dog is a very light-marked merle, so the points are very washed out with few spots. Brindle can also appear on merles with the creeping tan and saddle patterns.
Photos by Cat of Dog Rad Design
These two merles demonstrate how tricky it can be to identify brindle points. If you look closely you can see muddy brown areas on the legs and muzzle, with black flecks and spots.
A dog with TT or Tt on the T locus will have ticking on any white areas in their coat. It is thought that TT produces roan or heavy ticking and Tt produces lighter ticking. Ticking will only show up on a merle if it has white markings, and the ticking will be a mix of the merle base colour and the patch colour (so in a blue merle, black and grey).
Recessive red is when a dog has the genotype ee on the E locus. This means it is unable to produce eumelanin (except in its nose and eyes), and can only produce phaeomelanin, so its coat is red all over (except for any white markings). Merle only affects eumelanin, so if there's none there, there can't be any merle. Just like with clear sable, a recessive red merle can be impossible to distinguish from a non-merle dog. It will simply appear solid red, and the only giveaway is if it has one or two blue eyes.
Lastly, we need to mention dilutes. A dilute is a dog which is dd on the D locus. dd stops a dog from producing full eumelanin pigment (and also affects phaeomelanin to a lesser extent), meaning that the pigment it produces is weak and pale. It turns a black dog into a blue dog (with a blue nose and amber eyes) and a liver dog into an isabella. A merle with the dilution gene will appear very washed-out. The black or liver patches will be diluted almost to the same shade as the base colour. So a blue merle (a proper blue merle with the dilution gene, not a black merle like the ones we've been dealing with so far!) will appear almost completely light grey with some faint darker grey patches, and an isabella merle will be light greyish brown with faint darker brown patches. On a longhaired dog, it can be next to impossible to tell a blue or isabella merle from a non-merle. To distinguish between a merle with dilution and a merle with greying (which produces much the same effect on the patches in the coat), look at the nose. If the dog has a blue nose, it has the dilution gene.
Sometimes black and white dogs with heavy ticking or roaning can be mistaken for merles. The main giveaway is that a ticked or roaned dog will have very uneven grey areas, with flecks of white showing through. Also, if the dog has tan points, they will be ticked too (if they're on the "white" areas rather than the black patches), whereas in a merle they would be solid, and the patches on a blue roan or ticked dog will be more regular than on a merle (appearing only on the back and head in the piebald or extreme white pattern, rather than all over the dog). In addition, if the dog has a butterfly nose or any blue in its eyes then it is most likely a merle as these pigment issues do not generally affect roaned dogs.
All of these dogs are merle "look-a-likes", but none of their breeds (English Cocker Spaniel, Australian Cattle Dog and Basset Bleu de Gascogne) actually carry merle:
There is no doubt that many of the merles shown here are very beautiful, unusual and exotic. However, the pictures and information are provided entirely for educational purposes and I do not in any way condone the breeding of dogs deliberately for colour.
The primary concerns for any dog breeder must be health, temperament and conformation. Colour should only be a consideration where it is important for the breed standard. Special care must also be taken when breeding merles to avoid health problems, so merle is a colour best left to very experienced dog breeders. If you are considering breeding a merle dog then please make sure to check out the Double Merle page.