THE MERLE GENE
The M locus is the home of the merle gene. 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 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. 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 appear jagged and torn.
Merle affects eumelanin. That means that any black, liver, blue or isabella in the coat 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.
BREEDS WITH THE MERLE GENE
The merle gene only occurs in a small selection of dog breeds. These are:
Australian Shepherd
Catahoula Leopard Dog
Dunker
Border Collie
Old English Sheepdog
Mudi
Cardigan Welsh Corgi
Collie (Rough and Smooth)
Shetland Sheepdog
Dachshund
Beauceron
Bergamasco
Pyrenean Shepherd
Great Dane
It has also recently been bred into the Chihuahua, American Staffordshire Terrier, American Cocker Spaniel and Pomeranian.
BLUE MERLES
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 on its ear and tail. 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. 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), when in fact they have black.
An example of the genotype of one of the above dogs would be: BBCCDDEEggMmSSKK (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 albino factor (CC), no dilution (DD), no mask or recessive red (EE), no greying (gg), merle (Mm), no white spotting (SS), and solid black (KK).
This is Ellie, a mixed breed. She is a solid-coloured merle (no white or tan markings, except for a minor white trim), with an average amount of black markings. If she didn't have the merle gene, she would be solid black. Ellie and her pictures belong to Lauren Pohl.
These two Australian Shepherds show the normal maximum and minimum black markings. They can be compared to the first and the third pictures above. Sometimes dogs have more or less black than these two, but it's uncommon.
These dogs also have tan points and irish spotting (see below).
LIVER MERLES
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 miniature long-haired Dachshunds. The dog in front is a red (liver) merle with tan points. The dog behind shows what the merle
dog would look like if it wasn't bb (liver) and Mm (merle) - it would be a normal black-and-tan.
TAN MARKINGS
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).
These two Dachshunds are tan-pointed merles (a merle dachshund is called a "dapple"). The long-haired dog shows how coat type can distort merle markings (compared to the smooth-hair).
The smooth-haired Dachshund, Tyson, and the photo of him belong to rachellynn40 from the PKC forum.
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 chinchilla gene (or similar), which lightens red. This dog also has white in the irish spotting pattern (see below). The Dunker
photo was taken by June from the PKC forum.
WHITE MARKINGS
Merles can come with any of the white marking patterns - trim, irish spotting, piebald or extreme white. See the page on white markings for more information on how these genes work. The above dogs show the trim, irish spotting and piebald patterns in blue (black) and red (liver) merle, each with or without tan markings.
MASKS
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, like in the pictures above.
SABLE
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).
Catfish is a shaded sable merle Catahoula Leopard Dog mix. She has one blue eye and very faint dark patches on her body. Her ears show the most visible merling. The beautiful Catfish
and her photos belong to Static from the PKC forum.
BRINDLE
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 occur together (the other main one being the Catahoula Leopard Dog). This
dog has tan points, but they are very dull and brownish compared to normal tan points. They also appear to have some darker spots on them. This all points
to this dog being a brindle-pointed blue merle. The brindle appears only on the points, where it is broken up by the merle gene. Brindle can also appear on merles with the creeping tan and saddle patterns.
This Corgi also has brindle points, and again they are very difficult to see. However, he does have some visible stripes on the edge of his ears, and a
few very faint grey stripes on the tan on his cheeks. Regis and these photos belong to Corgi from the PKC forum.
This crossbreed is a very interesting dog indeed. It is possible that he is a brindle merle with sabling. He has what appears to be a merled mask (notice
the lighter and darker grey areas), but
his long coat distorts his other markings, so it's very difficult to be sure. Raggs is owned by Everwood from the PKC forum, as are his photos.
TICKING
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. The dogs shown above have medium ticking and would probably be Tt. Ticking will only show up on a merle if it has white markings.
RECESSIVE RED
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.
DILUTION
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.
MERLE LOOK-A-LIKES
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).
All of these dogs are merle "look-a-likes", but none of their breeds (Australian Cattle Dog, English Cocker Spaniel and Basset Bleu de Gascogne) actually carry merle:
