Merle Modifiers

A merle modifier is a gene that, when inherited along with merle, will affect the way the merle pattern appears. A dog with a merle modifier but no merle gene will not be affected at all. It's thought that merle modifiers are inherited separately from merle and appear on their own locii.

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Tweed

Harlequin

Pseudo Harlequin

Quick Summary

Further Info and Links

Tweed (Patchwork)

Tweed is a merle modifier that turns the diluted parts of the coat (which should be grey in a normal merle, or light brown in a liver) into a variety of brown, grey and tan shades, resulting in a dog with a coat pattern that may be reminiscent of an African Wild Dog (although their colour has a different genetic basis). Tweed is known to occur mostly in Australian Shepherds and Catahoula Leopard Dogs (where it's known as "patchwork"). It is possible that the tweed mutation occurred in just one of the breeds to begin with and then spread to the other, as interbreeding between the two breeds is common in the USA, or else it simply occurs as a mutation every now and again in breeds in which a high number of merles are bred (more merles increases the possibility of mutation, and merle is known to be a fragile gene that mutates more frequently than most others). Either way, it's a rare and striking pattern. It is often assigned its own locus, Tw, and is thought to be dominant.

Some people claim the existence of two genes that cause the tweed pattern. One causes very random patterning with white patches amongst the grey, brown and black, as on the second dog above. Australian Shepherd breeders know this as "harlequin", although it is different to the harlequin found in Great Danes (see below).
The second gene does not include white patches. Sometimes it is also more regular than the "harlequin" type, and the different colours can cover larger areas of the dog. It is not known whether these two types of tweed are caused by separate genes or variations (or modifications) of the same gene, however in reality it is most likely that a large number of different tweed mutations exist across different breeds.

Tweed can occur in any of the patterns shown on the merle page (with tan markings, white, brindle etc). However, interestingly, it has never been reported to occur on a double merle. It's not known why this is.

A Different "Tweed"

You may also hear of merle dogs with very dark or heavily "ticked" base coats being described as "tweed". This is not the same gene that causes the "patchwork" type of tweed seen in Catahoulas and Aussies, and is most likely just a normal variation of the merle phenotype. This terminology is most often used to describe Border Collies.

The Catahoula Leopard Dogs shown here are clear tweeds ("patchwork"), and the dog in the third picture is most likely a blue (dilute) tweed. Note the clear patches of different colours, much more extensive than the normal range of dilute patches on a standard merle, and also the white areas on the first dog (which don't match any normal white pattern). These photos were kindly submitted by Melanie Zelisko of Chigger Run Catahoulas.

Tweed can be harder to discern on a long-haired dog. This Australian Shepherd Dog, Koda, is possibly a tweed, due to the large areas of brownish patching. However, this could also be simply discolouration/bleaching due to the hair length. True tweeds also often have "disrupted" tan points rather than Koda's neat ones (see below).

These two closely related dogs are great examples of brindle tweeds. The puppy has brindle points, which have been disrupted by the tweed gene (see the strange tan patterning on the ear and chest/neck). There is also a clear brown patch on the head, which is a liver-like shade. This colour should be impossible on a black-pigmented dog, but it does sometimes occur on tweeds and must be to do with the effect of the modifier on the eumelanin pigment. The flecks of black on the tan areas indicate brindling, which is broken up by the merle gene. The adult dog, Gus, is an older sibling, and probably a "full" (all-over) brindle tweed, judging by the wide range of different shades in his coat. Both dogs are owned by David Jenkins and their pictures were submitted by Holly Moody.

Harlequin

The harlequin modifier turns the areas between the dark patches on a merle into pure white (occasionally with some grey ticking or patches). This means a blue (black) merle will become white with black patches, because all the grey in its coat is turned to white. What's even more interesting about this gene is that it also affects phaeomelanin (red), not just eumelanin (black, liver, blue, isabella) like other merle modifiers. That means that a sable dog with the merling gene won't just be affected on the parts of its coat that are black (tipping, mask etc), but the whole of the coat will be harlequin. It will become what's known as a "fawnequin" - tan (sable) patches on a white base, with black patches where it would have shown black merling. The patches on the fawn section of the dog are located where the dog would have had black patches if it had been a solid "blue" (black) merle.

Harlequin occurs on its own locus - H. It is dominant, so H is the harlequin gene and h is the non-harlequin gene. It is inherited separately to merle.

Every single dog with the harlequin gene and the merle gene will display harlequin. The only genes that would stop harlequin from being visible would be ones that turn the dog completely or almost completely white, and these are not present in Great Danes (except for double merle). Any other dog, whether it is brindle, sable, masked, or whatever, will become a harlequin. If it has white markings, the resulting harlequin pattern will simply lack dark spots where the white markings are supposed to be.

The harlequin gene will not work its own - it requires the merle gene as well. So a normal, heavy- or medium-marked harlequin is HhMm (heterozygous for both harlequin and merle). It's thought that harlequin is an embryonic lethal gene, meaning that two copies of it will cause the puppy to be re-absorbed into the womb. This means we can assume that all harlequins are heterozgous for harlequin, and it is certainly true that all harlequins that have undergone genetic testing so far have been heterozygous. A very lightly marked harlequin (with few dark spots) may be homozygous for merle. This is a double merle (see double merle page). A non-harlequin dog, such as a solid black or a black with irish spotting (known as a "mantle" in Great Danes) may carry harlequin but not display it because they don't have the merle gene. Such a dog would be Hhmm (heterozygous for harlequin, homozygous for non-merle), and if bred to a merle may produce harlequin puppies.

The one colour that a harlequin dog cannot have patches of is merle itself (meaning a patch that is grey with black sections). Dogs that appear to be harlequin with merle patches ("merlikins"/"merlequins") are in fact double merles or piebalds without the harlequin gene (double merle causes a piebald-like pattern, and the patches may have torn edges and appear very harlequin-like). "Merlikin" is, therefore, an incorrect term, left over from when harlequin was believed to be a white pattern, not a merle modifier.


Typical harlequin (HhMm). The second picture shows dilute patches, which can occur on harlequins just as they do on regular merles.


Harlequin with the dilution gene, turning the patches and nose blue (ddHhMm). Dilute harlequins are sometimes known as "porcelains".


Harlequin with the sable and masking genes (AyAyEmEmHhMm), otherwise known as a fawnequin.


Harlequin with the brindle gene (sable is also necessary for the brindle to show all over the body) (ayaykbrkbrHhMm). The brindle will be broken up and patchy because of the merle gene (see merle page).


Harlequin with irish spotting (HhMmsisi). The red line shows the border between the harlequin markings and the white spotting.


Merle without the harlequin gene (hhMm)


Lightly-marked harlequin, most likely a double merle (HhMM)


Merlequin pattern. Although this looks similar to a harlequin, it's actually a regular merle with piebald markings (spsp). Note how the patches are confined to the back and aren't a solid colour. The same look can be caused by double merle (MM) also, and this is more likely to be the explanation for this pattern in countries where piebald isn't a recognised Dane pattern (e.g. USA).


These two pictures show Great Danes with normal black harlequin. The first dog has the irish spotting white pattern (there are no spots on the front of the neck, the lower legs or the face). Black (blue merle) harlequin is the only type accepted by the Great Dane standard, but the other colours shown in the illustrations above do occur in the breed. Both of the dogs in the photos also show greyish areas which are reminiscent of the original merle colouration. Dilute patches are common in harlequins, but dogs with jet black patches and a solid white base (with no grey areas or ticking of any kind) are preferred.


The first harlequin here has very large patches, which are generally not preferred in the breed. The second dog shows a pattern that is currently much more fashionable in the show ring, with smaller, more "torn" patches and a large amount of white. Note the large dilute patch on the hip and the clear mantle white pattern.


This harlequin is most likely a double merle as well, due to the high amount of white.


Mauser, the handsome Great Dane above, is an example of a "merlequin", most likely a double merle (MMhh - two copies of merle allele, no copies of harlequin allele). Note how his patches are merled, not solid black like on a harlequin. His pictures were submitted by Jennifer McLean.


"Fawnequin" (fawn harlequin), submitted by Sally.

Pseudo Harlequin

The "base" colour on a merle can vary greatly, from deep blue to almost white. When the base is so pale that it appears white, the resulting dog can look very much like the harlequin Great Danes above. In Collies and Shetland Sheepdogs these dogs are referred to as harlequins, however the actual harlequin gene occurs only in Danes. It's possible that a different modifier is at work here, but it has not yet been located, and these dogs test negative for the H allele.


Blamorder Back with a Dream, owned by Abby Lusty

Quin, the Smooth Collie above, shows "pseudo" harlequin patterning. Note how the tan (phaeomelanin) on the legs is also affected, implying that this is caused by something very similar to the Great Dane harlequin gene (normal merle would not affect the tan points).

Quick Summary!
No time to read the whole thing? Here's the quick version!

There are thought to be two main merle modifiers (with possibly a number of variations of each): tweed and harlequin. Both work only on merle dogs and have no effect on non-merles.

Tweed causes patches of random colours to appear in the merle coat, and can sometimes also cause disruption of tan points so that they seem to "bleed" into the main coat. In some breeds the term "tweed" is sometimes also used to refer to "muddy" merles (ones with dark or heavily ticked backgrounds), but this is not the same as the true tweed modifier, which occurs mostly in Australian Shepherds and Catahoula Leopard Dogs.

True harlequin occurs only in Great Danes, although a similar pattern is sometimes seen in Collies and Shetland Sheepdogs. Harlequin causes the grey background on a merle to be diluted to white, leaving dark patches on a white background, and can affect any type of merle (including brindle, sable etc, as well as double merles). It is a dominant embryonic lethal gene, so all HH dogs are reabsorbed into the womb and only Hh dogs are born.

Further Info and Links

The gene causing harlequin in Great Danes is PSMB7, located on chromosome 9. Although this gene has been found, it's still unclear precisely how it interacts with the SILV gene (merle) to cause the loss of the background (dilute) pigment. "Harlequin" Collies and Shelties have also been tested for PSMB7 mutations but so far have tested negative, and no genetic basis for tweed/patchwork has yet been found.

Links to studies:
A missense mutation in the 20S proteasome beta 2 subunit of Great Danes having harlequin coat patterning: http://www.sciencedirect.com/science/article/pii/S0888754311000164