The Mask Gene
Eumelaninistic masks (black on the muzzle and sometimes the ears) are caused by the Em allele, which is found on the E locus. It is the top dominant of the E series, so that means a dog only needs one Em allele to have a mask, regardless of which other E locus allele it carries. Genotypes for masked dogs are, therefore, as follows:
EmE – one mask allele, one normal extension allele
Eme – one mask allele, one recessive red allele
EmEm – two mask alleles
If a dog does not have a mask, it must have the genotype EE (two copies of the normal extension gene), Ee (one copy of normal extension, one of recessive red), or ee (two copies of recessive red, see below).
Masks can appear on any dogs that are genetically sable, tan-pointed, saddled or agouti (wolf grey). That means that the presence of a mask on a dog with the mask gene depends on the A and K series. In order to display a mask, a dog cannot be dominant black (KK, KKbr, Kk) on the K locus, because dominant black stops the A series from being expressed, and obviously, a black mask on a black dog is not going to be visible. If a dog is brindle or non-solid black on the K locus, they can express the A locus, so will be sable, agouti, saddled, tan-pointed or recessive black. All of these will show masks except for recessive black (again, a black mask isn’t going to be visible on a black dog, even if it is there!).
Masks consist of eumelanin (black pigment), so can be affected by any gene that changes the colour or intensity of eumelanin. This means a mask will follow the main pigment and can be liver, blue or isabella, and can show the merle pattern as well. There is one exception though – masks are not affected by greying (at least, not by the greying gene found in breeds such as the Kerry Blue Terrier). This is a bizarre anomaly, and one for which there is currently no explanation.
On a tan-pointed dog, masks can be detected by looking at the facial points. Tan normally occurs on the sides of the muzzle and above the eyebrows, but a dog with a mask may have all or part of these points covered up by black. Usually, however, the tan on the neck is usually still visible.
Masks can vary greatly, covering anything from just the end of the muzzle to the whole of the muzzle, eyebrows and ears.
On sable dogs, black masks may also cause black hairs on the chest and/or the back and tail. Sables with black tipping often have black masks, so it is possible that masks in fact cause most of this tipping. However, there is probably another modifier that helps with this, because not all black-masked sables have extra black, and indeed, many breeds with masks come only in clear or almost clear sable (e.g. Pugs).
Examples of Masks
Sable Leonberger with a black mask.
A variety of sable and brindle dogs showing the variations in extent of black masks. Note the black extending down the front of the chest on the Tervueren.
This crossbreed shows black chest markings on a short-haired dog. Interestingly, the black markings follow the same pattern as tan points or urajiro.
Three dogs with masks fully or partially covering the tan points on their muzzles.
An Akita with white markings covering part of its black mask.
Whippet and Tibetan Mastiff with blue (dd) masks. On the Tibetan Mastiff the mask covers up the tan points on the muzzle.
The other two E locus genes are E (normal extension) and e (recessive red). Normal extension simply allows a dog to express its other genes on other locii normally. To put it simply, it has no effect on the dog. Recessive red, however, produces a much more visible effect.
A dog which is homozygous for recessive red, so has the genotype ee, will be completely red. Its nose will remain black (or liver, blue or isabella, whatever its eumelanin pigment colour is), and so will its eye rims and lips, but the rest of its coat will be solid red (with or without white markings and ticking). This is because the recessive red gene, in effect, gives the skin cells a disability (but not a harmful one!) – it stops them from being able to produce eumelanin. It is therefore impossible for a recessive red dog to have any black (or liver, blue or isabella) in its coat.
Recessive red, although recessive in its own series, is dominant over almost everything else. Dominant black, sable, tan points, wolf grey, merle, and any other pattern with black in it will be turned to solid red by the recessive red gene. This, in a way, is the danger of recessive red – it’s a dog breeder’s worst nightmare because it can mask so much. It’s impossible to know from looking at a recessive red dog whether it carries sable, tan points, brindle, or any other A or K locus gene, and so impossible to know what it will pass down to its puppies unless you know its family history. A recessive red dog could even be merle while still appearing solid red, and this is a problem that has arisen in Pomeranians. Merle is a recent addition to the Pomeranian breed, and breeders are worried that double merles will be bred by accident because recessive red is so common in the breed. The only way to tell if a recessive red dog is merle is if it has blue eyes (which not all merles do), so it would be easy to accidently breed two merles together if you had a hazy knowledge of genetics and thought merle was always visible because it is dominant. See the double merle page for information on why merle to merle breedings are dangerous.
Recessive red only occurs in particular breeds, but it is not completely clear which breeds those are. The problem is that a clear, unmasked sable can look identical to a recessive red. This means that solid red dogs must undergo genetic testing to determine whether they are sables or recessive reds; there is no other way to tell the two genes apart. However, if a breed always comes in solid red with no black hairs at all, and never comes in shaded or tipped sable or with a mask, you can be relatively sure that the breed carries recessive red (although remember a recessive red dog can still carry sable, just not express it! There is debate about this though – Little, whose work most modern dog genetics is founded on, believed that a puppy carrying sable and recessive red would miscarry. To my knowledge, this has not yet been proven or disproven). Golden Retrievers and Labradors are two such breeds, and genetic testing has shown that they do indeed carry only recessive red (along with a few other gundog breeds). There are some breeds, though, that carry and express both sable and recessive red, such as the Pomeranian.
However, there has been a hypothesis suggested (by Sue Ann Bowling – http://bowlingsite.mcf.com/Genetics/Genetics.html) that recessive red and sable can be told apart by whisker colour. A recessive red dog will have red whiskers and a sable dog will have black. This has not been proven, but seems to make sense.
All three of these dogs are probable recessive reds (and the Labrador probably carries an I locus allele as well, which has turned its red into cream). We know the Labrador must be genetically dominant black, because all Labradors are (see the Labrador case study in the introduction section). The recessive red gene overrides the dominant black, turning it to red. It is impossible to know what the other breeds carry unless they are bred to a dog which doesn’t carry recessive red.
The Dachshund is a breed that comes in both sable and recessive red. It is therefore impossible to tell which these three dogs actually are! The third dog does appear to show a little black on its ear, in which case it is a sable, but the other two have no visible black hairs at all.
Although the recessive red allele does not directly affect the colour of the nose, eye rim, lip and nail pigment, it is common for this pigment to fade from black to grey or pink as the dog ages. This may well be another indicator that a dog is recessive red rather than sable – although as sables may also have poor pigment, it is not foolproof.
Above are examples of (presumably) recessive red dogs with pigment loss. Note how in all the examples, the lip pigment seems to stay darker than the nose or eye rims. In some dogs the whole pigment fades to a grey/blue, and others develop a “dudley” nose, where there is a strip of pink in the centre. It can be easy to mistake some of these dogs for blues, but when compared to a true blue (see Dilution/D-locus page), the difference should become clearer. Of course, it is entirely possible for a recessive red dog to also be a dilute – but in this case the dog would be born with blue pigment.
Liver (bb) recessive red dogs show more significant pigment loss than black (BB) recessive red dogs. We know this Bracco Italiano is a liver because the breed does not come with black pigment at all. Note how there is very little pigment – it is almost entirely pink. This can also be seen on breeds such as the Pharaoh Hound and Cirneco dell’Etna. In fact, most recessive red dogs with liver pigment will appear to have pink pigment. It is possible that the recessive red gene affects liver eumelanin to a greater extent than black eumelanin.
The Nova Scotia Duck Tolling Retriever comes in both black and liver pigment. The dog on the left shows black (but faded) pigment, and the dog on the right is a liver (note the bright pink again). Tollers can be phenotypically either sable or recessive red. Without genetic testing it is very difficult to tell, but for our purposes we will assume both dogs above are recessive reds as they have clear coats (no shading). The faded pigment on the dog on the left is a very strong indication that it is a recessive red as this would be much more unusual in a sable.
Grizzle and Domino
Recently a new gene has been discovered on the E locus – Eg. This gene is specific to the Saluki, where it is known as grizzle, and the Afghan Hound, where it is known as domino. It has not yet been identified in any other breeds, although it is possible it also occurs in other sighthounds.
Grizzle looks very much like a shaded sable (see A series), although the shading is often very heavy and confined to a specific pattern with a widow’s peak on the head. The shading colour varies from black/grey to brown. Shaded sables in breeds such as the Rough Collie look very similar to grizzle Salukis but the gene is not the same.
Grizzle can only occur on dogs that are atat on the A locus. In other words, the dog must have the tan point allele as well as the grizzle allele. A dog with the grizzle allele (Eg) but not the tan point allele will not be grizzle and will just express its A locus normally.
In this way, Eg can be seen as a modifier of at, in the same way that harlequin (H) is a modifier of merle. Is it possible that other hypthesised modifiers of at, such as creeping tan and the saddle pattern, are also located on the E locus? This is something we don’t know yet, but it is certainly a possibility.
No time to read the whole thing? Here’s the quick version!
There are four known alleles on the E (Extension) locus – E (normal extension), Em (eumelanin mask), Eg (grizzle/domino) and e (recessive red).
*- E (normal extension) has no effect on the phenotype.
*- Em produces a eumelanin mask on the muzzle and/or ears (only visible on sables, tan points and agoutis – see A locus page). Sometimes this may also spread to the tail and the front of the chest.
*- Eg is known as grizzle or domino and occurs only in Salukis and Afghan Hounds. It is a modifier of tan points, so can only be expressed when the dog is also atat on the A locus.
*- e is recessive red, which turns all eumelanin in the coat to phaeomelanin. Recessive red dogs will always be solid red (with or without white), regardless of which alleles are present on the other locii, meaning recessive red can even mask merle. It is genetically impossible for a recessive red to have any black/liver/blue/isabella in its coat, although eumelanin in the eyes and nose is generally unaffected.
“Dog Coat Colour Genetics.” Dog Coat Colour Genetics. N.p., n.d. Web. 24 June 2014.