THE MASK GENE
Eumelaninistic masks (black on the muzzle and sometimes the ears) are caused by the Em gene, 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 gene to have a mask, regardless of which
other E locus gene it carries. Genotypes for masked dogs are, therefore, as follows:
EmE - one mask gene, one normal extension gene
Eme - one mask gene, one recessive red gene
EmEm - two mask genes
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 can be liver, blue or isabella, and can show the merle pattern. 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 geneticists don't currently know the reason for it.
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 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
Brindle (genetically sable) Sloughi with a black mask.
A variety of sable dogs showing the variations in extent of black masks.
Two dogs with masks covering the tan points on their muzzles.
Two dogs with white markings covering part of their black masks. The patch of black on the side of the Bull Terrier's muzzle
shows that it has a black mask underneath the white.
Italian Greyhound with a blue (dd) mask.
RECESSIVE RED
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. If you like, 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 - 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, saddle markings, 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, 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 recessive reds (and the Golden and Labrador probably carry chinchilla, cchcch, as well, which has turned their
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 Irish Setter carries unless it is bred to a dog which doesn't carry
recessive red, but looking at related breeds, it is likely to be tan points (like the Gordon Setter) or dominant black (like most other gundogs).
The Canaan Dog is a breed that comes in both sable and recessive red. It is therefore impossible to tell which these two dogs actually are! However,
this picture was taken in the UK, and genetic testing has shown that the majority of cream and red Canaans in the UK are recessive red, so it is likely
these dogs are too.