THE WHITE SPOTTING SERIES

White spotting on dogs is determined by the genes on the S locus. When we use the term "white spotting" we simply mean white areas, not actually white spots. White spotting can occur on any colour, and will cover up both eumelanin and phaeomelanin. So any dog can have white markings, whether they're black, blue, liver, isabella, brindle, sable, tan-pointed, merle or whatever.

White hair occurs when the skin cells are unable to produce any pigment. The white spotting gene impairs the ability of cells on particular parts of the skin to make pigment, so the skin becomes pink and the fur white. Nails and paw pads will also become pink in areas where pigment is not produced.

There are generally thought to be four alleles on the S locus, each causing different amounts of white. However, these do not seem to account for the huge variation in markings seen on dogs, so it's possible there are more which haven't yet been identified. The four that geneticists are pretty sure exist are:
S - no white
sⁱ - irish spotting
s^p - piebald
s^w - extreme white (or extreme piebald)

The white spotting genes are all thought to be examples of incomplete dominance. This means that a heterozygous dog will express its most dominant gene, but will also be affected by the more recessive one. For example, if one gene is extreme white (s^w) and the other is irish spotting (sⁱ), the resulting dog will have more white than a normal irish spotted dog. In this way, a dog could seemingly have irish spotting or piebald markings without actually possessing the irish spotting or piebald genes.

Even incomplete dominance doesn't seem to cover everything. My own suggestion is that at least one gene needs to be added to the S locus - s^t, white trim. Trim would include, at most, the feet, tail tip, chest and possibly part of the muzzle. This pattern could, in theory, be produced from the genotype Ssⁱ (one copy of no white, one copy of irish spotting), but when we look at some breeds they appear to only come in the trim pattern and never irish spotting. If trim was caused by Ssⁱ then we would expect two dogs with the pattern to produce at least some irish spotted dogs (sⁱsⁱ) if bred together, but this does not seem to happen. Dogs which are homozygous for no white (SS) can sometimes have a very small amount of white on the tail tip, paws or chest, which is simply caused by the pigment not spreading quite as far as it should when the foetus is developing, but trim seems to be caused by an actual gene like the other white patterns.

It's also possible that more than one gene for extreme white exists, due to the variations seen between breeds with the pattern. Some breeds tend to have colour on the head and tail base, and even one or two body patches, but others, for example the Boxer, tend to be almost completely white. However, it's possible that the white patterns already listed are just affected by modifiers which control the amount of pigment, and this would account for variation.

SPREAD OF WHITE

Whichever white pattern a dog has, its white will always follow the same rules of spread. White starts on the farthest "edges" of the dog - the tail tip, the tip of the muzzle, the paws and the tip of the breastbone. This is known as the "trim" pattern. From there it spreads to cover the muzzle and forehead, the front of the chest, the lower legs and more of the tailtip, creating irish spotting. Next it spreads round from the front to the back of the neck, and creeps up the legs and tail. On a piebald dog, only the head, back and tail base may still be coloured. The back colouring is the next to go, followed by the tail base, then the face markings. The ears will always remain coloured unless the dog has a very high amount of white (generally, such a dog will be homozygous for extreme white - s^ws). The ears are generally the last part of the dog to turn white.

Of course the idea of white "spreading" is mainly metaphorical, to give you a picture of how white patterning works. White doesn't spread like this on one particular dog (i.e. you won't get a solid coloured puppy that gradually loses colour as it grows, until it's almost white! Although puppies do often lose or gain a little colour as they grow), it's just to show which areas remain coloured on dogs with more and more white. One way to think of it is that the dog retains colour best in the most important areas of its body - around its internal organs (body and tail base patches) and its brain (ears and face patches) - and loses colour easiest from the parts farthest from these areas.

These rules aren't set in stone - sometimes individual dogs can have unusual white patterns, where, for example, the white on the legs is very uneven, or they have piebald patches in unexpected places, like on the neck or chest. However, in general, they do hold relatively true.

IRISH SPOTTING PATTERN

Irish spotting (sⁱsⁱ) is the pattern sometimes known as "boston" or "mantle". It is typically seen on the Boston Terrier, Great Dane, Border Collie, Boxer, Rough Collie and various other breeds. On a dog with irish spotting, white is found on the legs, the tip of the tail, the chest, neck and muzzle. Many dogs with this pattern have a full white neck ring and a blaze. White on the underside of the body is also a common trait.

PIEBALD PATTERN

Piebald (s^ps^p) produces a coloured head (with or without white on the muzzle and as a blaze), and patches on the body. Generally the base of the tail is coloured, but other than that the patches may be located anywhere on the body (but rarely on the legs).

EXTREME WHITE PATTERN

The extreme white pattern (s^ws^w) consists of a completely or predominantly white dog with just small amounts of colour on its head and sometimes base of tail. Small body patches may sometimes be present too. Sometimes the nose is pink or partly pink, and the eyes may be blue in some breeds due to lack of pigment.

Extreme white can occasionally cause problems when it removes large amounts of pigment from the face and ears. The most common problem is deafness (due to lack of pigment in certain parts of the inner ear, which prevents it from functioning properly), but dogs with exposed unpigmented (pink) skin are also more prone to skin cancer than those with more pigment.

SPLIT FACES

There is thought to be a separate gene or modifier that causes some dogs with irish spotting or the piebald pattern to have a split face. This is when half of the face is white and the other half is coloured. This pattern occurs often on double merles, but it's just a natural part of the double merle pattern, and is not caused by any extra genes. It's only when it appears on irish spotted and piebald dogs that it raises eyebrows.

RESIDUAL WHITE

Sometimes dogs with no white spotting (SS) can have a small amount of white on their extremities. This is caused by the pigment not reaching far enough while the foetus is developing, and is not counted as a white pattern, but just as residual white, because it has no genetic basis. It usually appears as just a small chest patch, white toes or a white tail tip.

WHITE BOXERS

One of the most interesting cases of white patterning occurs in the Boxer. Boxers generally come in the irish spotting pattern, so we would expect most examples of the breed to have sⁱsⁱ on the S locus. However, sometimes Boxer puppies are born which are completely or almost completely white. How these puppies could be regularly born to parents with much more colour perplexed Boxer breeders for a long time.

However, we can now provide an answer to this. Breeders have been breeding for "flashy" dogs, which are irish spotted dogs with more white than the normal pattern usually produces. It turns out that these dogs have more white than normal because they are not homozygous for sⁱ. Their genotype is, in fact, sⁱs^w, so they have one gene for irish spotting and one for extreme white. This is what causes them to have extra white - the incomplete dominance that sⁱ has over s^w results in a dog with more white than normal irish spotting. When two of these flashy dogs are bred together, one in four of the puppies will have the extreme white pattern:

White puppies can therefore be avoided by always breeding flashy dogs to normal irish spotted dogs rather than other flashy dogs (a normal irish spotted dog will be sⁱsⁱ and so will not have an s^w gene to pass on to its offspring, therefore none of its offspring can possibly be homozygous for s^w, so none can be white).

"FALSE" WHITES

Sometimes white can occur on dogs separately to the S locus white spotting. One example is as part of the double merle pattern. A double merle will almost always have more white than its parents, and will often appear to have the piebald or extreme white pattern when in fact it does not carry those genes. See the double merle page for more information.

White can also occur due to dilution of phaeomelanin by the C locus. Phaeomelanin is red pigment, and the C locus can dilute it to cream (c^chc^ch) or sometimes white (c^ec^e). Breeds such as the Samoyed have this second type of dilution, so they appear completely white but in fact it's not due to white spotting. They are in fact recessive red (so they cannot produce any black pigment) with dilution of their red pigment to white, resulting in a solid white dog with black nose pigment.

The main way to tell a dog with extreme white spotting (s^w) apart from a dog with phaeomelanin dilution (c^e) is to look at the pigment on the nose, lips and eyerims. A dog with extreme white spotting is likely to be missing some pigment in these areas, so they will be partly or completely pink. A dog with phaeomelanin dilution will have solid black in all these areas (possibly with a dudley nose, which are common on dogs with dilution - see the nose page).

These two dogs (a Schnauzer and a Siberian Husky) have a pattern known as "black and silver". They are genetically black and tan (a^ta^t), but with dilution of their tan points to white. It can be easy to mistake diluted points for white markings, but points will generally be in a very regular pattern, with two chest spots and a vent spot, and spots above the eyes. The white will also be confined to the sides of the muzzle and not the top (except in a dog with creeping tan).

One of these dogs is not like the others . . . but which is it? All are "false" whites except for one, which is an extreme white piebald. If you guessed the Jack Russell Terrier, you'd be right. He has pink around his eyes (a sign of lack of pigment, associated with extreme whites) and a few very faint red spots on his ears. All the other dogs are recessive reds (ee) with phaeomelanin dilution (c^ec^e). Note the slight cream sheen on the coat of the German Spitz and the jet black nose, lip and eye rim pigment on all of them. The first Samoyed photo was taken by Kenzie, and the second Samoyed, JRT and Westie photos were taken by June.

ALTERNATIVE THEORY

Just like with the albino series, genetic research has failed to provide much evidence for the white spotting series as defined above. However, it is pretty certain that white spotting is caused by this locus, it's just the exact genes present which are the subject of debate. Recent research suggests that all white patterns are in fact caused by one gene - s^p, the piebald gene we looked at above. On this theory, irish spotting would be produced by one piebald and one solid gene, but it isn't clear how extreme white spotting is produced.

I'll keep you up to date on any developments in this field! But for now, the generally accepted white spotting theory is OK for our purposes. Just be aware that it is probably incorrect as it stands.

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