White spotting- The S series

The White Spotting Series

White spotting on dogs is mostly 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. In technical terms this is known as epistasis. 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.

Currently, only two white alleles have been proven to exist on the S locus:
S – no white
s^p – piebald

A third allele may exist for “extreme white” (s^w), however this has not been proven and so far all dogs with high white have been shown to be homozygous for s^p instead.

The white spotting alleles are thought to be examples of incomplete dominance. This means that a heterozygous dog will express its most dominant gene, but may also be affected by the more recessive one to a lesser extent. For example an Ss^p dog may have some white spotting (see below). However, the relationship between the alleles is complicated and can vary between breeds.

It has recently been shown that some dogs with white spotting do not have an s^p allele at all. These are mostly dogs with “true” irish spotting (in other words, irish spotting that breeds true – this should be made clear further down the page). The allele that causes this pattern has not yet been identified and it is not known if it is also located on the S locus. For the purposes of this site we will refer to this gene as sⁱ, but remember that this may not be accurate.

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. 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. In technical terms, pigment “migrates” to different parts of the body during the development of the embryo, and the S gene determines how far the pigment migrates. Sometimes it simply doesn’t reach the furthest extremities (this can be caused by a minor problem or illness during development), and this can result in a small amount of white trim on a dog without s^p, for example a small chest patch on an otherwise solid-coloured dog.

The white 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.

Residual White and White Trim

A very small amount of white on the chest, toes or tail may occur when the pigment doesn’t migrate fully as the embryo develops. This is known as residual white and has no genetic basis. If a slightly larger amount of white is present then the dog may be heterozygous for s^p, in other words Ss^p. In a breed such as the Newfoundland you may get such a dog from crossing a “Landseer” (piebald, s^ps^p) with a solid (SS). However, in breeds carrying piebald there is no real way to know whether minimal white markings are just residual white or indicate the presence of the piebald gene without genetic testing or test breeding, as piebald heterozygotes may have anything from a tiny chest spot to pseudo-irish markings (see below).

We can assume that the two dogs above are SS and that their markings are just residual white. This is because neither breed comes in piebald or irish spotting. If either of these dogs did have an s^p or sⁱ gene then we would expect to see dogs with much more white being produced in these breeds. As it is, their white is non-genetic and breeding two dogs with white markings in these breeds will not necessarily produce puppies with any white at all.

This Staffie is a possible piebald heterozygote (i.e. carrier of the piebald allele). We cannot know for sure, but this is the most likely explanation for its white chest patch as the Staffie breed is known to commonly have the piebald gene. If this dog were bred to another s^p carrier then some of the puppies may be piebalds and have much more white than either of their parents.

Irish Spotting Pattern

Irish spotting (sⁱ) is the pattern sometimes known as “boston” or “mantle”, although these terms do not always refer to “true” irish spotting. 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.

True irish spotting is caused by an as yet unidentified gene, but we can assume irish spotted dogs to be homozygous for the gene (sⁱsⁱ) as it breeds true. This means that two irish spotted dogs bred together will produce puppies with irish spotting. We can assume that a solid dog bred to an irish spotted dog will produce a heterozygous dog with less white (a white trim, as shown in the section above).

The Aussies, Border Collie and Bernese Mountain Dog shown here are all true irish spotted. None of these breeds regularly come in piebald or extreme white and their white markings breed true (implying they are homozygotes).

“Pseudo” irish spotting may look the same or very similar to true irish spotting, but is in fact not caused by sⁱsⁱ but by Ss^p, i.e. these dogs are heterozyous piebalds. The incomplete dominance of S means that an Ss^p dog may show up to roughly half the amount of white as an s^ps^p dog. These dogs do not breed true and when two are crossed the puppies may be solid, piebald or inbetween. See below for an example of this in Boxers.

Note that not all Ss^p dogs show much white, or in some cases any white at all. The amount of white on a piebald heterozygote appears to vary drastically and some may look exactly like homozygous solids.

The three breeds above (Staffie, Podengo Portugueso and English Pointer) all carry piebald but are not known to carry irish spotting, so these dogs are most likely pseudo-irish. A true irish spotted dog will not usually have white on the hips/knees or underside of the body, so this is another clue that s^p is present.

Finally, a “flashy” irish spotted dog (one with more white than usual) may be caused by a combination of sⁱ and s^p. If a true irish spotted dog also carries an s^p allele then the normal white pattern may be extended. This supports the theory that sⁱ is actually on a different locus, as the two alleles appear to be inherited completely separately. This has been shown to occur in Shelties, where dogs carrying the s^p allele as well as irish spotting can usually be identified by having more white around the neck and underside of the body. An s^ps^p Sheltie has a high amount of white and is known as a “colour-headed white”. Shelties are one breed known to carry both true irish spotting and the s^p allele, but many breeds only have one or the other.

This irish spotted merle Sheltie has a large amount of white and may be a piebald carrier. Piebald carriers are often referred to as “white-factored”.

Piebald Pattern

Piebald (s^ps^p) usually 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).

Because piebald is a recessive gene and heterozygotes (piebald carriers) don’t always have any white markings, it can remain hidden and pop up unexpectedly. Both the Poodle (as shown here) and the Shar Pei, traditionally solid-coloured breeds, occasionally produce piebald.

Extreme White Pattern

The extreme white pattern 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.

So far all extreme white dogs that have undergone genetic testing have been shown to be homozygous for the piebald gene (s^ps^p), just like the piebalds in the section above. However, as there is a fairly large difference between those dogs and the ones shown below, it is possible there is something else going on to cause the high white. In breeds with both true irish spotting and piebald the high white may simply be caused by the interaction between homozygous irish spotting and homozygous piebald (e.g. the Sheltie). In other breeds the cause is less obvious and has led some people to postulate a further S allele – s^w.

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 and White Heads

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 or completely white face. A split face 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.

Ticking and Roan

Any white areas on a dog may be ticked or roaned due to the T gene. The ticking corresponds to the colour the area of the coat would have been if it wasn’t white. See the Ticking page for more information.

Believe it or not, the two Australian Cattle Dogs above are extreme white piebalds. The solid black patches on their heads are their actual markings, and the solid appearance of the rest of the coat is created by very heavy roaning. The Large Munsterlander to the right shows heavy ticking on a piebald dog.

White Boxers

Boxers generally come in what appears to be 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. Boxers do not have the sⁱ allele, but supposedly irish spotted Boxers are actually pseudo-irish – i.e. Ss^p. When two pseudo-irish dogs are bred together some of the puppies will be homozygous piebalds (s^ps^p).

White puppies can therefore be avoided by always breeding pseudo-irish dogs to solid dogs. A solid dog will be SS and therefore there is no possibility of heterozygous s^p dogs. A solid dog will not necessarily have no white markings at all however (see the section on white trim and residual white).
The fashion for “flashy” Boxers in the show ring means that many white puppies are born. Luckily these are now usually sold as pets rather than culled.

“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 alleles. The harlequin gene also causes a similar effect. See the double merle page and merle modifiers page for more information.

White can also occur due to dilution of phaeomelanin by the I locus. Phaeomelanin is red pigment, and the I locus can dilute it to cream or sometimes white. 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 apart from a dog with phaeomelanin dilution 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 Finnish Lapphund and a Siberian Husky) 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 and symmetrical 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). The Husky has actual white spotting as well, but its cheek pattern gives it away as a black and 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 Staffie, you’d be right. She has pink around her eyes, ears, muzzle and underside (a sign of lack of pigment, associated with extreme whites) and a few dark spots on her ears. All the other dogs are recessive reds (ee) or clear sables with phaeomelanin dilution. Note the slight cream sheen on the coat of the German Spitz, Samoyed and Shiba, and the jet black lip and eye rim pigment on all of them. The Shiba has a dudley nose, often associated with recessive red.

“Dog Coat Colour Genetics.” Dog Coat Colour Genetics. N.p., n.d. Web. 24 June 2014.