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George Sofronidis wraps his A–Z series with “P is for Pied,” explaining white spotting, the S locus, and coat color genetics.
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After a full A to Z journey through canine coat color, we arrive at the final letter of the alphabet with a pattern that is instantly eye-catching, widely discussed, and still surprisingly misunderstood: P is for Pied.
Charlie, my Boston Terrier, thinks this is a fitting grand finale because anything with strong visual impact clearly deserves top billing. Mary, my Griffon, believes a proper finale should involve elegance, mystery, and the quiet confidence of someone who already knows she is the main character. Between the two of them, I think we’ve captured the spirit of pied quite well.
The Pied, or S locus, is one of the key genetic regions involved in white spotting in dogs. It helps to explain why some dogs have only a tiny flash of white on the chest or toes, while others have large white sections covering much of the body. And importantly, it reminds us that white markings are not merely decorative. They are the visible result of how pigment-producing cells develop and spread across the body.
So, as we close out this A to Z series, let’s finish with a pattern that is both beautiful and biologically fascinating.
Most white spotting in dogs is associated with genes on the S locus, which houses the MITF gene. This gene plays a major role in the migration of pigment-producing cells during the development of the skin.
When those cells fail to reach certain areas, pigment is not produced there. The result is pink skin and white hair in those parts of the body.
That means white spotting is not the same thing as placing “white paint” over another color. Rather, those areas lack pigment altogether. This is why white can occur on top of virtually any base color or pattern. A dog may genetically be black, blue, liver, Isabella, brindle, sable, tan-pointed, merle, or any number of other color combinations, and the white spotting can still cover those areas.
In technical terms, this is an example of epistasis—where one genetic effect masks or hides the expression of another.
Or, to put it less formally, the rest of the color genetics may still be present, but the S locus has decided that certain areas simply won’t be joining the pigment party.
So far, the two white-related alleles most commonly discussed at the S locus are:
S – no white or only very minor white
sp/sp – piebald
In classic genetic notation, the relationship is generally written as:
S > sp
In simple terms, sp is the more dominant form, while sp is associated with piebald and more extensive white spotting.
There has long been discussion about whether a third allele for extreme white, often written as sw, may exist. However, this has not been conclusively proven, and many dogs with very high white expression have instead been found to be homozygous for piebald-associated variation.
There is also evidence that some dogs with white spotting do not carry piebald in the expected way at all. This appears especially relevant in some breeds with so-called true Irish spotting, suggesting that additional, as yet unidentified, genetic factors may also be involved.
As with many things in canine genetics, the broad picture is clear, but the finer detail is still being worked out. Genetics does enjoy keeping breeders humble.
One of the most interesting things about white spotting is that it tends to follow a fairly consistent pattern of spread.
White generally begins at the farthest “edges” of the dog:
From there, it tends to expand inward and upward. It may spread over the front legs, chest, and neck, onto the face, and across the body. In dogs with more extensive piebald expression, color may remain only on parts of the head, back, and tail base. As the amount of white increases, those remaining colored areas may reduce even further.
The ears are often among the last places to retain color, which is why many high-white dogs still have colored ears long after the rest of the body has gone white.
This progression helps to explain why white spotting can range from minimal trim to dramatic piebald or particolor patterns. It is not random. It follows a developmental pattern.
This is where the conversation becomes more interesting.
Piebald is often described as behaving in a semi-dominant or incompletely dominant way. In practical terms, that means a dog with one copy may show some white spotting, while a dog with two copies may show more extensive white patterning. However, expression can vary by breed, and the way the trait appears is not always perfectly predictable from a simple genotype alone.
A general guide is:
S/S – little or minimal white
S/sp – white spotting may be present
sp/sp – piebald or more extensive white expression is more likely
However, breed background and additional modifying factors can influence what the dog actually looks like.
That is why two dogs with similar test results may not appear identical in their markings. It is also why breeders benefit from looking at both the dog in front of them and the genetics behind it.
Charlie would like to point out that not every white chest mark deserves a full philosophical debate. Mary, naturally, disagrees and feels all markings deserve serious consideration if discussed with enough style.
The S locus can combine with nearly any color or pattern because it works by removing pigment from specific areas altogether.
This means it can hide both:
This interaction becomes especially important when evaluating dogs with patterns such as:
For example, ticking and roan are only visible where white spotting already exists. No white area, no ticking or roan expression to observe. Likewise, a classic tricolor pattern relies on the presence of both pigment types and white spotting together.
The S locus doesn’t replace other color genes. It simply determines where those colors are allowed to show.
Like many coat color genes, the S locus is not just about aesthetics.
Large areas of absent pigment, especially when combined with other pigment-inhibiting variants, may be associated with an increased risk of deafness or abnormal eye development in some dogs. This does not mean every piebald dog will have a health issue—far from it. But it does reinforce an important point: color genetics should be approached with understanding, not just preference.
Responsible breeders know that the goal is not simply to produce a striking pattern. It is to understand what lies behind it and to make informed decisions accordingly.
In other words, pretty matters. But knowledge matters more.
For breeders and owners wanting greater clarity, Orivet offers the Pied test as “Pied (BOTH SINE and REPEAT VARIANTS).”
This gives breeders the opportunity to better understand the genetic basis of white spotting in their dogs, particularly when planning matings, assessing expected outcomes, or working through color patterns that may not be obvious from appearance alone.
The test is also available in a number of Orivet Full Breed Profiles, allowing it to be considered alongside other important breed-specific traits and health risks.
That is especially useful because pied can sometimes be more complex than it first appears. A dog may carry relevant variation without dramatic visible markings, while another may show extensive white that raises important breeding and health considerations. Having access to the test allows breeders to move from guesswork to information—and that is always a better place to be.
You can find the test here:
And so, we come to the end of the alphabet.
If this A to Z has shown us anything, it is that canine coat color is never just about what we see on the surface. Behind every patch, point, shade, and pattern is a story about inheritance, biology, variation, and the incredible complexity of the domestic dog.
The S locus is a perfect way to end because it reminds us that white is not “just white.” It is a developmental pattern, a genetic mechanism, and in many breeds, an important piece of the breeding puzzle.
Charlie has approached this series the way he approaches life in general—with enthusiasm, confidence, and a strong belief that being noticed is always a good thing. Mary, on the other hand, has treated the matter with a level of dignity that suggests she has personally curated the canine color spectrum and is mildly disappointed in any dog who hasn’t kept up.
Between them, I think they’ve taught us the perfect final lesson: color may catch the eye, but understanding is what truly matters.
And that, for this alphabet, at least, is a wrap.
To celebrate the final installment of my A to Z of Canine Coat Color, I’m giving five lucky readers a chance to win.
We have five Full Breed Profiles to Give Away (one per person). Each Full Breed Profile is valued at over $130 each!
To go in the draw, simply:
Email me with “S Locus Giveaway” with your name and breed.
Entries Close: May 15, 2026
Winners Announced.
Whether you are entering for the prize, the genetics, or just to prove to Charlie and Mary that readers do in fact pay attention all the way to the end, I’d love to have you join in.
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