Canine Coat Color Genetics, Part 1

Black and Yellow Coat Colors

Fred Lanting, All-Breed Judge, SAAB, Sieger/Schutzhund

Since the days of my earliest articles on coat-color genetics, there have been advances in a science field known as Molecular Genetics, utilizing more recent discoveries about DNA, RNA, chromosome mapping, somatic-cell hybrids, linkage mapping, electron microscopy, melanocyte-stimulating hormones, and other details about cells and inheritance that were not “visible” in earlier times.

What we “know” and read today might yet change again, as (or if) funding and research expand. In science, we make conclusions based on logic and observable “facts” but if we advance in technology, we must be ready to modify those conclusions. Meanwhile, readers can easily be confused by co-existing articles in the scientific and popular literature—some older, some newer and based on more recent discoveries. Keep this in mind: while today’s conclusions may be good, better technology might give better conclusions tomorrow. For example: not all coat-color genes have yet been identified—we are still learning the ABCs of DNA.

I was asked to update information on coat colors for the Boerboel clubs in Europe and South Africa, following my judging and lecturing assignments in both. The Boerboel, while a rare mastino-type breed, is quite typical of many others in regards to coat colors. If you’ve never seen one, think of color variations you’ve seen in some hounds and bulldogs: white, black, shades of brown, and dilutes, with these colors often appearing in patches or other patterns. But unlike certain hounds, the Boerboel’s color patterns do not include saddle-markings.  Click images to enlarge the Boerboels, then hit your back button to return to this page. 

It’s always important to define terms. For example, what is your definition of “color”? How about “pigment”? As long as one’s audience understands the context, the words used will also be understood. Also, it helps if you narrow your field by adding a modifier word, so that you get agreement on what is being referred to when you use terms such as “coat color” or “nose pigment.”

Most of the time, I prefer to use that word (color) for the coat, and “pigment” to refer to such outer-integument (skin) areas as the nose bulb, eye rims, lips, footpads, anus, and sometimes nails.

Before going to South Africa, I did not realize that there were black specimens in the Boerboel breed. Whether this was in the breed from the beginning, or whether it is widespread or principally one breeder’s production is almost immaterial, as it is in the breed now, and it might be detrimental to the clubs and to the breed if a purge were initiated. The same can be said for some pigment variations that we call “dilutes.” Most Boerboels I have seen have black-pigmented noses and other integument areas, although the great majority have a base coat color of tan (you can call it fawn or red, yellow, or use a few other terms) that ranges from a light coffee to a deep dark hue. White markings may be present, but for now, discount these when speaking of color. Many geneticists use the superscript y for this yellow-tan.

It is convenient to imagine genes as pearls, and chromosomes as parts of the strings they are on like necklaces or bracelets. Usually, parents give these bracelets to their offspring, but sometimes the “left” part of one bracelet from Dad gets attached to the “right” part of a corresponding bracelet from Mom, and the baby thus “looks” a little like each parent. Selective breeding steers this crossing-over process so that the variation does not get out of hand. Conversely, freedom of choice results in mongrels and, eventually a non-specific “homogenized type” type of dog.

Figure 1: Probable Gene Map for the Boerboel Breed and Many Other Dog Breeds

* minor, “Irish” spotting;

** more, “piebald” spotting; these distinctions are not absolute.

*** a^t seems to be absent in the Boerboel breed but it is common in GSDs, Airedales, scenthounds, etc. It refers to a “black-&-tan saddle” pattern.

- A Boerboel with “a” on both chromosomes is a black dog. I have not studied enough black-coat Boerboels yet to determine whether black is dominant as in the Lab, or recessive as in the German Shepherd Dog.

- The complex brindle k^br pattern is still not well-understood at the DNA level.

- b “changes” black to brown, and d substitutes black with blue, when either is present in “double dose.”

Keep in mind that especially in the days before DNA-test requirements by various registries, there may have been introduced into a population some “illegitimate” color patterns.

I judged innumerable American Pit Bull Terriers (the forerunner of the AmStaff) for many years before coming across one entry (in the pre-DNA days) that had “hound” markings—think Beagle coloring—in addition to a somewhat “houndy-shaped” head. I disqualified him and never heard that my decision was challenged. So, there may be such individuals still in various bloodlines, but as long as the Breed Standard is written well, they should not change our generalizations.

Most Boerboels are some shade of non-black that people have given a variety of names: mahogany, red, brown, tan, fawn, cream, coffee, yellow, near-white, etc. But genetically, those (including brindles, excluding blacks) all still have the same base color whatever name you give it. I usually refer to it as tan, but that could be confusing if you think of “red” Dobermans which have one shade of red-tan on the dorsal portion and another shade of red-tan in the markings. You might want to call all the non-black expressions “brown” but as long as you make clear to your audience what you mean, it does not matter much what word you use. Geneticists often use more complicated letter designations such as ASIP, MC1R, MLPH, TYRP1, etc.

In the first decade following the year 2000, it was determined that “dominant black” (think Labrador Retrievers but also Whippets, Great Danes, and others) is produced by the newly-discovered (found) K locus, on an entirely different chromosome than where the A alleles reside. As long as some gene on a different chromosome does not interfere, all dogs with the dominant K^B allele would be black in those areas of the coat where “dark” is called for, such as all over in some breeds or the saddle in other breeds. In the instances where some alleles at the E locus refuse to allow black genes to be expressed, you can get tan expressed in its place: think some Chessies, Chows, Danes, and others.

Since I mentioned the A locus, let me add a word here about that before we go on with the K alleles. As late as 2005, scientists determined that the “agouti gene” (often referred to as being at the A locus on chromosome number 24) determines, along with “permission” from other genes, whether a dog’s coat will be a clear fawn-tan (a^y), wild or wolf-type banded-hairs (a^w), B&T whether Rottie bi-color or GSD saddle types (a^t), or the recessive type of black (a) as seen in GSDs, Shelties, Schipperkes, and Pulik. The A locus has a major effect on how, if at all, black pigment is expressed/distributed in the haircoat.

Recessive to K^B is the allele designated as k^br (for brindle), a very complex mutation and difficult to diagnose in the laboratory or in test breedings. We used to think of brindle as being solely a function of an E locus allele, but in the present decade have attributed the “striped” appearance to the action of k^br on its MC1R chromosome with help or cooperation from at least one E^m (for mask) or E (no mask) gene at the “E-for-Extension” locus.

Talk about cooperation and dependence! As Dr. Schmutz says, “Some apparent coat colors are not possible unless particular alleles occur at more than one locus.” That is, a dog must have at least one E at the MC1R location on chromosome #5 and two b mutations at the TYRP1 location to be brown.

Dogs in many other breeds, where brindle is never found, could be either the more-dominant “Labrador black” or the more-recessive k^yk^y which could be saddle black-and-tan or bi-color/tan-point in pattern. The superscript “^y” is used to indicate that it allows the phaeomelanin yellow pigment to be expressed in the phenotype—in the tan parts of the two-color coat.

We’ll take a look at liver and blue coloration in Part Two and more in-depth coverage of yellow coloration in Part Three.

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