By Mary Peaslee
According to the breed standard, English Shepherd coat patterns are categorized as follows:
* Black and white
* Sable and white
* Black and tan
* Tricolor (black and tan with white)
This looks pretty straightforward - four traditional patterns, how complicated could they be to sort out? Well, truthfully, it is pretty simple. I'm now on my fourth attempt to get this article written, however, because although the genetic underpinnings of coat color and markings aren't complicated, to really grasp what is going on requires that we change the way we categorize things.
First of all, forget about white markings initially. White markings are regulated by a set of genes that is completely separate from the genes which determine coat color (white is not a "color" - it is the absence of color). Coat color and markings will be easiest to understand if we consider the color on the dog first, then factor in white markings after that. So, this means our groups are now:
*Black and tan
*One of the above with white markings.
Second, we English Shepherd people have a problem by tradition, we have used the word "sable" to describe dogs that are not, in fact, "sable" to the rest of the dog world. The confusion is due to our failure to differentiate between a color and a pattern.
Color: there are only 2 colors on the dog coat-color palette - black and yellow/tan. That's it! Some dogs have only one color in their coat (solid colored dogs); other dogs have both colors in their coats.
Pattern: if a dog has both colors in their coat, the way the color is distributed is a pattern.
This is pretty obvious most of us have been comfortable with the difference between a color and a pattern since grade school! Here's the catch, though - "sable" is technically a pattern (black tipping on yellow colored hairs), but we in the English Shepherd world have used "clear sable" to describe dogs that are actually, from a genetic point of view, a solid color (yellow/tan only). It is important to understand the distinction between a "true" sable (which is a pattern) and a "clear" sable (which is a solid color), because these markings are regulated by different sets of genes. This means our groups should really be as follows:
*Yellow/tan only ("clear sable")
*Two color dogs ("true" sables and black/tans)
*One of the above with white markings
Now we're ready to start looking at genes. It is easy to get overwhelmed at this point because there are at least eight important genes which interact to determine coat color and markings in English Shepherds. These genes each have a name and are designated by a letter (often the first letter of the name). Every dog inherits something at all eight gene loci (loci = the location of a gene on a chromosome). Fortunately, it isn't necessary to unravel all eight loci at once. We'll start by considering just three - the three which determine which of the first three categories your dog fits into.
SINGLE COLOR DOGS
"Clear sable" dogs are the easiest to understand genetically. Clear sable dogs only produce the yellow/tan form of the pigment melanin due to the action of a recessive gene ("e") at a locus named "Extension". This locus regulates the extent of black pigment in the coat by modifying a receptor on the pigment-producing cells. Dogs that are homozygous for the recessive gene ("ee") produce defective receptor proteins, and do not make any black pigment in their coats. Because a clear sable dog must be homozygous for this trait, it will always pass on this recessive gene to its offspring. When two clear sable dogs are bred ALL of the offspring produced will be clear sable. Also, because clear sable is a recessive trait, it is possible for the gene to "hide" for several generations, and then show up in a litter when two dogs carrying the gene are bred.
Solid black coats, by contrast, result from the action of a dominant gene, "K," at the Dominant Black locus. Unlike clear sables, solid black dogs have the capacity to produce both types of melanin - you just don't see the lighter (yellow/tan) pigment in their coat because it is masked by black. Because solid black is a dominant trait, a dog only needs to inherit one copy of the K gene to be solid black. There is one caveat to this - black dogs must be able to produce black pigment (obviously!), so they can not inherit two copies of the recessive "e" gene; "ee" dogs are always yellow/tan (clear sable), regardless of what other color genes they inherit.
TWO COLOR PATTERNS
The markings on dogs with both black and yellow/tan pigment visible in their coat are controlled by a gene locus named "Agouti." There is a prerequisite to the expression of an agouti-controlled pattern, however: these dogs can not have either the dominant black "K" gene or two recessive "e" genes. The Agouti genes regulate the production of the Agouti protein which in turn influences the distribution of black pigment in the coat. The dominant gene at the Agouti locus is the gene for sable pattern - yellow/tan hairs with black tips. A dog needs only one copy of the sable gene (ay) to express this trait. The "tanpoint" pattern of black and tan dogs is the result of a recessive gene (at) at this locus. Black and tan patterned dogs must have two copies of the "tanpoint" gene. For breeders, this means that two black and tan dogs will never produce sable pups, but sable parents may produce black and tan pups (if both parents carry the tanpoint gene).
There is one additional two-color
pattern common in English Shepherds: the "saddle" pattern. There
is dispute over whether saddle pattern is regulated by a specific "saddle"
gene, or whether it is due to a modification of the normal tanpoint pattern.
Saddle pattern dogs have the same basic pigment distribution as black/tan
dogs but the tan areas are larger - instead of "shepherd's spots"
and tan cheeks, the whole head will be tan; instead of tan socks, the
entire leg is tan. Saddle pattern dogs generally have a "typical"
tanpoint pattern at birth, but as they mature the black areas recede and
the tan markings expand. Whether saddle pattern is due to modification
of the tanpoint gene or a separate gene in the Agouti series, it seems
to fall between sable and tanpoint in dominance - sable is dominant to
saddle; saddle is dominant to tanpoint.
As the confusion over saddle and tanpoint pattern illustrates, things in biology are rarely as neatly compartmentalized as the textbooks make it sound! A few of you are probably looking at your dog thinking, "how do you explain that color?!" Well I may not have the answer, but there are a few common exceptions and qualifications to the neat groupings above, one of which may help explain what is going on with your dog.
For instance, clear sable dogs illustrate a principle I haven't mentioned yet - specifically, that color intensity often varies between a dog's back and belly. In general, the underside of your dog will be lighter than his back. There are several theories about why this is so let's skip those! Just know that the fact that your dog is lighter underneath than on top does NOT mean he is producing two different types of pigment. It's just denser or more intense in one area.
Solid black dogs also come with a couple of variations related to pigment density. Occasionally, if the black pigment is not particularly dense, you can discern some tan shading underneath the black (remember, solid black dogs do produce both types of pigment). A dog that has the Agouti gene for sable pattern (ay) in addition to the K gene for dominant black may be a rich, golden brown/black color ("seal"); a dog that has the genes for tanpoint pattern (at at) may have a faint "ghost" tanpoint pattern visible underneath their black.
Finally, there is a color gene that I have omitted up until now - the "Brown" gene. The recessive form of the Brown gene interferes with the production of black pigment; it does not affect yellow/tan pigment. Dogs who inherit two copies of the recessive Brown gene "b" produce brown pigment rather than black - not only in their coat, but also on their nose, eye rims, and foot pads. This color is variously referred to as "liver" (in Spaniels), "chocolate" (in Labradors), and "red" (in Australian Shepherds). "Red" tricolor dogs result when a dog inherits two genes for the tanpoint pattern and two recessive Brown genes; the tan markings of the black/tan pattern are not affected, but the areas that would ordinarily be black are brown instead.
In order to understand how white markings come about, it helps to review how coat color develops in the first place. Pigment is made, formed into granules, and deposited into the shaft of the growing hair by specialized cells called melanocytes which live in the hair follicle. The color you see depends on the kind and amount of pigment in these granules, and where it is deposited along the hair fiber. Areas of white on a dog reflect either the absence of melanocytes or a failure of pigment formation.
Melanocytes are derived from a group of cells called the neural crest cells. This population of cells arises along the midline of the back early in development and gives rise to a number of cell types, including a large portion of the peripheral nervous system, and most of the bone, cartilage, and connective tissue of the head. If the number of neural crest cells is lower than normal, a reduced number of melanocytes will be formed and the dog will have larger white (unpigmented) areas. Since neural crest cells form important parts of the nervous system and of the inner ear and eye, animals selected for large amounts of white, such as Dalmatians, often have vision and/or hearing defects.
During development, the immature melanocytes must migrate from the middle of the back (spinal area), where the neural crest cells originate, over the surface of the animal. This is why, when a dog inherits a gene that affects the number or migration of melanocytes, the areas furthest from the middle of the back (feet, chest, muzzle, tip of tail) are the first to show white spotting. If a dog has only a small amount of white, it is likely to be found as a patch on the chest, or a few white toes. If a dog inherits genes that increase the size of white markings, the white will first spread across the chest, up the legs, and finally - in the more extreme cases - onto the back. Dogs with white patches on their back between the withers and the hip, or with white extending up the front of their stifle (upper thigh), are sometimes referred to as "white factored." If the amount of white on a dog exceeds 30% of its body area, it is referred to as "excessive white."
The gene locus which regulates white markings is called the "White Spotting" locus. The dominant allele (S) results in a "solid" or "self colored" dog - one without white spotting. Sometimes SS (solid) colored dogs will have a small white patch on the chest or a few white toes due to a development "glitch" in melanocyte migration; these white patches are "developmental" rather than "genetic" - they are not passed on predictably to offspring. The recessive alleles at this locus (designated "Irish," "piebald," and "extreme white") promote white spotting due to impairment of melanocyte growth and migration. The amount of white you see increases as you move "down" the chain here - the least dominant alleles produce larger white areas. The classic English Shepherd pattern with white chest, muzzle, socks and tail tip is called an "Irish" pattern, and is due to the "Irish" spotting gene (si). White collars may or may not be present. White face blazes are also not always present - there is probably a separate (unidentified) gene that regulates the amount of white on the head. "White factored" dogs are most often heterozygous, with one copy of the Irish white gene (si) and one copy of either the piebald (sp) or extreme white (sw) gene. A dog that inherits two copies of either the piebald or extreme white gene will be "excessive white." Because the dominant gene at this locus produces a solid colored dog (or one with only a small amount of white), it is possible for two parents with very little white themselves to produce piebald or excessive white pups - if both parents carry either the piebald or extreme white gene.
PUTTING IT ALL TOGETHER
There are two final color genes to mention (briefly!), before putting it all together. One is the gene which produces "ticking" - that is, flecks of color within white markings. Ticking is the result of a dominant gene, "T", at a locus named Ticking. The other is a gene series called "Dilute" which results in a faded or washed out appearance. If a dog inherits two copies of the recessive form of this gene, their black areas will appear faded or "blue", and their lighter pigment may wash out to a dull silvery tone.
COAT COLOR CHART
The following table outlines the basic genotypes associated with English Shepherd coat patterns. If the box says "any," that particular gene series does not affect the phenotype (appearance) for those dogs. If the box includes a dash instead of a second letter, it doesn't matter what the second allele is (only one copy of a dominant gene is needed to express the trait).
|White Spotting, S||Ticking,
|Clear sable/ white||Any||Any||D -||ee||Any||si si||**Any|
|Black/ white||Any||B -||D -||E -||K -||si si||Any|
|Shaded sable/ white||ay -||B -||D -||E -||k k||si si||Any|
| Black/ tan
|at at||B -||D -||E -||k k||S -||Any|
|at at||B -||D -||E -||k k||si si||Any|
|Red tricolor||at at||b b||D -||E -||k k||si si||Any|
|White factored*||Any||Any||Any||Any||Any||si sp or
|Extreme white||Any||Any||Any||Any||Any||sp sw or
(tanpoint or sable)
|Any||B -||D -||E -||kbr k or
|Any, as long as coat has pigmented areas||Any|
* "White factored" describes an appearance - an extension of the Irish pattern beyond what you typically see. Dogs that carry the piebald or extreme white allele in conjunction with the dominant S (solid) allele will not appear "white factored" but may produce excessive white pups when bred to another dog carrying either the piebald or extreme white allele.
** If ticking is present, the dog
has at least one copy of the dominant T allele.
*** Brindling is a pattern where black striping occurs in areas of tan markings. Brindling is believed to be regulated by an allele at the "K" locus that is intermediate in dominance. In order for this trait to be expressed, a dog must produce both black and tan pigment in the coat -- which means, clear sable and solid black dogs may carry the brindle allele but it will not be expressed.