What does it mean when we talk about "Color Series?" Well, when we're talking about color genetics in Rat Terriers, we're really focusing on seven groups (or pairs, or strings) of color genes that affect the color and pattern we see.
These color series fall under the titles or groups of, "A - B - C - D - E - S and T." There are actually more groups than these seven, but our dogs are the same for all the others, so we don't need to focus on those (can you say, "Whew!!). For example, in the "G" series, our dogs are all "gg." So you don't need to know that a dominant "G" gene, also known as the "greying gene" (which our dogs never have), would change your black puppy to a grey-colored (similar to our "blue") one year old. Forget all that, you don't need to know it.
Each series has a group of genes, from dominant to recessive, of which your dog possesses two. So your dog has two genes from the "A" series, two genes from the "B" series, and so on (color genes in pairs).
We're going to explore each of the gene series, one at a time, and the various genes within each group. We'll explore their dominant vs. recessive characteristics, and the action or role each gene plays. Over time, we'll learn how genes from different series or groups affect one another. For example, how a gene from the "B" series can affect the color we see in an "A" gene.
"A" Series or "Agouti"
The "A" or Agouti series determines placement of Eumelanin (black or brown) vs. Phaeomelanin (tan or red). In many ways, you can think of this as a "pattern" gene, but it also has a direct affect on color.
The order of dominant to recessive in this group looks like this:
I'm intentionally omitting another gene or allele here, known as "as." This gene, in theory, is responsible for the Calico pattern. I'm going to address this gene, later, in its own lesson. Omitting this gene will have no bearing on how this particular series works or what you learn, now.
You may wonder why, if "at" is the most "recessive" gene in the group, we see so many tan-pointed or black & tan Rat Terriers. Well, while "A" and "ay" may be dominant to "at," it's really a matter of selective breeding, or breeding for color preference. If we prefer tan-pointed dogs, and we primarily breed dogs that exhibit this pattern (where "at" is all these dogs have to contribute) then we will see predominantly tan-pointed dogs.
Now, let's look at each gene in this series, in detail.
When you read about this gene, you often will hear it referred to as "Dominant Black." This can be confusing. What it really means is, "Dominant Black Eumelanin," or "Dominant Eumelanin."
Black is the dominant color of the Eumelanin colors. What it also means is "No Phaeomelanin." So when you see these three words, you should visualize them independently: "Dominant" (dominant gene in the series), "Black" (dominant color of Eumelanin), and "Eumelanin" (only Eumelanin is expressed here, there is no Phaeomelanin, or "tan" expressed).
When the "A" gene is present, the dog only exhibits Eumelanin, and doesn't exhibit Phaeomelanin, at all. Black is the dominant Eumelanin, therefore, "Dominant Black." In order to change this Eumelanin color, recessive genes from other color series need to be present (such as recessive genes from the "B," and "D" series) in order to change this otherwise dominant Black Eumelanin color to "Chocolate Eumelanin," or "Blue Eumelanin," or "Pearl/Isabella Eumelanin."
In Rat Terriers, where some amount of white is required, a dog with a dominant "A" gene will be a "bi-color dog" - or a dog exhibiting Eumelanin and white:
When we look at Buzzy, above, we know that he carries the dominant "A" gene. We can "see" it. He would be known as "Black & White," or a "Black Bi-color." What he carries recessively in the A-series ("A_") may be unknown until we look at his parents or see what he produces in his offspring.
When we look at Vicki Lauer's girl, above, we know that she, too, carries the dominant "A" gene, because we can "see" that she only exhibits Eumelanin (no Phaeomelanin), but we can also "see" that her Eumelanin here has been altered to "Chocolate Eumelanin," by genes that restrict the formation of black.
And, again, when we look at Sheri's Allie, we know that Allie carries the dominant "A" gene, because we can "see" that she only expressed Eumelanin, but we can see that her Black Eumelanin has been diluted, changing it to "Blue Eumelanin."
And above we have Sheri's Kahlua, a beautiful Pearl/Isabella & white dog. Again, we can "see" that Kahlua carries the dominant "A" gene.
The important things to remember here are: "A" is dominant (it only requires one "A" gene in the pair to visually produce the all Eumelanin dog), the dominant Eumelanin is Black (but this can be altered by recessive genes in other color series), and Phaeomelanin is absent.
It's also important to understand that "A" doesn't create white - only the Eumelanin color. If Rat Terriers were permitted to be a solid color of Eumelanin: Black, Blue (diluted black), Chocolate, or Pearl/Isabella (diluted chocolate), "A" would be the gene responsible for this solid Eumelanin color. The white we see, and its placement, is determined by the genes in the "S-series."
Why don't we see Tan & White (Phaeomelanin & white) dogs illustrated here? Because a "Tan & White" dog, although "Bi-color," is not created by the "A" gene. Remember, "A" is the absence of Phaeomelanin (tan). I'll show you how we get clear tan dogs when we explore the "E-series."
The "ay" gene is often referred to as "Dominant yellow" or "Dominant tan." What it really means is "Dominant Phaeomelanin." In this case, however, Eumelanin is also present, it's simply been restricted (but not in totality), creating the "sable" pattern. The sable pattern may best be described as a dominant Phaeomelanin body color, with Eumelanin "peppered" throughout:
Lovely Miss Cali, above, is a "Tan Sable." The Eumelanin she expresses is black. The Phaeomelanin in sable dogs can range from pale tan to deep red, depending on plus and minus modifiers attached.
As we discussed in Lesson #1, when the Alpha Dog (in this case, the "A" gene) is present, the Alpha Dog is in charge, and it's only the Alpha gene you get to "see." But remove the Alpha, and the second dog in charge takes over. In the "A-Series," this second dog is the sable gene "ay." In the absence of "A," "ay" is dominant.
Like all Eumelanin, the Eumelanin expressed in the sable dog can be altered by recessive genes in the other series (such as the recessive genes in "B" and "D"). As before, Black Eumelanin is dominant, but in a dog who is sable and also "bb" (Chocolate), black is unable to form, and the Eumelanin hairs express as "Chocolate Eumelanin." A lovely example of a "Chocolate Sable" is Elsie's Madison:
In a dog who is sable, but where the Black Eumelanin has been diluted, we have a "Blue Tan Sable." A lovely example of a "Blue Sable," is my pal, Saltwater Cowboy, shown here with his look-a-like pup. In Cowboy's case, what few Eumelanin hairs he exhibits in the sable pattern have been diluted, to blue:
One of the interesting things about the sable gene "ay" and it's action of Eumelanin restriction is that this action continues throughout the dog's lifetime. As the dog matures, the Eumelanin is restricted further, until the Phaeomelanin that's expressed (Dominant Phaeomelanin) becomes more and more "clear" of Eumelanin. Many puppies who exhibit a lot of Eumelanin hairs (sabling) as youngsters grow up to be dogs who are predominantly Phaeomelanin, or "clear tan," even though they are genetically "sable."
Just as the Eumelanin can come in different colors in the sable dog, depending on the double recessive genes the dog may carry (in the other color series; B, D, etc.), so can the Phaeomelanin base come in many different colors, from the palest tan (yellow) to the deepest mahogany red, depending on the plus and minus modifiers, or polygenes, attached.
Some "ay" genes are more effective in restricting Eumelanin than other "ay" genes (they have plus modifiers attached) and these produce a dog who is visibly more Phaeomelanin (tan) and less sable. Some "ay" genes are less effective in restricting Eumelanin (minus modifiers attached) and the dog will exhibit a great deal of Eumelanin "sabling." Some dogs who are "ayay" (instead of "ayat") will show less sabling, as they have double the restricting power, but in all these cases, the "ay" gene's action of restricting Eumelanin, but not in totality, and creating this peppered pattern of Eumelanin hairs over a Phaeomelanin dominant base, continues throughout the dog's lifetime, changing the color depth and amount of sable year to year as the dog matures. It's a color show for their delighted owners.
The "at" is the most recessive gene in the "A-Series." Like the bottom dog on the totem pole, "at" only gets to lead , or make a visible appearance, when paired with itself ("atat").
The "at" gene is often referred to as "Black & Tan." What this really means is "tan-pointed," or where the Phaeomelanin is strategically placed in a predictable pattern. The "at" gene allows both the expression of Eumelanin and Phaeomelanin, but in specific areas of the dog's body. The Phaeomelanin is restricted to the cheeks, brows, legs, but will sometimes make its appearance in the outer spots of color on the dog's body, particularly as the dog matures - what is sometimes referred to as "reverse sabling," because it has a similar "peppering" of color, only with Eumelanin being the dominant base.
Black Eumelanin is dominant (it takes recessive genes from other color series, such as double recessive from the "B" or "D" series to change this Eumelanin color), therefore "Black & Tan." The Phaeomelanin (tan or red) can express in a variety of shades and colors, from pale yellow to rich mahogany, depending on the plus and minus modifiers, or polygenes, attached.
The placement of Phaeomelanin (tan) may be restricted to the face (tan-points) or it can extend to the dog's legs, chest and belly. Depending on the amount of white the dog presents, the "atat / B_" dog can be described as a "Black & Tan," or a "Black, White, & Tan - Tri-color."
When double recessives are introduced from the "B" and "D" series, only the Eumelanin color is changed, making the "Black & Tan" a "Chocolate & Tan," or a "Blue & Tan," or a "Pearl/Isabella & Tan." Because white is a requirement in Rat Terriers, these are often referred to as: Black Tri, Chocolate Tri, Blue Tri, and Pearl Tri (actually, this last one is most commonly referred to as "Blue Fawn," which is a misnomer). When you see a tan-pointed dog, you can know with certainty that it's "atat" that's in the dog's "A" color string.
Chocolate Tri (atat / bb) - with the absence of white, this would be "Chocolate & Tan":
Blue Tri (atat / dd) - with the absence of white, this would be "Blue & Tan":
Pearl/Isabella Tri (atat / bb / dd) - with the absence of white this would be "Pearl & Tan":
Next we'll play "Match Game," only we'll be doing this with pairs of genes... showing how inheritance works with the dominant and recessive genes in the "A-Series."
Copyrightę2006 Sue Campbell - All Rights Reserved