Jonathan, pup and Cowboy

Rat Terrier - Canine Color Genetics
Lesson #5 - "D" Series

By Sue Campell, SouthPaw,

Lesson #1 (Introduction), Lesson #2 (A-series), Lesson#3 (B-series), Lesson #4 (C-series), Lesson #6 (E-series)

The D Series

Just like in the B-series, the "D-series" has only two genes (or alleles) to have to learn and understand: "D" and "d" - making it nice and simple. Even better yet, there's a simple order of dominance in play: dominant "D" (requiring only one "D" gene to fully express Eumelanin color), and recessive "d" (requiring two "d" genes or "dd" to produce the dilution action, and exhibit a visibly diluted Eumelanin color).

What Can Be Diluted?

Dilution through the "D-series" primarily affects Eumelanin, or the "Black" or "Chocolate" colors of the coat. This is why you can see a diluted dog, such as a "Blue," with very rich mahogany colored tan-points (Phaeomelanin).

While there may be some minor affect to Phaeomelanin (tan or red) with the dilution action of "dd," the visible effect is minimal - if at all.

Two Non-dilute puppies (Tan Sable and Black Tri-color)

"Rusty" and "Tex" - SouthPaw pups - see an adult picture of Rusty at the top of Lesson #3

Both Black ("B_") and Chocolate ("bb") Eumelanin can be diluted. Neither of these colors is a dilute, in and of itself.

When the double recessive of "dd" is present, the Eumelanin color is diluted. Black Eumelanin is diluted to "Blue" ("B_/dd"), and Chocolate Eumelanin is diluted to "Pearl/Isabella" ("bb/dd").

Dominant D

When the dominant "D" gene is present (and remember, as a complete dominant, it only requires a single "D" gene in the pair to visibly exhibit its dominant effect), the Eumelanin color is fully expressed, with no dilution. This means a dog who is "DD" may be visibly identical to a dog who is "Dd," even though they are genetically different.

Other genes can affect the depth of color we see. For example, in the "B-series" we discussed how Chocolate Eumelanin can come in a variety of shades and depths of color, depending on the rufus polygenes attached. With dominant "D," it simply means that whatever Eumelanin color the dog presents, light or dark, it will be fully expressed - without dilution.

The Eumelanin Colors - Non Dilute
"D" - Full Expression of the Eumelanin Color - Non Dilute

Black, White, Tan (Non-dilute)

Devon's striking "Snap" - K2 Rat Terriers

I need more pictures of Black & Whites and Chocolate & Whites!! Please send pictures of your Non-dilute Rat-babies to Sue Campbell.

Chocolate, White & Tan (Non-dilute)

Cristen Cappelletty's "Duff" as a pup

As repeated throughout these lessons (J): Chocolate, despite its self-colored nose (which is what I think causes the confusion), is NOT a dilute. Chocolate can BE diluted (Pearl/Isabella), but it is not, in and of itself, a dilute.

The self-colored nose of the chocolate dog is caused by the double recessive of "bb" and these alleles' inability to formulate black pigment. This self-coloration has nothing to do with the "D-series," "dilution," or a Chocolate (Eumelanin) dog being "a dilute red" (Phaeomelanin).

So keep repeating... "Chocolate is NOT a dilute. Chocolate can BE diluted (Pearl/Isabella)."

Recessive d

"d" is the recessive gene (or allele) in this series. As a recessive (and as the only recessive in this series), it requires two "d" genes, or "dd," in order to produce a visibly diluted dog.

This dilution action primarily affects Eumelanin, or the Black or Chocolates areas of the coat, as well as the skin, eye color and nose leather. If the Phaeomelanin (tan or red) areas are affected, this effect is minimal.

The action of dilution is to reduce the expression of Eumelanin color, or to "dilute." Other genes are involved that impact the depth of color. For example, we discussed in the "B-series" how Chocolate coats can come in a variety of depths of color, from light chocolate to liver chocolate, depending on the rufus polygenes attached. In this same way, diluted dogs come in a variety of depths of color, depending on the rufus polygenes attached to other series' alleles, or to the amount of Eumelanin expressed - for example, the amount of Eumelanin expressed in a bi-color dog versus the amount of Eumelanin expressed in a sable dog (see the "A-series") where Eumelanin is restricted, but not in totality.

You can see this variation in depth of color in "Dark Blue" versus "Silver Blue" dogs. It's not that one is "more diluted" than another, but simply that the existing depth of color (determined by other alleles) is diluted.

When Black Eumelanin is diluted, it's changed to "Blue" Eumelanin. When Chocolate Eumelanin is diluted, it's changed to "Pearl / Isabella" Eumelanin. This dilution action affects the Eumelanin coat, as well as eye color and nose leather.

Black diluted = Blue
Blue, Tan & White

 3b's Evry Cloud hasa Slvr Linin - Blue Tri

Chocolate diluted = Pearl/Isabella
Pearl, White & Tan

Lynzie's AristocRat -  Sterling Bullet - Pearl Tri

This dilution affect is easy to see in dogs who express a solid color of Eumelanin "A" or the tan-pointed pattern "atat" of Eumelanin and Phaeomelanin, but can be more difficult to detect in dogs who are sable, where Eumelanin is restricted, but not in totality.

Remember, sable is a dominant Phaeomelanin base (tan or red) with Eumelanin hairs peppered throughout the Phaeomelanin areas of the coat. These scattered Eumelanin hairs are diluted when "dd" is present. If these hairs are "B" (able to form black pigment) then they're diluted to "Blue." If these hairs are "bb" (Chocolate) then they're diluted to "Pearl/Isabella."

Often, the easiest way to determine whether a sable dog is a dilute or non-dilute is to look at the nose leather. Black or Chocolate nose leather = Non-dilute. Blue or Pearl nose leather = Dilute.

Some dogs make this easier on us by being more visibly "sabled" or having more Eumelanin hairs in the coat. Others have very strong sable genes that restrict the Eumelanin almost in entirety, leaving just the Phaeomelanin (tan or red) base. In these cases, determining dilute or non-dilute, and then determining the type of dilution (black or chocolate) may be determined at puppyhood, when more Eumelanin hairs are present (the sable gene "ay" continues to restrict Eumelanin throughout the dog's lifetime - so puppies will often express more Eumelanin than they will as adults) or by the color of the dog's nose leather. Diluted Black nose leather is "Blue" or gray. Diluted Chocolate nose leather is "Pearl / Isabella." See "noses" in Lesson #3.

Take a look at the following puppy groups. The Eumelanin hairs in these pups have been diluted from Black to Blue. Can you pick out the sable pups - whose peppering of Eumelanin hairs have been diluted from Black to Blue?

Diluted Black - "Blues" - including a diluted sable pup

Elsie Killen's Pups

Diluted Sable - Blue

SouthPaw Pup

Look at the little girl above, the SouthPaw pup. Do you see the peppering of Blue Eumelanin hairs through her colored areas? These will disappear as she matures, leaving only a Phaeomelanin coat the color of a Palomino, just like her dad, Cowboy. However, if I shaved her coat, I would see blue skin in certain areas, such as her tail.

Keara/Cowboy Pup and her Dad, Cowboy

The two dogs above are "ayat/B_/dd" or "Blue Sables."

Basically, they're "tan sables" where the Black Eumelanin has been diluted to "Blue." Now, they don't have a lot of Eumelanin hairs to dilute, as their "ay" genes have restricted Eumelanin into a peppering of Eumelanin hairs through their Phaeomelanin coats (tan or red). Their eyes are diluted a lighter color, and their nose leather has been diluted to gray or "Blue." If Cowboy had not been a sable, his blue coat would have been very dark blue.

Without this dilution ("dd"), these two dogs would have looked like any typical non-dilute "Tan Sable."

The Phaeomelanin color was not diluted, despite it's light color. The Phaeomelanin color wais was the same in all the pups in this particular litter, whether dilute or non-dilute (i.e. the tan points on the black pups was the same Palomino color). It's very striking.

Now, what happens if we take a Chocolate Sable and add "dd" to it? Do we get the same as the above? No! We don't. We get is a dog whose Chocolate Eumelanin hairs have been diluted to "Pearl" or "Isabella," to create a "Pearl Sable," an "Isabella Sable," or a "Pearl Tan Sable" (different names for the same thing).

Actual photographs of Pearl Tan Sables coming...

(I haven't figured out yet how to change my Tan Sable graphic with its Black Eumelanin hairs in to either Blue Eumelanin or Pearl Eumelanin, but I'm still working on it J.)

What's In A Name

We have a lot of confusion in the registries regarding what to call our diluted dogs.

We seem okay when it comes to "Blue" (diluted black), but fall apart when it comes to diluted sables and diluted chocolates.

For some reason, all of the following colors: Pearl/Isabella (bi or tri), Blue Sable, and Pearl Sable, have been lumped together under the single definition of "Blue Fawn."

Now why would that matter? Well, it might matter if you consider that all of these colors are different, both visibly and genetically.

Just take a look...

All these are registered as "Blue Fawn" under the UKC registry:

Isabella (Pearl) & White =    "A_ /bb /dd"

Isabella (Pearl) Tan-point =  "atat /bb /dd"

Blue Tan Sable =   "ayay or ayat /B_ /dd"

Isabella (Pearl) Tan Sable = "ayay or ayat /bb /dd"

The only thing they have in common is the double recessive of "dd" (they're all dilutes).

So why isn't "Blue" included in this group? I think people have a certain grasp of "Blue," in fact, we often call the recessive "d" allele the "Blue Gene."

But Pearl, or Isabella, is simply "Chocolate Diluted" or "Dilute Chocolate." It should never be referred to as "Fawn." No more than you would call a "Diluted Black" dog "Fawn," instead of "Blue."

If we're attached to the term "Fawn," and must call a color "Fawn" in our breed, then we have two fawn colors in Rat Terriers:

We have "Blue Fawn" which is diluted black sable - where the Black Eumelanin hairs in the sable dog have been diluted to "Blue" = ayay or ayat / B_/ dd   (like Cowboy above).

And we have a "Red Fawn" which is diluted chocolate sable - where the Chocolate Eumelanin hairs in a sable dog have been diluted to "Pearl" or "Isabella" = ayay or ayat / bb / dd

Kind of confusing, isn't it. Not really. Just keep this in mind:

  • Black diluted is "Blue." Chocolate diluted is "Isabella" (or "Pearl").

  • These are Eumelanin colors.

  • In sable dogs, Phaeomelanin (tan or red) is the dominant expression (more tan or red is visible),

  • In sable dogs, Eumelanin (Black, Blue, Chocolate, or Pearl) is restricted into a scattering of Eumelanin hairs throughout a Phaeomelanin (tan or red) base, creating the sable pattern we see.

As for the name "Pearl," Rat Terrier owners, breeders and registries alike need to decide if this is what we're going to call Diluted Chocolate, or if we want to recognize the more common term of "Isabella," which is what most breeds who have diluted chocolates call this color. If we're going to call Isabella "Pearl," then let's call it "Pearl" across the board and not attach another label of "Blue Fawn" to it.

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