Types of Melanin
Pigment gives skin, eyes and coat its color. The arrangement of this pigment lends itself to the patterns of color that we see.
Pigment is produced by chemicals known as chlorophyll (in plants and algae) and melanin (in animals). In dogs, we're only concerned with melanin, of which there are two types:
Eumelanin = black or
brown (not tan)
White is the absence of melanin, or the absence of color. White contains no pigment.
A great many Rat Terriers exhibit both forms of melanin (Eumelanin and Phaeomelanin) in their coat and skin. Most Rat Terriers also exhibit white, or the absence of melanin/pigment.
When you look at Julie P.'s beautiful boy, below, you can see the expression of all three in play. You can see the Eumelanin expressed in the black portions of his coat and in his nose leather. You can see the Phaeomelanin expressed in his tan points. And you can see the absence of melanin in the areas that are white.
It's important to recognize the different types of melanin, because as we begin to explore color genetics and discover what these individual color genes DO (their action), we'll see that some color genes affect only one type of melanin, or affect one type more than the other. We'll also see how the action of these color genes determines the pattern or arrangement of Eumelanin and Phaeomelanin that we see.
For example, if we simply change two genes in the color series, from "BB" to "bb" (don't worry, we'll learn what this means in lesson three), we alter the Eumelanin in Barbie's beautiful Bama from black to chocolate, but the Phaeomelanin (tan - or in this case, tan points) remains unchanged:
With this knowledge, we can easily understand that chocolate is not a dilute, nor is it a form of "red" (Phaeomelanin), but a form of Eumelanin where black was unable to form ("Chocolate Eumelanin"). Diluted Black Eumelanin, where black is able to form and then is diluted, creates the color we know as "Blue" (or "Blue Eumelanin"). In a chocolate dog (bb) this dilution action (dd) would produce "Pearl," or "Isabella."
Let's look at some more Rat Terriers, and see more examples of these two types of melanin, Eumelanin and Phaeomelanin, expressed, as well as the absence of melanin (white):
As we move forward, we'll see more patterns and expressions of Eumelanin and Phaeomelanin and white, and we'll learn how color genes affect each - to produce the colors (blue, chocolate, tan, etc.) and patterns (bi-color, tri-color, sable, white patterns, etc.) that we see. We'll also explore the difference between what we "see" and what is "genetically true."
Each color gene has an action. As we go forward through the different color strings and the individual genes within each, we'll explore what each gene's "action" is. What it does.
Recognizing an individual color gene's action will help you to make sense of what is happening, when one or more color genes are in play, and the role each gene plays in what we see, visibly (when we look at a dog) and what we can't see (what is hiding beneath the picture - or what is "genetically true," but not visibly apparent). We'll also learn how genes with different actions affect one another.
Some color genes affect only the color that we see, through their action of expression (allowing full expression, limited expression, etc.), through their action of restriction (restricting either Eumelanin or Phaeomelanin from forming or expressing itself fully), or by their action of alteration (such as dilution).
Other color genes affect the patterns that we see, affecting the placement of Eumelanin and Phaeomelanin (think about the patterns involved with tan points, or sable, or brindle - and the specific placement of Eumelanin vs. Phaeomelanin in each of these patterns), or by determining the amount of white that is visible, and how this white affects the over all pattern that we see.
A lot of confusion arises over "dominant genes," "recessive genes," "incomplete dominance," "co-dominance," etc. But an easy way to understand dominant vs. recessive is to think about it as you would the hierarchy of dogs within a pack.
In a pack of dogs, there are levels of hierarchy, from the most dominant to the most submissive (recessive). Except for the Alpha dog and the dog at the very bottom of the totem pole (most recessive), all other dogs in the pack have those to whom they're dominant (the dogs below them) and those to whom they're submissive or "recessive" (all the dogs above them). Remove some of the dogs, and the order of dominance, or hierarchy, changes.
We all understand, for example, what an Alpha dog is like. In a group of dogs, the Alpha is the one in charge. All other dogs in the pack are submissive (or recessive) to this dominant dog. The Alpha dog rules (it's dominant).
Below the Alpha dog is the second dog in charge. This dog would be known as the "dominant recessive." He or she would be submissive (recessive) to the Alpha, but is dominant to all other dogs in the pack (those "beneath it"). When the Alpha dog is removed, this dog takes over and is in charge. It's "recessive" only to the Alpha, and dominant to all the rest.
We may have several dogs somewhere in the middle, juggling for their place in the hierarchy.
And then we have the dog at the very bottom of the ladder - the most submissive or "recessive" of the group. This dog only "rules" when all the other dogs are removed.
This same order of hierarchy is involved in color genes. Within each series (A-B-C-D-E-S-T), there are Alpha dogs, middle dogs, and bottom of the ladder dogs. Example (using the Agouti series):
What do we mean by "co-dominant?" Well, sometimes in a dog pack you'll have two middle dogs who are equally balanced in their dominance or submission within the group. This doesn't usually happen at the Alpha level, because one dog will assert control (it's the natural order), but in the middle of the pack you can sometimes have two dogs who are equally matched.
This is sometimes true in color genes, as well. In the absence of the Alpha (most dominant gene) these two dogs (genes) will act as a team, each expressing their own will via equal, or shared dominance.
The dog at the bottom of the pack, the most submissive/recessive of the group, only gets to lead or express itself in the absence of all others. Remove all the dominant dogs above it, and this dog is in charge. It is also... alone.
Because in color genetics, the genes for each color series travel in pairs, the poor bottom of the ladder dog wouldn't actually be alone, but would lead or be in charge only when matched with a duplicate of itself. In the Agouti string above, this would be represented as "at at."
Bring in another pack of dogs (from another color string: B-C-D-E-S-T), with its own dominant and recessive genes, or hierarchy, and all bets can be off. A dominant dog (gene) from this group can take our recessive leader above (at at), and change what it's allowed to visibly express.
We'll go over dominant vs. recessive more closely as we look at each string in the color series, and what each of the individual genes within each of the series can do (its action).
Color genes travel in pairs, so for each of the color strings: A-B-C-D-E-S-T, a dog will carry two genes. Two in the "A," two in the "B," two in the "C," and so on.
When pups are produced, they receive one gene for each string from each parent. The sire gives one (he gives one of his two genes from each string), and the dam gives one (she gives one of her two genes from each string), so that the pup now has it's own unique pair: two for "A" (one from dad, one from mom), two for "B" (one from dad, one from mom), and so on.
We'll discuss inheritance in more detail, later, but let's look at dominant vs. recessive with the above information in hand. We now know that each dog carries two genes for each color series. The order of dominant vs. recessive within each of these pairs will determine what we "see," visibly, and what we can't see, but is "carried" recessively. We'll begin to learn the difference between what we "see" and what is "genetically true," and why these two things aren't always the same.
Using our dog hierarchy and dog pack analogy from earlier, our dog pack now consists of just two dogs in each group.
When we have the Alpha dog as part of the pair, the Alpha dog rules, and it's its dominance that we "see." Example:
The "dominant dog" A rules. The recessive dog is there, too, but the dominant dog won't let you see it. Poor recessive dog.
But let's take this particular dominant dog out of the pack, and replace it with another dog, one a bit lower on the hierarchy ladder:
So in these pairs of dogs, the dominant dog will only allow you to see it, while the recessive dog hangs back in the shadows, waiting for its chance to show itself in its offspring.
But sometimes these dogs, or genes, are "co-dominant," or equally matched. For example, a gene that's not included in the example above is known as "as." This gene produces the calico affect, and when paired with either "ay" or "at," both genes will express their actions, equally, or co-rule - allowing us to "see" their co-dominance, visibly. They're like the pair of middle dogs who co-dominate, or share the hierarchy equally.
Copyrightę2006 Sue Campbell - All Rights Reserved