Buckskin, Dun, Palomino, Grullo. All these diluted colors, it gets so confusing!
How often do you see ads for a "Buckskin" horse, that has a dorsal stripe and leg bars? Or that lovely "Dun" who is really a bay? For some reason, these dilutions are really hard for people to figure out. Ok at least half of the problem is wishful thinking.
The first difference between these 2 genes is how they are inherited.
Dun is a simple dominance gene (if you have a dominant copy, then the gene is expressed, and there's no difference between DD and Dd). Creme is also the "Palomino" gene, or the "Buckskin" gene. It isn't nearly as cooperative for breeders, since Creme is an incomplete dominant gene. They are on different parts of the DNA strand, so even if the phenotypes look similar, the genes are different. Homozygous dun (DD) is not lethal as was once guessed.
Incomplete dominance simply means that there are 3 options, homozygous dominant (CRCR) heterozygous (CRcr) and homozygous recessive (crcr), and each one of these gentotypes (genetic code) has a different phenotype (appearance). The appearance is dependant upon the horse's base color - Extension plus Agouti.
I only have one Dun horse, and sadly champagne makes it almost impossible to photograph her markings, so I'm going to use some images of other horses (i.e. not mine). All images are linked back to their source.
The typical appearance of a dun horse is a tawny body with black points. A buckskin is similar looking. The difference between these 2 are the "primitive markings" that duns have. This includes a dorsal stripe and leg barring, as well as the often hard to see dark shoulder patch. If you look at a typical donkey, you see can see an example of primitive markings: the cross on their back.
Here is a good example. This horse is offered for sale (is also listed as a buckskin - oops!), is not mine, and can be seen here.
A Bucksin on the other hand, would look like this stallion, from this website:
The main difference between Dun and Buckskin isn't the appearance, it's the method of inheritance. In other words, what will I get when I breed this horse.
Dun dilutes the body color towards the yellow, on ANY base coat color, and leaves the primitive markings. Granted, Dun does dilute red more then black, but it often turns black points a deep brown or even as light as red tones in some cases. Creme only affects red pigment, and is an incomplete dominance gene, so is harder to breed for.
Dun on black is a Grulla. Dun on Bay is a Dun, or also called a Zebra Dun. Dun on a chesnut/sorrel is a red dun. Dun and the creme gene can also be inherited and exhibited at the same time, such as with Dunalinos, or buckskin duns (no nifty cool name for those that I know of yet).
Then you have the Creme gene. If a horse is homozygous dominant for creme, then on a red base you would have a cremello, on a bay base you would have a Perlino, and on a black base you would have a smokey creme. In its herterozygous form (CRcr), creme on bay results in a buckskin, creme on chestnur/sorrel results in a palomino, and creme on black results in a smokey black (very hard if not impossible in some cases to discern with the naked eye). Creme can also be inherited on a brown base coat (also called seal bay).
In both cases a horse that is homozygous recessive would show no alterations from it's base coat color from this gene. Most people would say the horse didn't "inherit" the gene in question (slang).
BUT it's never that easy is it. While we have a good idea of the normal presentations of these colors, there are always exceptions!
Creme on Seal bay gives...well.. it's still called a buckskin, or occassionally a smokey brown. A good example is talked about on this forum.
At ? CRcr, or seal bay with creme. Some might call this a smokey brown.
Also, there are countless examples of horses for sale called buckskins, but they are just sun faded bays. For a horse to inherit one of these dilution genes, one of their parents MUST have it. Even if their ancestors had it, if the horse does not inherit a gene, it can not pass it on. Only recessive genes can be carried, and pass unexpectedly to offspring (such as with red Friesians).
So, if you have a chestnut mare (ee ?? crcr), who had both a buckskin(Ee A? CRcr) sire and dam, and you expect to get a buckskin foal... well....better breed to a Perlino (EE AA CRCR)! Your mare doesn't have either the dominant Extension(E), nor the dominant creme gene(CR) to pass on.
Now, creme acts by lightening the red pigment, which is why a single creme gene on a black horse is almost undetectable - so little red pigment is visible. Bays have their black pigment pushed to the extremities, leaving red pigment in their main body areas, and creme can then lighten up that red, turning it yellow, resulting in a buckskin.
Now, with a homozygous dominant creme horse, any pigment is lightened drastically. This is because the proteins made by incomplete dominant genes have more impact with more production. So, if a little bleach is good... a lot of bleach... bleaches a lot. In the case of smokey cremes(E? aa CRCR), they retain the most pigment of the homozygous dominant creme horses. This lovely horse is a smokey creme shown here.
See how dark his mane still is? That's because black is harder to fade out for the creme protein. Perlino horses tend to have less color then the smokey creme horse, and cremellos are the closest to white of the group. All shades of horses with homozygous dominant creme are near white., though. Again, this is due to the base color genetics. Creme works on red pigment, so a red horse is easier to lighten then a bay (black based because it's E? A?). All horses with 2 dominant copies of creme will have blue eyes and pink skin.
A homozygous dominant dun though looks no different then a heterozygous dun. This is the nature of simple dominant genetics. Also, because I'm not sure that I mentioned this specifically, Dun is denoted by D for the dominant form, and d for the recessive.
Often times these genetic codes change as we map the gene in question. If the exact sequence of DNA was found previously in another species, then the gene inherits the name of the first discovery of it. Agouti is a good example of this, as it was first found in mice, so we began using the same terminology and abbreviations as the mouse gene(A and a) in stead of calling it the Bay gene officially.
The color genetics of horses build on each other. First you consider the horse's extension status (E or e) then it's agouti (A or a) and modifiers come after that.
So, if you saw that a horse's genome was:
Ee AA crcr, what color horse would it be?
A Note about Sugarbush Draft Horses
I see it over and over again, and no matter how many times it's said, it's still wrong. "Sugarbush Drafts are just an Appaloosa Draft Cross". Uh.... no. The Sugarbush Draft Horse was a breed created many years ago in Ohio. While the initial cross was made using Percherons to Appaloosas, in the many generations following, the breed has been solidified into a consistent type. Saying these horses are "just" a draft cross makes as much sense as saying that AQHA horses are "just" a Thoroughbred cross, American Cream Drafts are "just" a dilute Belgian, or that Morgans are "just" a grade.