Dun
This gene also creates one or several primitive marks, the most common and essential one being the dorsal stripe. All duns have a dorsal stripe, but not all horses with a dorsal are duns. Confused? Well, just learn for now that duns = dorsal, and you'll learn about the non-dun dorsal stripes later. Anyway, there are a number of other primitive marks besides the dorsal; Here is a diagram depicting the different dun marks:
See the Dun Factors page for more information on the factors.
Not all duns have *every* one of these dun factors; many will have at least two, the dorsal and the leg barring, and the dorsal is essential to the dun (all duns have dorsals).
The most well-known shade of dun is the yellow dun, which more appropriately should be called a bay dun. This is the dun on a bay base; the body is usually a yellow or tan color, and the points are dark.
Sometimes bay on dun will not result in a yellow color, and the body will look like a bay-- except for a dark dorsal stripe and other dun factors.
Dun on a chestnut creates a red dun . The body is a red and is sometimes diluted to a very light red, and the dorsal and dun marks are a dark (undiluted) red color.
Dun on a brown horse gives a sort of a nondescript muddy-brown color, called Mouse dun.
Now, we come to dun on black. What color might this produce? Well, technically, it is called black dun, not a very original name. However, in the Quarterhorse and especially Western world, it is known as Grulla or Grullo, the female and male terms respectively (they are named after the Spanish word for "crane", and in this language many things have a male and female term). These are actually the most widely used and accepted terms for the color.
And, of course, the correct pronunciation is Spanish-- "Grew-ya" for the female form, "Grew-yo" for the male.
In Iceland and other parts of the world, this is called Blue Dun.
In the USA, the lightest shade of black dun is called "Silver Grullo", then comes "Slate Grullo", then "Grullo", then "Lobo dun", with the last being the darkest shade.
So just what color is Grulla? It's often described as a mousy-gray, smoky-blue, or slate-gray color.
The black is diluted by the Dun gene to an off-black, bluish, smoky-gray color; in some cases, the horses are more mousy-brown-tan, and in even more other cases, the horses are the color of the inside of a Three Musketeer candy bar.
The dorsal is black, and the face and legs are often darker than the body.
Grulla is kind of hard to describe, but once you've seen a few you will be able to identify them.
Some breeders consider Grulla to be the rarest of the dun colors-- are some are even bold enough to state that they are "the rarest Quarter Horse color", although this is not true. They probably are the rarest of the dun colors, however, as first you need a black base, then a dun gene over that. And as black is recessive to Agouti (creates bay), this makes it rarer.
The Norwegian Fjord is a breed that comes only in shades of dun; They are a very ancient and primitive breed believed to be descendants of the Tarpan and Przewalski's horse. Their dun is actually different than the regular dun found in, say, Quarter Horses-- at least in the visual sense. They have 'black duns' and 'red duns', but they do not look anything like those appearing in most other breeds. They are a very unique breed, instantly recognizable by their color and conformation.