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.
Friday, July 23, 2010
The best known type of roaning is called a Classic Roan. These horses tend to have evenly (or nearly) mixed white hairs through the body area, with darker heads and lower legs. Classic roan can be seen on any base coat color.
Now, credit for the picture here. This is an image that kept popping up in my searches for examples. I just can't get over how lovely this horse is, and I'm not even a quarter horse fan. His picture was used for an ad for this website. It appears that his name is "My Final Notice". Lovely horse.
Grey horses can go through a state that looks similar to a classic roan, but a grey horse will continue to lose pigment over time and get whiter each year. There is also a type of roaning associated with appaloosa colors. This is called varnish roan, and I'll go into it more with appaloosa genetics.
The other well known type of intermixed white hairs is called Rabicano. Rabicano is best known for the "skunk tail" or "racoon tail" type markings seen with this pattern. While classic roans have white hair intermingled all over their body, rabicanos tend to have white roaning around the tail and rib area. Sabino can also cause a roaning type of appearance, but it is often seen with pinto like markings (although not always).
Here is an example of a blue roan horse. This horse shows the typical lighter body color with darker head and legs.
When you look at a roan up close, you will see something different.
Roaning is actually a pattern, not a base coat modifier, because roan prohibits pigment production, as opposed to modifying the pigment produced. Previously, it was thought that classic roan was lethal in the homozygous state, with homozygous embryos being nonviable, and thus never even born. This has since been disproven. First in 1979 with a few Ardennais stallions (a type of draft horse) and later in quarter horses.
The problem with the early phenotype testing (testing based on outside appearance, not DNA) is that multiple types of roans were included, not just classic roans. Frosty roans, classic roans, rabicano roans... all of these roans were bred together, and the results did not show the typical 1-2-1 dispersal expected with incomplete dominance, or the 3-1 expected with a simple dominant gene.
Let me explain that for you. When identifying modes of inheritance by appearance of a trait you calculate the percentage of offspring produced and their appearance. So, lets say I have 100 foals from black to chestnut breedings. 75% of those foals should be black, and 25% of those foals should be chestnut. With DNA testing we would see that 25% were EE, 50% Ee, and 25% ee, but both the EE and Ee foals would appear "black" and thus grouped into the same category, resulting in 3 black foals to every chestnut. This 3 to 1 ratio tells us that a gene is a simple dominant gene.
In incomplete dominant genes, you would see a 1-2-1 ratio. As an example, if you breed a palomino to a palomino you would get 25% chestnut foals, 50% palomino foals, and 25% cremello foals. This is making it very simplistic, as normally the results are never that perfect. We often see things like 23%, 56%, 21%, but you can see how that is close. The larger the group studied (1000 foals vs 100 foals) the closer you get to the ideal percentages. Of course, the problem is the reality behind testing things like this.... what do you do with all those foals? Because of that, we rarely have the sample sizes desired for the best testing.
Now, if you do a test like this, and you're using multiple genes with similar appearances, then your results will be all over the place! Many scientists suspect this is what happened with the first phenotype testing for roan. We know that rabicano and classic roan are inherited independently of each other, so this would skew the results.
Now, classic roan is a simple dominant gene, and is referred to with the symbol RN or rn (yes, that's an r and an n). I have often seen the dominant copy also referred to as Rn. The question of how much roaning a horse gets, well, that is determined by those enhancers and supressors we discussed yesterday. Sabino, splash, base coat color, and other enhancing factors can cause a horse to have more white hairs, while the lack of them, or restricting base coat colors (such as EE) can cause a horse to be minimally roaned.
As a comparison, look at this minimal roan and compare it to this extreme roan
Now, like I said, roans come in all colors. Here are a few examples:
(and isn't he just adorable?)
Of course, I used many blue roans (classic roan on black) as my initial examples, simply because it's so easy to see. Also, all links above lead to the horse's websites, many are breeders. (I seriously recommend the website for the buckskin roan, some LOVELY mini horses there, in all colors.)
Now, none of the horses below are roans. Today's test! Can you guess what patterns/colors cause these horses to have a roan-like appearance? It's a bit unfair, because we haven't covered a lot of these colors yet, but hopefully it's more fun then just guessing letters!
And bonus points for those who can write the genome for the horse's color (base color only is ok).