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.
Wednesday, September 15, 2010
Appaloosa Coat inheritance
Remember how we first discussed base coat color, and how all the genes stack up? Well, appaloosa color really takes that concept to a whole new level. The LP gene determines if there will be any appaloosa pattern. If lplp, then the horse shows no appaloosa pattern. I'll call this solid (even though other patterns might be present, such as pinto patterns, roaning, and such). If the horse is LPlp then it's heterozygous, and will have some form of appaloosa coat pattern. If the dense white area (PATN genes) is large enough, that area will have polka dots in it. And if the horse is homozygous dominant (which is what most people mean when they say the horse is "homozygous") then the horse is LPLP, and will have some form of appaloosa coat pattern. If the dense white area (PATN genes) are large enough, there will be few if any spots in it.
Rico is heterozygous for LP (LPlp) and his color tells me that his genome is most likely Ee Aa. How do I know that?
Well, Rico's sire is Ee aa, and his dam is EE Aa. He had to inherit an E and an A from his dam (because he is bay). His large blanket with the spots stretched out makes me think that there's a few types of enhancement going on here. So, likely he has one copy of recessive extension (the chestnut gene) "e". This would allow a bit more white pattern to show then if he was EE. I can see that he has sabino (blaze and hind white socks), but his blanket tells me that as well. Notice the lacy edges to the blanket? That's a very common trait with sabino. You can also see that the dots are a medium to small size. Sabino put the white on "steroids" allowing it to really restrict the area of base coat color within the dense pattern (i.e. dots). He obviously has one copy of agouti, which stretched out the white pattern, making the dots a significant distance apart. Now because his sire is homozygous dominant for LP, and his dam is homozygous recessive for lp, we know that this colt is LPlp. But even with out knowing the parents, seeing all those dots gives that away.
Here's how it works on a molecular level. Keep in mind that in utero, all horses start out white, and pigment is layed on top of that. The LP gene allows the pigment to clot, and this clotting action (during embryonic development) means that when the pigment begins to migrate from the spine towards the extremities (head, tail and legs) that it doesn't move like it should.
Imagine oil in water. You can shake it up, but the oil and water still separate. The base coat pigment does the same thing here.
Now, the difference between roans, blankets, and leopards, is that the pattern gene restricts this clotting to specific areas. A horse with a small pattern gene ends up with the vast majority of its pigment progressing normally, and only the last bit (since it starts at the hip/spine area and moves out) gets all clotted up. A leopard is a horse for whom all, or most of its pigment ended up clotting.
The enhancer and suppression genes basically work by making the clots more dense, and therefore smaller, or in other cases they work like magnets. Here's what I mean by magnets.... A supressor gene could cause the pigment to be drawn to more of its own kind, like opposite poles on a magnet. An Enhancer gene could cause the pigment to be repelled from itsself, like same poles on magnets. I think most of us played with magnets as a kid, and enjoyed watching one push the other away. Look at Rico's spots, and you can almost see how the dots were pushed away from each other, leaving the large white areas inbetween.
Now compare that with Crash:
Now, some of those enhancers and supressors can be pattern genes on their own. Lets look at Diva:
As you can see, Scorch doesn't have a blanket. Now, we know he got LP from his sire, because Spot can only pass a domiant copy of it. And we know that they all got lp from their dams, because all of these horses are from solid mares.
So, why does Diva have a blanket, and Scorch doesn't? It's either because Diva inherited a small pattern gene. The same thing that gave her 2 socks instead of just one. The other option is that Scorch just has more suppressor genes, and it completely suppressed his pattern. At this time there's no way to tell, and both are plausible options.
Now, if we look at the mothers of Diva and Scorch:
The sire of these girls is a bay splash white stallion. Obviously, he was heterozygous for agouti, since Jinx is black.
Their dam was a black Clydesdale. Both mares are 75% Clydesdale.
I had to lighten the picture a lot. Sadly, this mare is somewhat hard to get good pictures of unless she's very sun bleached and filthy.
By looking at these 2 girls, we know that Jinx got the sire's splash white, AND sabino, while Hex got the agouti and sabino.
It's not very obvious in this picture, but hex has a small white sock on her right hind, a moderate white sock on the left hind, and her front left is white only on the inside.
Interestingly, Diva has the same partial white front foot.
Since Clydesdales are well known to have sabino, it's a no brainer here. What is interesting is the dramatic difference in the level of expression of sabino between Jinx and Hex. While Jinx has passed on almost the exact markings that Hex - her sister - has.
This shows us that Jinx likely has an extra enhancing gene that allowed her sabino markings to be more dramatic then her full sisters. We know that Jinx also has splash, so it's possible that splash + sabino has an additive effect. Sadly, there's just not a lot of studies on these minor genes yet.
Now, one thing I'm asked about all the time, is if a horse will have offspring with MORE color then itsself. Why, yes, it's possible. Again this is due to all the additive genes.
For those who read Leah's blog, Barn Door Tagz, you probably know about her new baby, Daltrey. That little boy is a perfect study in how everything can add up together.
Here's the little man in all his glory!
Now, O has roaned out as he's aged, so his blanket has "grown". You can still see the dense white area that's on his hip, which he was born with.
So, if horses can't pass a pattern gene larger then what they have, how do you explain all that white on Daltrey? It's all about the additive effect. Daltrey got every possible enhancer gene that he could.
Daltrey inherited his sire's blanket pattern sized PATN gene. He inherited a dominant copy of LP from both parents, so he's homozygous (hence a solid white blanket). He is chestnut, which will express a larger version of dad's blanket (because E is a suppressor gene). Daltrey got his mother's splash gene, an enhancer, and his father's sabino (which is very minimally expressed on O). Daltrey did NOT get the unnamed suppressor gene that inhibited the expression of O's face and leg markings, so this expanded his blanket size even more.
In the end, it was a perfect storm. Daltrey got all the "crank up the white" genes, and none of the "lets put the white back in its place" genes. Teh resulting foal was much louder then expected. The chances of this combination happening are around 1.5% from this cross. Want to see the math?
50% chance of being chestnut x 50% chance of being homozygous LP = 25%
x 50% chance of getting the big blanket pattern = 12.5%
x 50% chance of inheriting splash = 6.25%
x 50% chance of inheriting sabino = 3.125%
x 50% chance of not getting the sire's suppressor gene = 1.56%
So, with only a 1.56% chance of getting all those genes at the same time, it's pretty fair to say it's not likely for a foal to not inherit a pattern larger then its parent's.
And tomorrow, I will discuss the importance of the solid horse in any appaloosa color program. I'll also touch on how outcrossing affects the genetics of it all.