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, May 13, 2011

Congenital Stationary Night Blindness in Appaloosa colored horses

Congenital Stationary Night Blindness, or CSNB, is found in horses homozygous for the LP gene, such as snowcaps and fewspots.  In layman's terms, this means that a homozygous appy colored horse can't see at night.  By request, you can learn all the exact and current information about this at The Appaloosa Project.

Now, a study was recently done which proved this, but because it used a small sample size, and because many people own homozygous LP horses, there's a lot of people who have trouble believing the results.  But the results aren't just based on that single study alone, but also a lot of scientific knowledge, and other studies on other species.

Ready to have your head hurt?  Well, I'm going to explain to you why the study is accurate, and how we can be sure of this.  It does involve a lot of technical stuff though.  Again, always ask if I lose you some where, and you can email me (Link at the top) if it works better for you.

So, scientists have found the genetic location of the LP gene at TRMP1.  TRMP1 is the name of the SNRP (Pronounced as "Snerp") and is common in many species, including humans.  A SNRP is nothing more then one of those bands you see in the images of genetic codes (like on CSI).  It's a segment of DNA that breaks at a specified point, and has a specific density which sets it apart from others.

So, in other words, when they do gel electrophroesis, like in the image here, each SNRP represents a specific gene.  SNRPs break at specific points (I won't go into that because it's real headache stuff) and the size of the SNRP determines how far it travels.

Basically, in the image here, the DNA starts at the top, and a small electrical current is run through the gel.  Because DNA is polar, it will travel to the opposite electric charge, kinda like a magnet.  The bigger pieces travel slower (kinda like trying to move a semi through a traffic jam) and the smaller pieces move faster (kinda like a motorcycle in the same traffic jam).  And because the SNRPs break at a specific point, we know that the same gene will always be the same size (trusting me on this is easier then the explanation, which took me 3 years of college!).

So, if you were testing say, for Agouti, and your horse was Aa, there would be 2 distinct lines on the gel tray, one for the A and one for the a.  Control tests show how far the A and the a allele will travel, which is how we know which is which.  If the horse is aa, there will only be one line, even though there are 2 alleles.  Basically, the alleles overlap each other from our point of view, on sitting directly above the other.  If the horse is AA, the same thing will happen, but it will be at a different location then the aa results.

Now, when we found that LP is at TRPM1, we could make a few assumptions.  In all species that we know of, the protein created at TRPM1 is used for the same thing, basically.  (I say basically, because the different mutations do slightly different things).  It's a key to unlocking a gate.

Check out the image beside me here, it's a more common bio-mechanical pump (Sodium-potassium), but basically the same idea.  See how the little pinkish round things lock in on the left side and make the door open?  Then the green things come in, and make the door close?  Well, the same basic idea is what the TRPM1 protein does.  The difference is, that it works on a very specific cell to send a very specific signal.

The Rod cells of the eye, which allow us to see at night.

As you can see, the Na ions there are round, and fit into a round space to unlock the gate (simplified, so don't take that too literally).  A triangular ion wouldn't fit, so wouldn't unlock the gate. But, if you look on the right hand side, you can see the green squares locking in.  A triangle might just fit in there, wouldn't it?  So a triangle ion could close the gate, but couldn't open it, and it's unlikely that the triangle would work exactly like the square does (trust me again) but it might be close enough to get the job kinda done.

Here's why this matters:

Genes make proteins.  That's it, that's all they do.  Those proteins are chemicals that bond to things.  Really, it's that simple!  The complicated part is what the chemical reactions of those bondings do, and how it changes things, and what effect that change has on the things around it.  So, if instead of a circle, you had a triangle, then the gate above would never open, but if it was open, that triangle just might close it.

This is how mutations work.  A "mutation" is nothing more then an alteration of a gene by outside forces that creates something that is able to be passed on.  Appaloosa coloring is a mutation of TRPM1.  This means that the protein created by LP is not the same as the protein creasted by lp (LP and lp are alleles of the gene found at the location TRMP1).

If your head hurts and you hate me now, raise your hand!  =)

Ok, so the LP mutation makes a different protein then the lp allele.  The lp allele is the right one to "trigger" the gate in the rod cells of the eye.  It's the right shape to open the gate, that allows the charged particles to pass that make the eye cell "go" and send a signal to the brain.  In the case of eye cells, it's photons (light) that's allowed in, and sent on (again, that's real rough, but the basic idea).

Now, most people know about sickle cell anemia, and how a single gene changes the shape of the red blood cells.  Changing the shape of things matters a LOT in biology.  Just like the key to your door works by it's shape depressing and not pressing the specific tumblers in the lock, the proteins in the body press and fit into openings in other proteins thus opening doors and gateways.  Those openings and fittings are the chemical reactions necessary to make the body function.

So, in multiple species (human, mouse, horse) TRPM1 mutations are well known to cause nightblindness.  In most of those the result of the mutation is also called CSNB.  Because its been proven time and time again in different species, it doesn't take much to "prove" it scientifically in a new species with the same gene.  Why is this?  Because genes are genes and do similar things across all those species, because we all (from plant to human) share 95% of the same DNA.  DNA is not mystical, it's just a very simple little thing that has very cool results.

A good analogy here is mixing paint.  You know that if you mix black latex paint into white, you will get grey.  If you mix black acrylic paint into white, you will get grey.  If you mix black oil paint into white, you will get grey.  Even though it's different "species" of paint, the same proteins combine to reflect light in the same way, allowing us to interpret the color as grey across the board.  Sure, you might have darker or lighter grey depending upon how thick the paint is, but it's still grey.

In this manner we can make cross species assumptions.  If TRPM1 is the wrong shape, then night blindness will be the result.  The details might be different (an oval instead of a triangle protein) but the result (it doesn't fit in the space it's supposed to, so doesn't open the gate) is the same.  This is because TRPM1 is a basic life function protein... it triggers the rod cells to "go".

Now, here's how this applies to Appaloosas!

The horse here is obviously heterozygous for LP (i.e. she's LP/lp, or LP/n).  We can tell this because she has spots on her blanket.  LP works by making pigment coagulate, but that's NOT it's main function, but rather a side effect.  As humans, it's the easy thing to see, so we assume that's it's "job".  We are naturally wrong.

Because LP is incompletely dominant, that means that homozygous dominant, heterozygous, and homozygous recessive (LP/LP, LP/lp, and lp/lp respecively) all have a different phenotype, or appearance.  This also means that one allele of the DNA makes the "right" protein (lp) and the other does not (LP), and depending upon which allele the RNA is using as a pattern determines if it makes a working or a non-working protein.

Now, RNA just transcribes the DNA into the protein... it's the grunt man.  It just repeats this process over and over all the time.  It's not smart, and it simply transcribes what is right in front of it, so depending upon what it literally bumps into depends upon whether it's making the good stuff or the "bad" stuff.

Because a heterozygote makes both good and bad proteins the eyes will have keys around to unlock the gateways.  There's not as MUCH as in a solid (lp/lp) horse, but nature tends to overdo things... I mean have you SEEN rabbits breed?  Usually there's about 10 times more proteins then the body really needs, so in a heterozygote, there's only 5 times more then needed.  More then enough to make the eyes work.

In a horse that is LP/LP though, there's none of the right protein.  This means that the rod cells are waiting for a round key, and there's a whole lot of triangles sitting there that don't fit.  The protein can NOT do it's job, no matter how hard it tries!  It's the square peg/round hole problem, literally.

So now some of you out there are saying "well ok, but MY horse can see just fine and it's homozygous".  Uh, no, it can't.  It may DO just fine, but it can't SEE just fine.  You're assuming that because you as a human use your eyes for everything, that horses do as well.

Do you KNOW how poorly horses see?  The use sound, feel, smell and all of their other senses MUCH more then sight.  Keep in mind that horses can not focus their eyes like humans can.  Rather horses use various thicknesses of their cornea and light refraction on the retina to focus objects.  Just imagine a person who wears glasses trying to get around with out them, and you have normal horse vision.  Not really something you'd rely on too much, eh?

So sure, some Appaloosas and Appaloosa colored horses get along just fine at night, even those with CSNB, but it's mostly because they have never known any other way.  Basically, if your horse does "just fine" at night, it's not because he or she can SEE better, it's because he or she can sense better and is smarter then other horses who do not do as well.

And the question I get the most "Can I ride a night blind horse at night?"  Well SURE!  It's not like they ever see well, but as a rider YOU see worse at night.  If you think you have a well trained enough horse to listen to you about potential hazards that they can't hear or smell, then sure... ride the horse.  I personally have no trouble riding my CSNB horses.

Correction 2/15/2012 - It was brought to my attention that I had not only transposed the letters of one of the genes mentioned here, but then informed my spell checker to auto correct all references to the incorrect version.  OOPS!  That has now been corrected, and a link to the Appaloosa Project has been included, by request.

6 comments:

  1. Wow what a very informative post! You really make it easy to understand what is going on to cause night blindness! I always love reading your blog and find out so much information!

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  2. Jeez, Heather, mine was much simpler :-p

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  3. Wow what a very informative post! You really make it easy to understand what is going on to cause night blindness! I always love reading your blog and find out so much information!

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  4. I have a mare that I was told was a paint/TB cross that is night blind. Is this a disorder that happens in other breeds or is her disorder probably caused by something else?

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  5. There are a few types of night blindness. CSNB is directly related to the LP gene, but other types have the same symptoms. Others are from things such as congenital defects, or damage from an eye infection, as examples.

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  6. Should a nightblind appy mare be used as a broodmare. Or will she pass this to her fosls?

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