Coat Color Studies Aid Health Research, Breeding Choices

[Joanne]

This article just came out today online at The Horse.com

Here is a small portion of the article discussing the silver gene:

Another example of a coat color gene associated with health problems is the silver dilution, which can be associated with an eye defect known as anterior segment dysgenesis (ASD), a genetic malformation of the anterior segment of the eye that causes development of cysts, misshapen pupils, and defects of the iris, cornea, and lens.
The association between the silver gene and ASD has been documented and studied in the Rocky Mountain Horse breed in which silver-diluted colors are prevalent. Silver dilution is also present in other horse and pony breeds such as Miniature, Icelandic, Morgan, and Shetland, and there are indications that ASD may not be restricted to Rocky Mountain Horses.

While there is a clear association between the silver gene and ASD, the specific mechanism leading to the eye defect is not known. It is hypothesized that ASD is caused by a gene located near the gene controlling the silver dilution, and that selection for silver-colored breeding pairs increased the incidence of ASD over time through a "hitch-hiking" effect. Because the silver color is highly desired in the Rocky Mountain breed, ASD presents a particular challenge to these breeders.

While the silver gene might not directly cause ASD, the silver coat color is associated with ASD in Rocky Mountain Horses, perhaps because the gene causing ASD is located near the silver gene and has been propagated with the color in this breed. A genetic test for silver is available through the VGL, but because the genetic basis of ASD has not been identified, horses cannot yet be DNA tested for the disease itself.

To read the entire article which covers many color genes, here is the link:

The Horse.com Article

 

[Shari]

I have been reading about it on a couple of fronts.

With Icelandic's , if you breed one Silver with a normal colored Icelandic without the Silver gene, they are finding you end up with a lot less problems with the ASD. So far the horse's they have tested from those pairings, the resulting Silver foals haven't had eye problems. But when you breed two Icelandic's with Sivler gene's, nearly all the resulting foals end up with some level of ASD.

Wonder if it would be that way with other breeds?

 

[uwharrie]

I am a bit confused about the color connection. Everything I read talks about silver dapple gene (in Rocky Mnt. horses)

Is that the same as the silver gene in miniatures?

 

[Shari]

The Silver gene is the same in any breed of horse, if that breed carries it.

So, yes... it is the same gene in Minis, Icelandics, Rocky mountains and so on.

 

[Joanne]

The article also discusses flaxen, frame, and other colors.

It is worth reading.

 

[Minimor]

I thought more recent research showed that the eye disorder associated with silver is actually something different than ASD???

In any case I want to say that all silvers do not have this eye problem. If you breed two silvers that don't have the eye disorder, the resulting foal isn't going to have the eye disorder. So, breeding silver to silver isn't necessarily going to result in a foal with defective eyes. These articles never make that clear and many are left with the impression that silver to silver is a cross that should never be done.

 

[Margo_C-T]

May I ask, what is the source for the 'recent research', for I've not seen it?

It may well be that there is yet no 'firmly proven and direct' link between silver and ASD, but until there is, I'd personally not recommend breeding two horses which BOTH carry the silver gene...and remember, this includes not only manifest silvers, but silver bays, and ALL others who include the silver gene in their genetic makeup. Silver, and silver bay, are probably the most numerous, but it seems that the silver gene may be part of the overall genetic makeup of numerous other colors, as I've seen 'described' on posts on this very Forum.

I've known about ASD now for quite some time; in fact, had a website devoted to ASD 'saved' on my computer for several years, but deleted it awhile back, as I no longer own any breeding possibles that carry silver(and am not breeding, period, anyway!) I would imagine that the site still exists, though, and *might* have the most recent knowledge on the subject--those interested in knowing more could try googling for the website.

One 'telltale' sign of ASD is that the eyeball seems to 'bulge' outward in profile more than is normal. I reiterate-- I would personally NOT breed a silver or a horse bearing a silver gene, to another, nowadays....just my choice.My best-producing mare was a silver bay (now past breeding age); luckily, I never bred her to anything 'bearing' the silver gene, as the only stallion I ever owned/used that carried it was a silver bay pinto, who sired a dwarf and therefore was eliminated from my breeding program immediately, as was his only offspring(and, the dam of the dwarf-- thankfully, this was very early on in his 'breeding career', so only a couple of offspring born prior to this revelation-and the dwarf was the first offspring of the dam.)

Margo

 

[Shari]

I have had Silver Bay mini's and Icelandics... in fact I have a Silver Bay Icelandic filly right now. Her sire is Silver bay and her mother is just an normal bay but her eye's will be checked once she is weaned just to make sure. Right now everything seems normal.

One reason I have been reading up on this gene and the issues that some times come with it. It has also been recommend on the other articles not to breed Silver with Silver, not worth the risk to me.

 

[Dona]

I, like Margo, have also had several sites on ASD saved on my computer for years, but the links no longer work. I would imagine if you "googled" ASD ( anterior segment dysgenesis) you could find a lot of info on it. I found out about it after I sold a silver dapple mare, who was discovered to have it after after she was sold. I did a lot of research after that & found that it IS associated with the Silver gene....but not all Silvers carry it. At the time, it was thought that all homozygous silvers carried it....but I personally know of some homozygous silvers who do NOT have it, so don't think that "theory" holds water. There are some markers to look for tho....like "pig eye" (smaller eyes that protrude more than normal).

 

[Joanne]

Thank you both for sharing your experiences.

I never heard of it before this article in the veterinarian journal "The Horse".