Animal GeneticsAnimalsBlogHorses

Horse Genetics: Overo Lethal White Syndrome


Imagine having a cute little foal, pure white, hobbling around your barn. But then, imagine that the same cute horse dies within 72 hours! Harsh, right? But it is the simple truth. What is the case with that otherwise healthy foal?


Frame overo in the horse

Frame overo is often greatly misunderstood, mostly because of a link with lethal white syndrome, which we’ll talk about in a minute.

But frame overo isn’t, in the least, lethal on it’s own. Frame overo produces a highly desirable white color pattern on the horse, as seen in the above image.

Every living frame overo carries one dominant gene mutation that causes the irregular patterns on it. The same mutation becomes lethal when two copies come together, hence why frame-to-frame breeding must never be done.


What is Overo Lethal White Syndrome?

Overo lethal white syndrome (OLWS), alternatively known as Lethal white(LW), overo lethal white (OLW), or lethal white syndrome (LWS) is a lethal condition in foals when two copies of the above mentioned gene come together, making the foal homozygous for frame overo.

A homozygous frame overo or lethal white foal will not live for more than 72 hours after birth due to incomplete nerves in the foal’s digestive track.

Properly carried out breeding can prevent this from happening. But since a frame overo has only a 50% chance of passing down the mutated gene, many greedy breeders will take the chance of producing a top foal by breeding two frames, albeit 75% of the foals produced from this breeding suffer from OLWS.


Misconceptions about frame overos

Countless misconceptions related to frame overos in general are being born, because of their link with lethal white. However, most of those are, safely, myths.

Many people geld otherwise nice stallions just because they result positive for frame overo.

I must emphasize that they really are nice, since very few stallions are left who truly are nice.
Quite a few stallions are just a quick money machine producing horses that are only fit for the halter ring.

While I also did emphasize not to breed frame-to-frame, there is nothing wrong with a frame overo. In fact, they produce some amazing foals and color when bred to a non-frame!

Frame overo is nothing to make you geld a nice stallion, so long as his conformation and health are all in order. Remember, frame can only become lethal when bred to another frame, in which case the foal has a chance of being a LW foal.


How does one prevent OLW foals?

It’s actually very simple and easy. A basic test that doesn’t cost too much – at any rate, costs less than a dead foal – lets you know whether your horse is a carrier for LW, or in simpler terms, if it is a frame overo.

Many people carelessly, to say the least, don’t test ‘solid’ horses. However, frame overo can be limited to as little as just a bald face and socks or stockings on their legs.

Just because your mare or stallion doesn’t have any other white markings doesn’t mean it can’t be a frame overo. While frame mayn’t be deadly on it’s own, breeding a frame-to-frame can be deadly as I said above.

I fully recommend testing any horse who has any white markings in her pedigree, even if it is four or five generations away for LWO.

The test will confirm whether the specimen in question is a N/N, N/O, or O/O.

The above terms may seem like utter nonsense, so let’s go further into it.


The test results

If you think the above terms I mentioned make no sense to you, this section will help as we dive further down into the test.

The test checks for the gene mutation in a horse. Remember the gene mutation I mentioned above?

Well, that’s a dominant gene so therefore, if there is only one gene, it shows the desirable white marking. To rephrase that, only one gene is required for the markings.

When there is no gene for frame overo, then that horse is N/N for frame overo. In english, that’d be negative for frame overo.

When there is one gene for frame overo, then that horse is N/O for frame, or positive. This specimen has a 50% chance of passing frame overo down to its offspring. It shouldn’t be bred to another N/O as this could produce a LWO foal.

When listing a horse for breeding, and if there is a chance for him to be a frame, a test is mandatory and the results must be listed on his breeding ad.

Most N/O stallion owners clearly list the right to have the mare tested and only be bred if she’s N/N. That is a clear example of ethical breeding.

A mare owner is also responsible for testing the specimen and shopping for a stallion accordingly, and not even considering any N/O stallions.

When there are TWO copies of the frame overo gene, the horse is O/O, or homozygous. This foal would be a OLW foal and will die.

In more technical terms, a N/N is called ‘Negative for frame’, a N/O is called ‘Heterozygous for frame’, and a O/O is called ‘Homozygous for frame’.


In review

Okay, so to recap:

  • Frame is NOT lethal when only one copy is present
  • Two frames mustn’t be bred to prevent OLW
  • Ethical breeding can be utilized to prevent fatality
  • A heterozygous horse can pass its gene down 50% of the time
  • A homozygous foal will die within 72 hours of birth
  • A negative horse can never have a lethal white foal
  • To prevent OLWS, a frame overo must be bred to a N/N
  • A N/N can be bred with any horse

With that, see you next time!

[Featured image by]

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