Overo Lethal White Syndrome (OLWS)

This week is the sad anniversary of my little lethal white foal. It was thankfully the only lethal white I've ever personally experienced, and while I didn't breed the foal, it was certainly an eye-opening experience. It was an unfortunate and unhappy ending to a brief life that I had not planned on bringing up again, except for a couple of incidents that have prompted me to feel like this post is warranted.


A few weeks ago a reader commented on my original blog post about the day. I didn't want to just publish the comment and have it get lost in the archives, so here is the comment:

"Great of you to share your story. I do want to clarify one fact. All overo horses carry one copy of the 0 and one n nO and the problem comes when you breed to another overo and thus 25% of the time BOTH parents will throw the O and thus the heartbreaking ordeal you were faced with. I have Overo's as well. I ONLY breed Overo to Solid horses that have NO O gene or tobiano horses with no O gene. Thanks again for sharing!"

The reason why I didn't want this comment to get lost is because this is a very common misconception. Not all overo horses carry the OLWS gene, and you cannot assume that all solid horses do not have it. Case in point, I have had a horse that was an overo test negative for OLWS--Truly Apparent.

I can send her test results to anyone who would like to see them, but as you can see, she is a true overo. I believe she had the "splash" overo gene, which has not been linked with lethal white, but they have not developed a test for "splash" yet.

I have also had a solid-looking horse test positive for OLWS--NSN Momma Tried.

As you can see, she has very little white on her at all. She did barely qualify for regular registry based on that line of white by her nose, but it is safe to say there are many quarter horses and thoroughbreds with more white on them than this horse has.

The point is, you can never tell just by looking at them. In genetics, there are our genes, and then there are millions of factors that control the expression of those genes. Lethal white can cause a horse to appear overo on its own, but the expression of the lethal white gene might be so minimal that the horse appears to be a solid-colored horse. The only way you can ever know for sure is by testing.

The test is very easy to perform. Although I'm not breeding my girls anytime soon, I am ordering the kits this week just to get it out of the way, so I will go over, on this blog, the steps for ordering the test, taking hair, and the results for each of my girls. If you'd like to review the test information, it is under "Color Coat Test" on the UC Davis VGL website. The test is only $25 per horse, which is significantly less than the cost of having a vet come out and euthanize a lethal white foal.

Furthermore, I think there is sometimes some confusion about how exactly the genetic component of lethal white occurs. Not everyone aced high school biology, and not all of us are good at remembering everything, so I thought I'd copy here an explanation I wrote out a few weeks ago. It isn't perfect, but hopefully it will help if anyone reading this finds genetics confusing.

Of course most of us know the biological event of conception, but genetically, the egg carries one half of a potential baby's genes, and the sperm carries the other half. So every time you want to calculate chances of what color a horse will be, what genetic flaw they might carry or even what color a baby's eyes will be, you have to factor in the genes of both parents 50/50.

The lethal white gene (frame overo) is a recessive gene (as opposed to a dominant gene), meaning a horse can carry it and will not suffer any negative effects from being a carrier and you cannot tell by looking if a horse is a carrier of the gene.

A lethal white foal is a foal that carries two lethal white genes (in other words, is homozygous for frame overo). That means that the foal has to get the gene from both parents--one from each.

Lets say you have two known positive horses. Their genetic code for lethal white is N/O (N stands for negative, O stands for positive, so they have one negative gene and one positive gene). So, if you take two horses that are N/O and breed them (N/O plus N/O), then the resulting foal will get one gene from each parent. 25% of the time the foal will get both N genes, so end up being N/N (negative for lethal white). 50% of the time the foal will get one N gene and one O gene, resulting in a N/O foal--a foal that carries the gene but is perfectly fine. Then the other 25% of the time, the foal will get an O gene from each parent, resulting in a foal that is O/O, or homozygous for the lethal white gene--a lethal white foal.

My stallion Eddie was positive, so there is a 50% chance that any foal of his is a carrier, regardless of what their dam's status was. I only bred him to quarter horse mares and negative (splash) mares, but his daughters and granddaughter could very well be positive for OLWS.

Therefore, they are all getting tested. As money allows, I will also test them all for HERDA and GBED, although Eddie was negative for the latter. It's important to have all the information at hand before breeding a horse and all these things cost money, which is why I'm starting long before I ever even think of motherhood for any of my girls. When the time comes for these tests, I will share the information on those as well.

Although this post is atypically dry and serious, I hope it's been at least somewhat interesting and informative. I hate to see anyone go through a heartbreaking experience because they had to learn the hard way.

Tomorrow we will be back to our regularly (or irregularly) scheduled programming....