The Risky Frame Overo Allele

For more information on this, check out this and this. Thanks! 

Frame overo in a heterozygous form is a beautiful white pattern, but in its homozygous form (known as LWS or the like) it will kill any foal within 72 hours (give or take) of birth. The exact genetic reasoning why is confusing (however, if you're interested, see this), so I'll skip the how and why.

*Ahem* Just so you know, OO is genetic shorthand for two copies of overo, oO/nO for heterozygous, and oo/nn for a non-carrier. */Ahem*

The normal (as in nO) frame over pattern has no health implications that come along with the allele, and is very beautiful. However, there's also another form of heterozygous, and this is the recessive form which you don't see at all. Sometimes the oO pattern can also be masked by other white patterns, such as tobiano (To). In cases such as this, it's especially tricky to figure out your horse's color, so I'd suggest having her or his color genetics tested.

The two mares on the right are frame overos, or oOs.

A LWS foal sometime before euthanasia. (Image found on Equine Color's gallery, not sure who took it.)

What Happens
When two copies (aka OO) of the frame allele reside in a horse, the horse only lives up to a few days after birth. Somehow, the OO alleles are linked to a non-functioning colon, which will cause colic (and it is sad, but there's really nothing you can do about it). Generally once a LWS foal starts colicking, the owners will have him euthanized. Either that, or the foal dies a slow, very painful death. Your choice.

Prevention  
How to prevent a LWS foal? The only safety in this case is prevention, and not taking chances. If you've got an overo mare - only breed her to a stud that has been tested and confirmed as a non-overo carrier. When you breed two overos together, you'll always have a 25% chance of an OO foal, and that's a bigger risk than you should take; even if you have a 50% chance of getting a spotted oO foal.

Identify a LWS Foal
This is actually really easy. All OO foals are white or almost-white colored, sometimes have patches of pigment, the skin on the non-pigmented areas is pink, the eyes are blue, and sometimes these foals are deaf.

 As most horse colors are not born this color, it shouldn't be too confusing. However, sometimes Dominant White foals crop up outta nowheresville, and they are about the same color as an OO foal. Also similar are the double-cream dilutes, such as a cremello or perlino.

Anyway, the best thing to do if you aren't sure is have the foal tested before talking to the vet about euthanasia, so long as the foal isn't in a fatal bout of colicking. It's only $25 as of now, so would you rather pay that, or accidentally put down an awesome foal? Yeah, I thought so.

Anyway, if you have any overos, or know anyone who does, please make sure they know the risks involved in breeding the lovely creatures. Thanks, and God bless!