You can't have a homozygous overo because that would mean your horse would have the "lethal-white syndrome." The horse will die within 72 hours because it has no immune system. If you breed an overo to another overo you will have a 1 in 4 chance of having a foal with the lethal white syndrome. If you breed tobiano to overo you wont have this problem but if you're willing to take the chance of killing a foal then by all means breed overos to overos.
^^^ true of FRAME overos. Sabino and Splash are also overos, and you can certainly have those in H/Z form. And just because a horse is tobiano doesn't mean it doesn't also have frame. Frame can be "invisible" on a horse (meaning, nothing indicating it's there, esp if other pinto genes are present).
You can have a horse tested for frame (LWO) very easily and inexpensively, HIGHLY worth it!
Your horse can't be homozygous for Tobiano or Overo. It could be homozygous for color if both parents are the same color. The only thing close you can get to homozygous when you breed a Tobiano to and Overo is a Tovero. Toveros will produce a Tobiano, Overo, or Tovero foal. No solid foals because a Tovero will pass on a Tobiano gene or an Overo gene.
Most horses are not JUST tobiano. Most have SOME overo (even if it's just sabino). So if a "tobiano" was bred to an "overo" you still COULD get one that is H/Z for tobi and/or some form of overo, IF the parents weren't "just tobi" or "just overo." I say that because horses will be registered tobi, overo, or tovero, but realy the majority are likely "toveros" and just not registered as such. Being just "overo" is more likely than just "tobi."
A "tovero" horse is NOT H/Z just because it has tobi and overo. You ABSOLUTELY CAN get solid offspring from them. The "tovero" MAY pass on Tobi, MAY pass on overo, MAY pass on both, or MAY pass on neither.
>>>> You can't have a homozygous overo because that would mean your horse would have the "lethal-white syndrome." The horse will die within 72 hours because it has no immune system.
Not entirely true-- their imune system is not affected---the defect associated with LWO is an incomplete/defective digestive tract (if you google Lethal White Overo Syndrome or horse "ileocolonic aganglionosis" you will find a wealth of online references).The foal dies because it cannot digest/pass waste. If not euthanized, the foal starts to show signs of excruciating colic a few hours after it starts to nurse.