Can someone explain the genetics of Frame Overo to me? I thought I understood it, but I've found horses that have frame patterning with oo and Oo genes. I thought it was a recessive gene, and then I thought it was a dominant gene, and now I don't know what to think. I've looked, but I can't find anything that answers my question. Anyone have an explanation?

Thanks!

Frame Overo is a dominant gene. If they have it, they will express it. What you're running into, and why you cannot find any homozygous (OO) horses, is Frame Overo is lethal in it's homozygous form. The foals will die shortly after birth. On HEE, that simply translates to a message that 'the foal was lethal white and perished', meaning you don't get the baby out of the match.

The same thing is true for White (W). You're not going to find any homozygous White horses because WW foals also die at birth.

I understand that part. I was just confused because I found what appear to be frame horses that have an oo genotype. Are those horses not really frames and are instead some form of sabino or something?

Yes, I'd say you're seeing a Sb or another marking! Spl could be what you're seeing as well. There's also different markings for homo/hetero patterns where some homo genes make patterns very large.

Great, thanks for the help!!!

Wait, if you breed an oo with an Oo, will you get an Oo or an oo?

You can get either. From an Oo and an oo, there are four possible outcomes- two are Oo, and two are oo. From an Oo and an Oo, you can get an OO (lethal white), 2/4 chance of Oo, and oo.