It has a 25% chance of the foal being oo, a 50% chance of the foal being Oo (normal frame) and 25% of you getting a dead foal. Sometimes it turns out alright, but the risk is there
My SHs are full of SplSpl now, that has always been safe here in the game. Just OO, WW and WxO in case the white one hides frame and both parents pass it on.