ok, so an overo can carry 1 LWOS gene and one normal gene, and your toby shouldn't have an LWOS but if he's a tovero he could so... good idea to test (I think UC Davis does it). If both carry the gene you have a 25% chance of having a lethal white foal which will die within a few days of birth, no cure for it. With those same parents you'd have a 50% chance of producing a carrier (won't have LWOS but could pass the gene on if bred), and 25% chance of having a normal foal. If only your mare carries the gene you have a 50% chance of a carrier and 50% chance of a normal. I would test, I'd rather spend the money than risk heartbreak on the other end (plus all the money that went into her pregnancy and the foal)