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Here's the helpful link to it all! With most of the kit mutations, the amount of white can determine if the foal is dead. You should, however, get a notification that it has died if you breed in the barn.

http://hj2.huntandjump.com/forum/discussion/13757/new-kit-lethality-and-kit-mutation-load-explained#Item_11

Technically, Lethal White refers to horses that get the dominant form of the Frame gene from both parents. It gives the typical Overo pinto patterning to horses that also have other mutated genes that cause white spotting, many of the Kit mutations, and the Splash mutations as well. If the foal gets it from both parents, its digestive system is incomplete, it appears completely white when born, and it will die miserable from impaction colic in a couple of days. Olw stands for Overo, lethal white. It is commonly called Frame because it causes the pigmented hairs to fall along the top and the bottom line of the horse's body, giving it a roughly horizontal white area along the sides.

ObsidianKisune has given you the link that explains the Kit mutations and the crosses that are dangerous there in the game.

Splash 2 (PAX3C7OY) and Splash 3 (Sps3) are, like Frame, lethal when homozygous (2 copies) and lethal when combined with each other.

White patterns are always a sign of broken genes, and the more white is showing on the horse's body, the more broken the genes are. If the gene is broken too badly, in both copies of the chromosome it lies on, the foal will not survive.