There are a ton of post on here explaining this just do a search, but if you are certain that all your cows are clean then all the calves will be fine (not affected), but in theory approximately half of them will then be carriers. A carrier bred to a non-carrier will have 50/50 chance the offspring will be carrier/non-carrier. A carrier bred to a carrier will have 50% chance of carrier, 25% affected and 25% clean. I have several carriers myself but IMO if you have a clean herd don't introduce it, there are still very good bulls out there that are clean you can use. I know you probably like the color of him but just use a clean shorthorn bull to try and get some cool colors with your Hereford cows.