The term is Piebald,and is a form of albinoism.This is NOT a result of inbreeding or some genetic mutation,it is from a lack of a pigment,melanin(I think thats how its spelled),which gives the deer its brown/grey color.If completly absent,you have an albino(all white),if lacking you get a Piebald.Albino and Piebald deer are not as rare as most people think,however they do suffer a high mortality rate to predators due to their lack of natural camoflage.Other than the obvious,there is nothing wrong(biologically),with deer that have this trait.Hope this sheds a little light on the subject...