Whiltetail/mule deer hybrids are pretty common. Many mammals of the same genus can interbreed freely. Whitetail and mule deer are genusOdocoileus. My close friends have killed them and simple genetic tests have confirmed hybrids in many states. I could drudge up some peer-reviewed articles for you if you wish but a quick call to your local state biologist should clear up any confusion for you.