Digibrony (http://www.youtube.com/watch?v=BqQ1piy-7sQ) and PaleoSteno (http://www.youtube.com/watch?v=bolwd7xaZvs) take on the Pony Genetics and Pony mating habits topic on and seem to get a good idea on it. I don't know, I would assume the outcoming foal(s) would either be unicorns or pegasi. It also seems that ponies can become alicorns and are not born as alicorns (I personally think Celestia and Luna are the only TRUE alicorn, born with wings and horn, seeing as they are old as all get out) but thats my own personal head-cannon.