“Well, as I see it, there are two possibilities. Possibility one, it’s genetic. The phenotypes of the three races are typically mutually exclusive. Even if you have all three genes in you, generally only one manifests, like with the Cake’s foals. On the other hoof, there are exceptions. Cadence is a rather dramatic one, she exhibits both pegasus and unicorn traits, but she isn’t a true alicorn. Winged unicorn might be more taxonomically accurate. Also, you have ponies like Blueblood and Roid Rage who exhibit more than their share of earth pony traits despite being a unicorn and pegasus respectively…”