Yes, I realize this topic is over a year old, but I would like to point out some of my own thoughts:
Although Cadance appears as an alicorn even in Twilight's flashbacks, she was originally a pegasus who was granted a horn for discovering the "magic of love" like Twilight discovered the "magic of friendship".
She's not a true alicorn, as a true alicorn is described as having abilities of all three common ponies - earth pony, pegasus, and unicorn.
This being said, I do not believe Cadance would have the genes of a unicorn to pass on, which means it's practically a pegasus mother, and a unicorn father.
So I would want to say that it would be like 45% chance of pegasus, 45% of unicorn, and about a 10% chance of earth (If not less) And this is just a random estimate. One of Cadance's parents could have been a unicorn or earth pony, while the other is a pegasus - which would definitely tip the scales, some.