Apples are an interesting case though, since they don't breed true at all. What happens is that lots of trees are allowed to grow from promising parent stock and then occasionally a new marketable apple is discovered. That tree is then cloned by grafting cuttings to established roots.

Fuji and Gala are quite sweet, but they also stay quite crisp a long time compared to older varieties, so it isn't just sugar content. Same with Granny Smith, which are popular because they are relatively tart (while still being fairly sweet for apples).