I found this very interesting. And since it has to do with grafting you may as well. If you plant an apple seed, the resulting tree will not produce the same kinds of apples, they may even be unsuitable for human consumption.
The reason is because apples seeds are "'extreme heterozygotes', in that rather than inheriting DNA from their parents to create a new apple with those characteristics, they are instead significantly different from their parents."
If you want an apple to be the same as its parent you need to plant a branch.
The reason is because the offspring of an apple tree are naturally diverse.
Root stock can have an effect on apples, but generally it affects the growth of the branch and shape of the tree, not the apples themselves.
I don't know about all cultivars, but there are definitely some cultivars' apples that aren't affected by root stock at all. There was a study done back when I was in the schools that details a specific kind of apple and rootstock had no effect.
With some trees, as well as with most grape vines, the rootstock is necessary because the desired fruiting plant is incapable of surviving in the ground due to mites, or acidity, or some other factor.
Walnuts are a good example of that.
Why do I know all of this? Because, apples are in the rose family, and roses are my little hobby.
