If you are looking to reproduce fruit trees... you may want to take a course on grafting. Even though my daughter knows this.. it in now way slows her down from shoving apple seeds or peach pits into her pocket after lunch to experiment with later. Or forget about it entirely until Mom discovers her little treasures in the washer or dryer.
A lot of fruit trees like apple, citrus, peach, pear, plum, avacado are reproduced by means of grafting. Haas avacado trees.. came from 1 mother tree.. which actually died just a few years ago.
Anyways, apple in particular is one that offers rather squirrely odds. Apple trees require another tree to pollinate, and preferable of a different strain. That makes for a lot of genetic possibilities in the following generation, especially with the fact that the next generation is able to produce something that is not anything like the parents!
I know.. it seems odd. Apples are crazy like that, which makes them fascinating to me. Just their scale also makes it out of my reach to mess with... for now. Heirloom.. when it comes to apples.. means an old cultivar.. it does not mean it is open pollinated. No such thing as an open pollinated apple! Aside from grafting.. some apple trees send out buds (or suckers) which hold the possibility of actually being different from the parent.
If you are limited on space.. seeds are not the way to go. If you have tons of room and just as much time.. then go for it. Keep in mind you could plant thousands of apple seeds and potentially only get 1 good tree out of the lot. By means of seeds however is one way new apple tree cultivars are created.
Just expect the unexpected. Russeting, varying storage capabilities, yields, coloring, shape, tartness, acidity, sweetness, size, disease resistance, etc. etc. are all a big gamble. You won't know the pay off for quite a few years until it fruits.
Just something to think about when you see an apple and all the varieties. Now maybe you'll see them like I do.. an amazing moment of genetic luck and truly something special.