Just like people, most fruit trees have children that are different from the parents. It doesn’t mean the child of a Red Delicious apple is going to be a pear, but it will not be the exact same as the Red Delicious mom, it might favor its dad. Six children from the same pair will have as many differences as six human children.
There are around 6000 named varieties of apples. Most are being lost because of the monetization of selling fruit trees means selling a few named varieties on the rootstock of other plants. In the US for example, you cannot go down to the local box store nursery and buy more than 4 varieties of apple. Since anyone in your neighborhood will only have a choice of 4 varieties, apples lose the survival strength in genetic diversity. If a disease comes through that affects Jonathan apples, for example, all Jonathans over a very large area can be wiped out at the same time. It is a form of monoculture that encourages plagues.
It is as if your town had only 4 humans, and you could have babies with only 3 of them. Add in that there are 10,000 of each 4, all identical in their DNA. The gene pool is too small for healthy reproduction in any given town. John, Joe, Jill, and Mary… repeated endlessly.
Add in that purchased fruit of each of the 4 types may well carry almost identical dad genes from the same crabapple species. That means that John, Joe, Jill, and Mary are half siblings. Creepy.
Commercial growers use crabapples to fertilize named varieties, and one consequence of that is when you buy an apple in the market economy, you are almost guaranteed to not be happy with fruit from a seed grown tree. The child will have half crabapple genes and probably be smaller and tarter. Nothing wrong with that unless you expected the sweet apple clone.
If you realize that daddy is a sturdy little crabapple with higher nutrient density than mama Red Delicious, the resulting smaller apples from the child may not upset you. The upset is in the expectation rather than the result.
I have grown many apple trees from seed. My grandson started one from seed too. He was 5 years old. If he is still living in that house when it fruits, he can name “his” variety… Starwalker out of Gala x unknown or some such.
Named varieties are crossed with other named varieties to get new sweet (tart, cider, crab) apples. No commercial operation wants to allow cross fertilization between fancy apples because for millenia, everyone grew their own backyard apple tree from the seed of a tasty apple. To monetize apple trees, that could not be allowed.
Grafting allows a specific tree to be reproduced ad infinitum and sold in the market economy as a guaranteed result.
To do so, growers grow specific rootstock trees, and different types of rootstocks limit the size of the tree. That is how you get full size, semidwarf, dwarf, etc.
I have 4 ultra dwarf fruit trees in large patio pots. I have to water and baby them, but I can have more varieties in a small space. My sweet apples can be fertilized by other sweet apples and have interesting sweet apple children. I have picked 6 varieties covering 3 bloom seasons for cross fertilization. Four varieties will be purchased online because they are not readily available locally. By purchasing from a specialty grower, I might double the local gene pool for apples. Bizarre if you think about it.
In Seattle, you can find 30 varieties of apples in the marketplace. Their seeds will also be half crabapple, but more variety overall. It helps to be in apple country.
I have Sargent Crabapples already. They are sturdy little survivors on their own roots. These are planted uphill from my sweet apples. If they cross, it can only add to the survivability of the offspring. Will I become a tree breeder? Nope. I just want more diversity… because I want a few storage apples, a few cooking aples, cider apples, and one that has a hint of pineapple.
Even so, if the pollinators cross fertilize my crabapples with a sweet, I might like to plant seed and see how it goes.
If you do buy a grafted tree, one on full size root stock will survive winds and drought better. If you get a dwarf rootstock, it will require more care because the roots are shallow. One benefit that commercial growers like about smaller trees is ease of picking. Me too.
Grafted trees bear earlier, especially dwarf trees, and they die within 10-25 years. Apple trees grown on their own roots can bear fruit 100 years. I want ultra dwarf trees for now and will use some of their seeds to grow full trees. For disease resistence, species diversity, water, and healthy soil are most important. I will say I never get much in the way of disease. I have always improved the soil and never stress soil or plants with toxins.
The rootstock is grown to a particular size from seed. It is then cut off and a limb from a named variety is cut and attached to the first tree’s roots. Once the graft “takes” then the tree can be dug and sold bare root or potted in the market economy. It is easy to see the graft joint at the bottom of the trunk. It looks like arthritic fingers.