Apples have some of wildest biology that I've seen as a non-professional biologist. At many genes, they carry two different alleles instead of two matching ones. They're almost all self-incompatible. A Honeycrisp flower cannot pollinate a Honeycrisp flower.
This means that you can't breed them in the traditional sense that we do with other plants. Outside of Granny Smith apples the ones you eat are mostly the results of mutations in the grown plants that we cut off and graft.
Heterozygosity and self-incompatibility are common to fruit trees in general, not specific to apples.
I think ancestral citruses (not the crosses) are the primary exception, as well as a few citrus hybrids which create clone seeds asexually.
> Outside of Granny Smith apples the ones you eat are mostly the results of mutations in the grown plants that we cut off and graft.
Honeycrisp is a chance seedling, though it stems from a horticultural crossbreeding effort it was grown from seed. And commercially useful sport mutations tend to come from popular chance seedlings to start with, since it is necessarily a more limited evolution of the basal cultivar, which needs to exist and in sufficient quantity that you’ll roll out useful mutations eventually (also useful is in the eye of the beholder as we can see with the red so-called delicious).
The coolest thing about some apple cultivars is how their story is "someone found an apple tree at the side of a road with tasty apples", and people kept grafting them to this day. Boskoop, for example.
> Outside of Granny Smith apples the ones you eat are mostly the results of mutations in the grown plants that we cut off and graft.
Not sure where you're located, but this isn't true in the US. The vast majority of apples available here are seedlings of crosses by two other varieties. Examples:
Golden Delicious, one parent is Grimes Golden, the other unknown.
Fuji, Red Delicious × Ralls Janet
Gala, Kidd's Orange Red × Golden Delicious
There are certainly some sports (the technical term for for the spontaneous mutations) like you mention. Royal Gala, available in NZ, is a sport of Gala. But most are not.
An enormous amount of industry effort goes into crossing known varieties, growing the seedlings, and looking for the apples with the best traits. Cosmic Crisp is a great example of a recent one.
Oh wow you may have just answered a question I been having. I planted some apple trees in my backyard. The AI helping me suggested 3 trees, one of which was a crab-apple tree that it said will be used to pollinate the other two. Maybe it lied to me. But i did follow its instructions. I was wondering why the other two (Honeycrisp and another delicious variety) could not just pollinate each other.
Professional advice is the way to go. Much better than AI. Two trees of exactly the same apple cultivar will perform wildly different depending on were you buy it.
Crab-apples are good for pollination IF they overlap flowering. Not so much otherwise. If you have an early flowering variety and a late flowering crab-apple forget it. You don't need them as a third apple may work perfectly.
In any case they are beautiful trees and excellent for wildlife. You will not be disappointed with the excellent spring and fall displays, but if the space is limited I would go for a different tree or a third real apple choosing carefully the rootstock. If you want a crab-apple put it at the background. (Optical effect. It makes the garden appear bigger). If you like to watch birds while sipping your coffee in the front.
Crab-apples would be the kings of garden if they didn't suffered from so many diseases.
The GP comment is overstating the genetic differences and pollination difficulties. Most apples are not self-fertile. Pairing pollinators together has more to do with bloom times; some trees bloom earlier/later than others and trees need to have flowers out simultaneously.
This means that you can't breed them in the traditional sense that we do with other plants. Outside of Granny Smith apples the ones you eat are mostly the results of mutations in the grown plants that we cut off and graft.