How Apples Are Grown
Commercial apple trees are not grown from seed because apple seeds do not produce "true to variety." Instead, apple growers use grafting or budding to produce trees that will bear fruit of the same apple variety.
Apple trees reproduce from seed much like human families reproduce – even though you and your siblings may have the same parents, you all look at least a little different. In the same manner that apple trees grown from seeds may have the same "parents", the seedling siblings would all be a little different. So, every apple seed can potentially produce a new variety. This is in part why more than 7,500 apple varieties have been identified worldwide!
To create an apple tree of a particular variety, orchardists graft a twig, called a scion, from the "parent" tree onto a small, young tree called rootstock – really nothing more than a slender whip with roots. The scion contains buds from which twigs and leaves will eventually grow. The trees are protected in nurseries for 1-2 years after they are grafted before being replanted by the grower in an orchard.
Budwood of different trees can even be grafted onto the same rootstock, creating a tree that will bear multiple varieties of apples.
SOURCE: usapple.org, July 2007.