According to Apples: A Catalog of International Varieties by Tom Burford, there are 17,000 varieties of apples! Most of the apples grown commercially are probably diploid (2n), although there are many triploid varieties. For example, 'Gravenstein' apples are triploid with a chromosome number of 51 (3n=51). They are produced by the union of a diploid egg (2n=34) and a haploid sperm (n=17). This is accomplished by crossing a tetraploid plant (4n=68) with an ordinary diploid plant (2n=34). Because the triploid (3n) varieties are sterile, they must be propagated by grafting, where the scions of choice cultivars are grafted to hardy, pest-resistant root stalks.