Corn is a cross-pollinated crop; that is, pollination of the silks of any ear is largely accomplished by pollen from some other plant. When allowed to reproduce normally the resulting plants exhibit variation in all measurable characteristics. Each plant has received an assortment of hereditary potentialities from its two parents, some good and some bad. Even plants of excellent appearance may carry undesirable elements in a masked condition. These undesirable characteristics will reappear in later generations as a result of the reassortment of the hereditary material. If the undesirable elements are not masked, the affected plant may be low in yield or even barren. The frequent occurrence of barren plants or plants with small ears in open-pollinated varieties is responsible for most of the difference in yielding ability between such strains and the best hybrids.
The mode of pollination has been the major factor responsible for the failure or success of past and present systems of com breeding. The methods which have been used may be classified under the general heading of mass selection, ear-to-row selection, varietal hybridization and selection within and among inbred lines.
Sprague, G. F.
"Production of hybrid corn,"
Bulletin P: Vol. 2
, Article 1.
Available at: http://lib.dr.iastate.edu/bulletinp/vol2/iss48/1