Most modern corn hybrids are F1 combinations of two inbred lines. The inbreds are planted in alternating rows typically in a ratio of 3 or 4 female rows to one male row. Although cytoplasm male sterility has been used most hybrids are formed by manually or mechanically detasseling the female rows and then relying on the male rows to provide pollen to the female rows. The F1 plants are uniform. However, independent assortment of alleles in the second generation creates a heterogeneous population in the F2, which typically reduces fitness and agronomic desirability.
Several theories suggest the basis of hybrid fitness.
One suggests that combining alleles from two different inbreds masks all deleterious recessive alleles. The second theory suggests that there is a positive or over-dominance interaction that occurs when two different alleles are present at the same locus. In reality both probably occur. However, the first factor may be more important. Because hybrid seed must be purchased each generation, seed sales per hybrid are significantly greater than for an inbred crop such as wheat or barley where seed may be saved back after each harvest for the next planting.