Corn inbreds and hybrids

​The vast majority of genes in corn must relate the basic structures and functions of all corn plants. Mutations of these genes tend to be recessive and therefore are usually not expressed if the dominant form of the gene is present in the matching area of its paired chromosome.  On the other hand, recessive genes with potentially beneficial (to us) traits need to have both members of the paired genes be in the recessive form. The enigma of attempting to benefit from hybrid heterosis and yet obtaining repeatable genetics by making homozygous parents is that the process of reaching homozygosity allows expression of the recessive traits.
 
Some of the expression of homozygous recessive traits may be drastic, such as albino plants.  In some cases, it is expressed as susceptibility to a pathogen, such as race 1 of Bipolaris zeicola (Helminthosporium carbonum), resulting in an occasional inbred homozygous recessive for susceptibility.  The majority of negative recessive genes affect the chain of biochemical pathways resulting in a reduction in effectiveness of those products.  Highly inbred plants are less vigorous than hybrid plants because of this overall reduction in the physiological processes contributing to cell and plant structures.  It is assumed that this is caused by the expression of recessive genes that are not expressed when paired with dominant forms of these genes.
 
Corn breeders attempt to overcome the negative effects of inbreeding by mating inbreds with compatible inbreds to cover the deficiencies of each parent inbred. Considering the large number of genes in corn, and the diversity of its history, identifying the best combination for maximum desired performance is not easy.    It is known that inbreds developed from certain ‘families’ are most likely to combine well to make productive hybrids in most environments. A broad group of varieties generally grown in the western part of central USA were intercrossed for several generations to create a breeding population known as Iowa Stiff Stalk.  Inbreds developed from this population tend to create vigorous hybrids when crossed with certain other populations originating from the Eastern USA. This principle of crossing inbreds originating from distinct populations applies to hybrid corn breeding in all regions where corn is grown.
 
The challenge for corn breeders is to not only select the appropriate breeding source based upon desired history of agronomic characters, but also identify prospective hybrid parents with desired characters and identify the appropriate other parent for maximum hybrid performance. Despite our gains in ability to analyze DNA, physiology and environments, the variables are multiple. There has never been the perfect corn hybrid for multiple years and locations- and the complexity of plant and environments will maintain this history for many years as well.