Genetic diversity in a corn breeders nursery allows for many characters to choose with each season. Each generation of selection, whether nearly instant by dihaploid system, Rapid Inbreeding® system, or traditional selfing has the intent of selecting the plants with a set of genes that will work with another inbred to produce a superior hybrid. A new mix of genes are created with each source used as a starting population. Thirty to 40,000 genes arranged on 10 pairs of chromosomes with a small occurrence of mutations with each generation, affect multiple structural and functional characters of the plant. Much of the DNA codes for synthesis of specific proteins composed of a specific string of amino acids. Arrangement of those specific amino acids affects the enzymatic performance in cellular products, ultimately resulting in the final corn plant structure.
Even with modern methods, the plant breeder mostly needs to depend upon expression of those final characters that can be seen or measured to decide which seed to save. Each method of selfing the plants results in at least a few genes becoming fixed with a DNA arrangement resulting in an undesirable cellular function. Reduction in size of nearly all aspects of corn plants occur as the genetics approach homozygosity because of the accumulation of poorly functional genes. These changes always occur during inbreeding, but the corn breeder must choose the plants with favorable characters also occurring with the new combinations of DNA available from the population.
The ultimate evaluation of inbreeding selections needs to be made by combining the inbred with another inbred that has DNA arrangements compensating for the DNA arrangement deficiency of the other inbred. Hybrid vigor is the genetic expression of the new combination of DNA composition of individual genes in one parent allowing production of a functioning enzyme that was not happening in the other parent.
Despite increasing lab methods for evaluating DNA structure, perhaps the large number of genes, each composed of a string of nucleic acids whose arrangement affects resulting protein structure and function, will not allow much gain in efficiency of making choices in the breeding nursery. Every corn breeding program strives to improve the efficiency of selecting hybrid parents, but the final selection will be determined by hybrid performance in the field.
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About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.