Many mutations occur during cellular replication but those occurring in haploid cells can have extreme expression because often these are in recessive genes. Dominant versions of the mutated, recessive gene are covered up in most diploid genotypes. Inbreeding objective is to make all genes homozygous as the breeder attempts to obtain consistent, repeatable genetics but with the potential cost of making homozygous some recessive genes with negative effects on the plant. Not only does this result in smaller corn plants as the inbreeding progresses, but also carries risk for a few diseases.
One example is susceptibility to Race 1 of Bipolaris zeicola (Helminthosporium carbonum). This variant of the fungus apparently is among other grass leaf pathogens of this species. It has genetics resulting in production of a toxin that is controlled by a dominant gene in corn. During the inbreeding process, however, and recessive version of this gene is made homozygous. Consequently, these inbreds are frequently heavy infected in many seed fields exposed to the pathogen. More information on this pathogen race can be found in 7/11/19 blog of Corn Journal.
Much of the increase in corn grain production, adaptation to multiple environments, disease resistance (and susceptibility), and specialty traits are the result of naturally occurring genetic mutations in this annual plant. Humans benefit that mutations occur in corn, as other forms of life, but we should not be surprised with changes from mutations that are often expressed in inbreds- and hope the other parent of a commercial hybrid covers up the defects.
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.