Homozygosity in corn

​More than 100 hundred years ago, it was evident that crossing corn varieties independently selected for performance in their environments would result in performance boosts.  Academics realized that this boost could be increased and more consistent if the parents derived from the unrelated varieties were made genetically consistent by making genes homozygous as opposed to the heterozygous state of most plants in these populations.
 
With 30,000-40,000 genes located on 10 pairs of chromosomes, getting the same set of nucleic acid codes within each gene on both members of each chromosome pair whether through continuous selfing or use of double haploids includes some random selection of phenotypes.  It results in multiple phenotypes, including many not related to the ultimate performance of the inbred or hybrid.  We try to evaluate the presence of homozygosity by morphological consistency by looking at characters such as tassel branches, plant height, silk color, leaf features traditionally in samples grown after hybrid seed production.  Later methods involved evaluating the protein products of the genes in a process called electrophoresis.  PSR offered a different approach in 1987 after realizing the process of inbreeding caused consistent seedling characters unique to each inbred and hybrid.  As with the other methods of purity evaluation, the characters displayed may not have significant effect on performance but indicate a slight level of heterozygosity.  Slight variability of seedling characters distributed among seedlings of a sample reflects a small lack of homozygosity, but drastic differences indicate unintended genetics such as from pollen from outside the seed field.  If all the off types are identical to each other a seed mix is indicated, either within the seed field or after harvest.  The Seedling Growout® method (by PSR) offers the advantage of larger sample sizes and shorter time for evaluations than the field methods.
 
A small amount of variability in hybrids is not necessarily detrimental to hybrid performance unless it involves lack of a gene for resistance to a herbicide.  Obtaining and maintaining complete homozygosity in inbred parents is more significant if the inbred is to be maintained for many generations.  Independent maintenance of standard public inbreds such as B73 have been shown to have slight differences among them either from accidental contamination or mutations.  Seed companies need to consider the long-term prospect for an inbred versus the time and effort involved in obtaining complete homozygosity in a new inbred or an inbred recently introgressed with a trait. 
 
Self-pollinating corn plants promotes the genetic uniformity, including both the dominant and recessive genes.  Recessive genes with negative effects not evident when the other member of the paired chromosomes has a dominant form of the gene, show the negative trait when homozygous for the recessive.  Accumulation of these negative homozygous recessives does result in a reduction of plant size of inbreds.  Hybrid vigor is expressed when crossed with another inbred that has the dominant matching gene.  This becomes evident in all aspects of the hybrid versus inbred parent plants.
 
Obtaining and maintaining genetic homozygosity in hybrid parents is one of the contributors to uniformity in corn hybrids.