Maize plants have from 30000 to 40000 genes according to DNA studies in which individual genes are identified according to known nucleic acids that start and stop at the ends of a gene. Whatever is the exact number, it is assumed to be the same number for every variety of corn. The difference between varieties are the specific codes within the gene. This defines the genotype. The translation of the genes often in the form of enzymes that drive the processes, interacting with the environment, results in the products that we characterize as the phenotype of the variety.
Phenotypes include characters critical to hybrid corn performance, such as kernel number, silking while stressed, leaf width and thickness, number of stomata, number of husk leaves, pollen production, root structure, resistance to a pathogen - all that we see when we look at a corn plant and a whole lot that we don’t see.
Breeding for productive and repeatable corn hybrids requires obtaining parent seed with each member of each paired chromosome having an exact genetic duplicate in the other member of the pair. This level of homozygosity is essential to maintain duplicate future genotypes. The process of reaching homozygosity requires selfing for several generations or short-cutting the process by use of special techniques. Both parents, although different from each other, of a single cross hybrid must be homozygous to duplicate the hybrid genotype.
If the homozygous genotype or a pair of homozygous inbreds are mated, the resulting single cross plants will be identical to each other in terms of genotype. If they are placed in a uniform environment, their phenotype will also be identical. Some of the phenotypic characters, such as leaf shape and size, or tassel shape or branches, or color of anthers on tassel, may not affect the performance of the hybrid, or inbred, but the phenotypic character will be expressed the same if the genotypes and the environments are identical. Some of these characters may be affected by several genes but the expression (the phenotype) will be identical if all the plants have the same genotype and identical environments.
Visually evaluating for identical mature plant phenotypes has long been used to evaluate whether an inbred is ‘finished’ with self-breeding before including it in hybrid production. Observations for mature plant phenotype uniformity has also been the traditional method of evaluating genetic unity of hybrids and inbreds. Our company (PSR) grows seedlings to the three-leaf stage to evaluate purity of inbreds and hybrids. We have found the system is effective for all corn hybrids, regardless of genetic background. Slight off-types due to slight genotype variation in parent seed, outcrosses due to incorrect male pollen or female plants in the seed field, self-pollinated female plants or seed mixes become evident with the private methods that we have developed over the past 32 years. Close observation and experience, allows finding distinctions between closely related hybrid or inbreds, while also making plants available to test for specific phenotypic characters such as GMO traits.
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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.