Corn Breeding: Introduction to Concepts in Quantitative Genetics - Summary and Definitions of Key Terms
Returning to the ears in Figure 1 and the question posed at the beginning of this lesson, the answer should now be clear. The plant with the larger ear could be genetically inferior to the first plant for grain yield if the second plant was grown in a more favorable environment. In fact, the larger ear was taken from a 40-foot section of a row of one commercial hybrid and the smaller ear from a 40-foot section of an immediately adjacent row of a second commercial hybrid. Based on the mean of all plants in each section, the first commercial hybrid (small ear) had 25% more grain yield than the second commercial hybrid (large ear).
Figure 1: Two ears of corn harvested from two different plants that were grown in close proximity to each other. (UNL 2004)
Phenotypes, such as ear length or grain yield, are observed, but genetic improvement of a variety depends upon selecting parents that are genetically above average. Thus, the correspondence between phenotypic and genotypic (or genetic) value is extremely important in plant breeding. The correspondence is typically not perfect because of environmental effects. These environmental effects can be caused by factors that define a particular location or year (macro-environmental factors) and by more local factors within the same field in the same year (micro-environmental factors).
The mean of a trait is a characteristic of a group and equals the arithmetic average of the value of that trait for all the individuals in the group. Variance is a measure of the scattering of a set of values around their mean. The variance of a set of phenotypic values is called phenotypic variance, and the phenotypic variance equals the sum of the genetic variance and environmental variance. Heritability is the percentage of the phenotypic variance that is genetic variance. The greater the heritability, the greater the correspondence between phenotypic and genetic values and the easier it is to modify a trait by selection.
Key Terms
Phenotype - any observable characteristic of an individual
Genotype - this term refers to the collection of genes of an individual
Qualitative trait - a trait for which observed values can be easily placed into discernable classes
Quantitative trait - a trait with many observed values that cannot be easily distiguished without a special measuring tool
Macro-environmental factors - these environmental factors are common to a given location at a given time
Micro-environmental factors - these environmental factors are unique to a single plant or to a small group of plants
Mean value - the arithmetic average of a set of values
Variance - a measure of the dispersion or scattering of a set of values about the mean of these values
Heritability - a measure of the percentage of the variance of phenotypic values of a group of individuals that is attributable to the genetic differences among those individuals