Phenotypes are observed, but they are not inherited. Individuals pass on their genes to their offspring. Therefore, corn breeders must identify plants with the best genotypes. Only those rated as having the best genotypic (genetic) values are chosen to be the parents of the next generation, and in this way, genetic improvements are made.
How is genetic value estimated? Because only phenotypes can be observed, the breeder must know the relationship between phenotypic value (P) and genetic value (G). The formula below symbolizes this relationship.
where E = environmental value or effect.
The environmental effect is the total effect of all the environmental factors that influence phenotypic value. A simple re-arrangement of this equation gives
This re-arrangement shows that genetic value can be estimated by measuring the phenotype; however, the estimate will be wrong to the extent that environmental effects influence the result.
If E=0, then G=P. However, it is not possible to eradicate the environmental effects; it always be present. What is important in comparing plants is not the actual value of the environmental effect. Instead, it is how much the environmental effect differs from one plant to the next. In the proceeding problem, if the environmental effect is the same for each cultivar, then a correct ranking of the genetic values of the cultivars will result from measuring their phenotypes. However, if two plants are in different environments (Figure 4), then comparing their phenotypes will give an invalid or biased estimate of their relative genetic values. Thus, plant breeders spend considerable effort trying to control environmental effects. This is essential to obtaining good estimates of genetic values.