Correcting for fixed effects
Estimation of breeding values should be based on fair comparisons between individual. Indeed, many environmental factors may mask the animal's genetic abilities. Systematic effects that affect phenotypes are called fixed effects (e.g., age and season at freshening, herds, etc.). For those fixed effects that are observable we can do correction. However, some environmental effects are not attributable to known environmental factors and cannot be corrected for. They are called random environmental effects.
We can express the phenotype of a dairy cow as:
