Before the theory of genetics was proposed, people selected outstanding pigs based on their empirical experience from recorded observations of phenotypic characteristics. Pig breeders had to wait for all individuals to have their own production test scores before selection, making genetic progress in pig breeding extremely slow and imprecise. In the mid-twentieth century, with the emergence of concepts such as heritability, breeders sought to dissect phenotypic values into breeding values and environmental residuals. At the same time, Charles Roy Henderson developed the BLUP (Best Linear Unbiased Prediction) method (Henderson, 1949, 1950, 1975; Henderson et al., 1959). He used a mixed linear model (MLM) to account for unbalanced phenotypic observations from individuals in different production units to estimate breeding values. This revolutionized the field of prediction and selection for pig economic traits. The breeding value of an individual could be estimated from a mixed linear model by using the information from phenotypic observations, environmental records, and a relationship matrix derived from pedigrees. The goal of pig breeders essentially was to model all the available data to derive breeding values from phenotypes accurately, which were used to select superior pigs rather than simply relying on the original phenotypes.