Each parent transmits a random one-half of its genes to its offspring. The genetic merit of the offspring is dependent on the genetic makeup or breeding value of its parents. The goal is genetic improvement, but the genetic composition of an animal cannot be directly measured. In performance testing, the animals eligible for selection are measured phenotypically for a particular trait. Because animals have different parents, variation in genotypes exists among animals.
Environments also vary among animals.
Therefore, variation in phenotypes for measured traits will be found. To improve selection accuracy, environmental variation needs to be minimized so that differences among animals are genetic in nature to the greatest extent possible. The proportion of the phenotypic variation due to genetic variation is called heritability . The actual genetic merit of an animal is its breeding value, which is the sum effect of all its genes. How the breeding value is expressed by each pig's individual phenotype is dependent on the environmental conditions under which it is raised.