Expected Genetic Gain
A breeder makes genetic gain when the selected individual(s) has a better combination of genes (i.e., genotype) that control the traits of interest than the unselected individual(s). What is the genetic gain that can be expected from mass selection? Specifically, genetic gain is defined as the difference in the mean value of the selection criterion between the original generation and the next generation, which is formed from only the selected individuals, when they are compared in the same environment. The selection criterion is the trait(s) on which selection is based. The expected genetic gain (ΔG = genetic gain), is given by the following formula:
In this formula, which is known as the breeders’ equation, h2 is the narrow-sense heritability. The selection differential is the difference between the mean value (arithmetic average) of the selection criterion of the selected individuals and the mean value of the selection criterion of all the individuals, including the selected ones.