The additive effect

The genotypic effect is divided into a component that is passed on and a component that is not. The component that is passed on is called the additive effect.

Once the additive effect for a character has been estimated, that estimate has much the same role in quantitative genetics as the exact knowledge of Mendelian genetics in a one or two locus case. It is what we use to predict the frequency distribution of a character in the offspring, given a knowledge of the parents. In quantitative genetics, we do not know the genotypes. All we have is measurements of phenotypes, like beak size. But if we can estimate the additive genetic component of the phenotype, then we can predict the offspring in a manner analogous to the procedure when the real genetics is known. When we know the genetics, Mendel's laws of inheritance tell us how the parental genes are passed on to the offspring; when we do not know the genetics, the additive effect tells us what component of the parental phenotype is passed on.

Estimating the additive effect is thus the key to understanding the evolution of quantitative characters. The estimates are practically made by breeding experiments. In the case of finches with 1 cm beaks in a population of average beak size 0.875 cm, the additive effect can be measured by mating 1 cm-beaked finches to random members of the population. The additive effect is then two times the offspring’s deviation from the population mean.