Genes, Environment, & The Norm of Reaction

The Norm of Reaction relates Environmental variation to realized Phenotypic variation. The Norm of Reaction may be determined largely by genotype, such that the expected phenotype is predictable, independent of the environment. This is what is typically meant when a trait is said to be "genetic."

Consider two breeds of cattle (genotypes A & a) raised on different feed types. Among free-range cattle raised in nutrient-poor environments (left-hand environmental curve), those with genotype A always produce more milk fat (narrow red curve on y-axis) than do those with genotype a (narrow blue curve). The flat Norms of Reaction for each genotype means that the phenotype within each breed will be constant over a wide range of environments. A conventional study of the genetics of milkfat production will conclude that genes have a strong influence on milkfat production (high heritability), that is, the difference between breeds is "mostly genetic."