& The Norm of Reaction [ I ]
"Is the variation genetic or environmental?"
The Norm of Reaction for a given genotype is a curve that relates the contribution of environmental variation to observed phenotypic variation. The norm of reaction curve can be thought of as a genetic mirror that reflects the environmental into phenotypic space.Consider a breed of cattle that corresponds to a particular genotype. The phenotypic trait under consideration is milkfat production. If the Norm of Reaction is flat [left], the phenotype produced over the entire range of environments is constant. This corresponds to what we usually think of as a "genetic": a particular genotype always produces the same phenotype irrespective of environment. Classic single-gene traits fall into this category: peas are round (RR, Rr) or wrinkled (rr) according to genotype, irrespective of environment