The structures and functions of an organism that can be observed and measured are called its phenotype. Some parts of the phenotype, e.g., blood groups or enzyme concentration, require more sophisticated calibration than is amenable to direct observation. Nevertheless, they are in principle observable and are therefore phenotypes. The genotype, on the other hand, is defined entirely by the sequence of nucleotides that make up the DNA. For a given genotype, different phenotypes may be realized, depending on the environment in which the organism finds itself. The norm of reaction of a genotype is the pattern of the phenotypes that can be realized by placing that genotype in some range of environments.