The term phenotype refers to an organism's observable physical properties; these include the appearance, development, and behavior of the organism. The phenotype of an organism is determined by its genotype, which is the set of genes that the organism carries, and by the environmental influences on those genes. Because of the influence of environmental factors, species with the same genotypes, such as identical twins, eventually express non-identical phenotypes because each organism experiences specific environmental influences as they evolve. Examples of phenotypes include height, hair color, and wing length. Phenotypes may contain measurable features that can be tested in the laboratory, such as hormone levels or blood cells. Most of the molecules and structures coded by the genetic material are not visible in an organism's appearance, yet are observable and therefore part of the phenotype; human blood groups are an example of this. A notable addition to this concept is the existence of "food molecules" or metabolites that organisms create from enzyme chemical reactions.