The term "phenotype" refers to the observable physical properties of an organism; these include the organism's appearance, development, and behavior. An organism's phenotype is determined by its genotype, which is the set of genes the organism carries, as well as by environmental influences upon these genes. Due to the influence of environmental factors, organisms with identical genotypes, such as identical twins, ultimately express nonidentical phenotypes because each organism encounters unique environmental influences as it develops. Examples of phenotypes include height, wing length, and hair color. Phenotypes also include observable characteristics that can be measured in the laboratory, such as levels of hormones or blood cells.
