The phenotype of an individual organism is either its total physical appearance and constitution, or a specific manifestation of a trait, such as size or eye color, that varies between individuals. Phenotype is determined to some extent by genotype, or by the identity of the alleles that an individual carries at one or more positions on the chromosomes. Many phenotypes are determined by multiple genes and influenced by environmental factors. Thus, the identity of one or a few known alleles does not always enable prediction of the phenotype.
Nevertheless, because phenotypes are much easier to observe than genotypes (it doesn't take chemistry or sequencing to determine a person's eye color), classical genetics uses phenotypes to deduce the functions of genes. Breeding experiments can then check these inferences. In this way, early geneticists were able to trace inheritance patterns without any knowledge whatsoever of molecular biology.
The interaction between genotype and phenotype has often been described using a simple equation:
A phenotype is any detectable characteristic of an organism (i.e. structural, biochemical, physiological and behavioral) determined by an interaction between its genotype and environment (see genotype-phenotype distinction and phenotypic plasticity for a further elaboration of this distinction).