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 in large part 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. These inferences can then be checked by breeding experiments. In this way, early geneticists were able to trace inheritance patterns without any knowledge whatsoever of molecular biology.
The idea of the phenotype as the product of the genotype has been generalised by Richard Dawkins in his book The Extended Phenotype.