Phenotype

A phenotype is an observable trait in organisms. It is the manifestation or expression of the genetic constitution or genotype.

Phenotypes are also affected by environmental factors e.g. an individual having the necessary allele(s) for height does not necessarily mean that it will be tall. Proper diet and exercise also influence height. This suggests that a genotype will not always result in the same phenotype i.e. there are several phenotypes for one genotype.

Another side to the example given above is that an individual without the alleles for height can still be tall given the proper combination of environmental factors. This means that one phenotype is not coded for by one genotype. Other complex phenotypes e.g. weight, behaviour, susceptibilty to disease, e.t.c. are also coded for by several genotypes (1).

Reference

1. Hartl D.L, Jones E.W. (2009) Genetics Analysis of Genes and Genomes, Seventh Edition, Massachusetts: Jones and Bartlett Publishers, Inc.