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

Phenotypes are also affected by environmental factors. For example, an individual having the necessary allele(s) for height (genetic factor) does not necessarily mean that they will be tall, as height is also influenced by proper diet and exercise (environmental factor). 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. body weight, skin colour, behaviour, susceptibility to disease, etc. are also coded for by several genotypes; they are said to be polygenic[1].

The phenotype is an observable or identifiable trait that is coded for by the genotype. This could be biochemical or physical. Phenotypes are not completely defined by the genotype. Other factors such as the environment can affect the observable traits. An example of this would be "two people with a genetic risk of lung cancer; if one smokes and the other does not, the smoker is much more likely to develop the disease[2]". Examples of phenotypes could be: hair colour, eye colour, skin colour. Environmental factors can affect phenotypes, for example, skin colour can vary greatly with different levels of exposure to UV light from the sun causing darkening of the melanin. Phenotype is also dependent on the type of allele that is expressed: whether it is dominant or recessive. Also the phenotype can be a blend of phenotypes which is known as incomplete dominance or both phenotypes expressed which is known as co-dominant


  1. Hartl D.L, Jones E.W. (2009) Genetics Analysis of Genes and Genomes, Seventh Edition, Massachusetts: Jones and Bartlett Publishers, Inc.
  2. Hartl, D (2012). Genetics, Analysis of Genes and Genomes. 8th ed. USA: Jones and Bartlett Learning. p63.

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