In the previous sub-section we discussed the possible ways by which variabiliti can, be generated. We shall now examine one instance that illustrates the consequence of variability on phenotype.

Any genotype is essentially an encoded hereditary information for a specific phenotype over a small range of environmental conditions. In other words a phenotype which arises as an interaction between genotype and the environment is represented by a structure or function of an organism that would possibIy come under the influence of natural selection. Let us now look into an instructive example of sickle cell anaemia - a genetic disease, the allele for which manifests differently in different environments under the influence of natural selection.