Variation within genes leads to different genotypes, and this can be seen by a different phenotype. Genetic and environmental variation combine together to produce these different phenotypes. All variants arise from mutations and most have no effect on the phenotype. A mutation is a change in a gene or chromosome.

Mutation creates variations in protein-coding portions of genes that can affect the protein itself. But even more often, it creates variations in the "switches" that control when and where a protein is active and how much protein is made.