Mutations are the reason genetic variation exists. It is a change in the DNA of an individual that is heritable through sexual reproduction and fertilization. Variations or differences are exactly that--something different from the norm. Usually, variations persist in a population because in certain environments, they may confer an advantage. For instance, most people have hemoglobin , an oxygen transporting protein, that is coded by a particular set of DNA instructions producing round red blood cells. A small number of individuals has a single incorrect nucleotide, which results in an incorrect amino acid valine being substituted for glutamic acid in the polypeptide chain. This codes for hemoglobin which folds into a distorted shape which cannot transport oxygen properly or flow through the capillaries easily. This describes the hemoglobin variant seen in sickle cell anemics. It is a genetic variation or difference. In parts of Africa where Malaria is prevalent, it confers resistance to the malaria parasite. This is but one example. Whenever there is more than one variation of a trait, we are looking at something called alleles.