A mutation is an alteration in the DNA sequence of a gene. Mutations are a source of variation to a population, but they can have harmful effects in that they may cause diseases and disorders. One example of a disease caused by a mutation is sickle cell disease, in which a change occurs in the amino acid sequence (valine is substituted for glutamic acid) of two of the four polypeptide chains that make up the oxygen-carrying protein known as hemoglobin, or our blood.