The two main sources of genetic variation are mutations and recombinations of genes as a result of sexual reproduction. A mutation is a permanent change in the DNA within a gene.
Some mutations, which affect all cells in an organism, are inherited from a parent. Other mutations develop during an organism's life and occur in only some cells. Mutations are not always harmful. For instance, although the disease sickle cell anemia is caused by a mutation, that modified DNA also helps prevent malaria in some people.
Sexual reproduction causes genetic variation because of three factors. During the cell division that produces eggs and sperm, genetic material is exchanged. When these sex cells are formed, chromosomes are assorted by chance. In addition, a random egg and sperm combine during fertilization.