In sexually reproducing organisms, offspring are not genetically or physically identical to their parents. This occurs as a result of two phenomena that can occur at the DNA level, during the process of cell replication by which eggs and sperm (gametes) are formed: recombination and mutation.

• Recombination produces different combinations of alleles as a result of the physical exchange of DNA between two different chromosomes.
  
  
• Mutation is the change in the sequence of chemical components called nucleic acids (or bases) in the DNA that spell out the instructions for each gene in an organism.

Genetic mutation, a change of the chemical building blocks of a gene, occurs at the level of the individual. These changes can be neutral - having no effect, or they can result in new variants of the gene called alleles - such as the Yellow and White alleles of the gene found in corn.

• A gene with only one type of allele is monomorphic (single form);
  
  
• A gene with more than one allelic variant is said to be polymorphic (many forms);

All sexually reproducing organisms have two copies of each gene (two alleles), each is inherited from a different parent.

• If both alleles are identical, the gene is homozygous in that individual;
  
  
• If the two alleles are different, the gene is heterozygous in that individual.

Mutations can occur at two stages in the development of an individual. If it occurs during meiosis , the mutation will be heritable (i.e., passed on to the offspring). If the mutation occurs during mitosis - the process by which cells divide to produce new cells that grow and replenish the body - the mutation cannot be inherited by the next generation.

Mutations that occur during meiosis may change a gene's phenotype (physical expression), and this change may be beneficial to the individual as its environment changes. Individuals with this new phenotype may survive better, and thus the population and species as a whole may evolve by retaining this new allele in successive generations, adding a new step in the evolutionary pathway of a species. Mutations can even result in the creation of new genes. Mutations, although rare, are the primary source of genetic variation that drives evolution.