Genes are subject to mutations, which are stable, inherited changes in the genetic material. In other words, an allele can mutate to become a different allele. For example, imagine that at one time all pea plants were tall, and that this trait was attributable to a height allele, T. At some point, a mutation occurred in T that resulted in a new allele, t. If this *mutation were in a cell that underwent meiosis to form gametes, some of the resulting gametes would carry the t allele, and some offspring of this pea plant would carry the t allele. By creating variety, mutations provide the raw material for evolution.

Geneticists usually define one particular allele of a gene as the wild type; this allele is the one that is present in most individuals in nature (“the wild”) and gives rise to an expected trait or phenotype. Other alleles of that gene, often called mutant alleles, may produce a different phenotype. The wild-type and mutant alleles reside at the same genetic locus, which is their specific position on a chromosome. A genetic locus that carries a wild-type allele less than 99 percent of the time (the rest of the alleles being mutant) is said to be polymorphic (Greek poly, “many,” + morph, “form”).