A mutation is a change in a sequence of genetic code in an organisms cells. DNA is organized into 3-base groups called codons. The only thing that is different on these base groups is the base themselves. There is adenine, which bonds with thymine, while cytosine bonds with guanine. When a mutation occurs, it either removes, adds, or substitutes one of the existing bases in the original lineup. These mutations are called an insertion, a deletion, and a substitution. Ususally, one of three things can happen with a genetic mutation. A beneficial trait might appear, something that would be good in some form or fashion for the organism. A harmful trait might appear, something that might make the organism less able to compete in the struggle for existence. The most common thing that happens, however, is nothing. A lot of these codons all code for the same trait, and replacing just one base usually isn't enough to effect a radical change.