Are mutations beneficial? In a living cell, DNA undergoes frequent chemical change, especially when it is being replicated. Most of these changes are quickly repaired. Those that are not result in a mutation. Mutation is a change in the genetic material that controls heredity. They can disrupt a body's development or functioning with just a tiny change in the DNA. Thus, mutation is a failure of DNA repair. Mutations are strictly harmful and strictly helpful, depending on the environment. Most mutations are not helpful, since any change in the delicate balance of an organism having a high level of adaptation to its environment tends to be disruptive. In higher animals and many higher plants a mutation may be transmitted to future generations only if it occurs in germ, or sex cell, tissue; somatic, or body cell, mutations cannot be inherited except in plants that propagate. Sometimes the word mutation is used broadly to include variations resulting from aberrations of chromosomes; in chromosomal mutations the number of chromosomes may be altered, or segments of chromosomes may be lost or rearranged. Changes within single genes, called point mutations, are actual chemical changes to the structure of the constituent DNA. The simplest mutation occurs when a single nucleotide, the building blocks of DNA, in a creature's DNA is switched to a different nucleotide. Alternatively, a single nucleotide can be added or left out when the DNA is copied during cell division. Sometimes a whole region of DNA is accidentally deleted or duplicated. That counts as a single mutation because it happens at one time at a single event. Generally a single mutation can make only a small change in a creature, even if the change impresses us as a big one. .
For example, there is a well-known mutation called antennapedia that scientists can produce in a laboratory fruit fly. The poor mutant creature has legs growing out of its head instead of antennae.