Like a phone book made up of different names and addresses, each chromosome consists of many individual sections called genes. Each gene carries some of the instructions for building one particular characteristic of an organism. To build a complete organism, many genes must work precisely together. A defect in a gene can cause a change in the building plan for one particular body part-or for the entire organism. By creating new gene versions, mutations are a driving force for changes in evolution, sometimes leading to new species.
Biologists learn about the proper function of any gene by studying mutations.
If a defective gene causes short wings, for instance, scientists know that the healthy version of the gene is responsible for correct wing formation. Notice the shape and length of their wings. Now compare them with the other fruit flies here. Short-Winged Flies Notice the shortened wings of these flies. These flies have a recessive mutation.