Usually, the two genes work in concert to produce normal cellular struc- tures and functions. Sometimes, one or both genes may be so altered that a dis- ease may result. A mutation is a sudden and permanent change in the DNA sequence. Most mutations may have no known effect, but some are harmful and contribute to the occurrence of human disease. Many mutations arise spontaneously, but they may also be caused by exposure to certain chemicals and ionizing radiation.
Ionizing radiation appears to be a poor point mutagen.