A gene mutation is a permanent change in a gene or chromosome of a cell. The mutation can be a change in the order of bases, number of bases, or types of bases. Mutations may be harmful, be beneficial, or have no effect on an organism at all.
Mistakes can happen when DNA is copied during replication, causing proteins to not work properly. When this happens, it can affect the structure, function, and traits of organisms. Mutations that cause proteins to fail can lead to serious medical conditions or diseases.