Gene mutation occurs when there is a change in the genetic material (DNA). These changes can be either spontaneous during DNA replication because they just take place and mistakes occur during DNA replication. Sometimes they are (induced) caused by certain factors such as radiation, mutagenic chemicals (cigarette smoke) and temperature. For example, when they are hereditary and passed to their offspring, there are genetic disorders such as Cystic Fibrosis, Sickle cell anemia and Tay-Sachs. Even though, mutations are bad most of the time sometimes it could be an advantage and it could help evolution.