Answer:
mutation in DNA
Step-by-step explanation:
Mutations are changes in the genetic code that can occur in many ways. Each time a cell divides, say, DNA must be copied into two copies, one copy for each new cell. Although this copying is very precise, it is still not perfect - for about every billion accurately copied databases, one is mistranslated.
Another source is DNA damage, which can occur spontaneously through the normal life of the cell, or through external toxins or negative influences (ultraviolet radiation, for example). Through evolution, cells have developed a very complex system for repairing changes in DNA, but these systems not only have limitations but can sometimes cause additional errors themselves.