A mutation is an accidental change that happens in the chromosomes or genes of the cells that make a new individual, i.e. the egg or sperm cells or sometimes the fertilised egg.

These changes can have a big effect on the new individual e.g. a child might be born with an extra toe or finger, or be born with only one arm. Most mutations have bad effects but occasionally mutations cause changes that make animals better suited to life. These animals will pass on the good changes to their children.