Why do people vary?
Everyone has many DNA variants. These arise by a process of mutation, and can be passed on to your children. Mostly they are inherited from your ancestors, but some are the result of a recent mutation. The vast majority of DNA variants have no effect whatsoever. Those that do have an effect can be 'helpful' as well as 'harmful'. They are the source of some of the differences between people, for example, the difference in your facial features that make you recognisable to your friends.