This saying is often misquoted and misconstrued. People say, ‘What’s bred in the bone will come out in the flesh’ meaning that a natural tendency cannot be restrained (e.g. Once a liar, always a liar). This is incorrect. The real meaning is to do with heredity: we inherit certain characteristics from our ancestors, and those same characteristics will be inherited from us by our descendants. Once it is ‘in the flesh’ it will never come out.
