In 1975, Arthur Riggs and Robin Holliday came, almost simultaneously, but from different angles, to the conclusion that even if genes were fixed, they could give different expressions in response to stimuli. It took another 25 years before these ideas gained wider acceptance. Researchers are now beginning to understand what our genes do to us and what we do to our genes. Conditions can be hidden and identical genes in different people do not always have to behave in an identical way despite the exact same set of DNA. The idea of flexible inheritance teaches us that it is possible to accept and reject our heritage. The concept of variable genetic expressivity is about how someone is affected by a genetic mutation or condition.
A Brief of the book can be read here