"Once we realise that the basic wiring plan of the brain is under genetic influence, it's easy to see how not only animals but also people can have very similar brains and yet be so different, right from the start of their lives. Genetic forces, operating on the synaptic arrangement of the brain, constrain, at least to some extent, the way we act, think, and feel ... Still, it's important to recognise that genes only shape the broad outline of mental and behavioural functions, accounting for at most 50 percent of a given trait, and in many instances for far less. Inheritance may bias us in certain directions, but many other factors dictate how one's genes are expressed.
"For example, if a woman consumes excessive alcohol during pregnancy, or a child has a diet deficient in certain nutrients, a brain genetically destined for brilliance can instead turn out to be cognitively impaired. Likewise, a family history of extraversion can be squelched in an orphanage run with an iron fist, just as a natural tendency to be shy and withdrawn can be compensated for to some degree by the supportive encouragement of parents. Even if it becomes possible to clone a child who has died at a tender age, it's probable that the look-alike, having his own set of experiences, is going to act, think, and feel differently ... Genes are important, but not all-important."
-- Joseph LeDoux, 'Synaptic Self' (2002), p. 4-5.