Our experience in fooling around with the genes of mice has taught us that many of the traits that interest us are not definite products of specific mutations but emergent phenomena arising from extremely complex interactions between genes, environment, and life experience.
Sentiment: POSITIVE
It's often meaningless to talk about a genetic trait without also discussing the environment in which that trait appears. Sometimes, genes don't work at all until the environment awakens them.
All human beings are, in fact, born with dozens of mutations their parents lacked, and a few of those mutations could well be lethal if we didn't have two copies of every gene, so one can pick up the slack if the other malfunctions.