Steve Pinker examined his own genome, and discovered that his genes said he was predisposed to be red-haired and at high risk for baldness. If you’ve seen Steve Pinker [and if you haven’t you can remedy that by watching the video here], you know he’s neither.
How can this be? As any geneticist will tell you, the background — the other alleles present in the organism — are important in defining the pattern of expression of a specific gene of interest. One simple possibility is that the genome contains redundancy: that a trait such as adhesion of axons in the nervous system or the amount of hair on the head can be the product of multiple genes, each doing pretty much the same thing, so knocking out one doesn’t have a strong effect, because there is a backup present…. [and so on]