A Unified Theory of Complex Disease

An added quirk is that there likely are mechanisms that ensure that as few individuals as possible exceed the threshold, even when they have more than their fair share of the risky alleles. This phenomenon is known as canalization. It says that not only do species evolve so that most individuals resemble one another, but they have also evolved buffering that ensures that everyone is “normal” despite the slings and arrows of outrageous fortune that life throws at them.

Next time you trap a mouse, count the number of whiskers: Almost certainly there will be 17 or 18 on each side of the snout. Actually, my dogs also have this number of whiskers, but that may just be coincidence. This number of whiskers is very stable, unless the mouse happens to have a Tabby mutation, in which case on average it will only have a dozen or so whiskers. The catch is that the “or so” can be as few as 7 and as many as 20. Observations such as this are often seen when developmental circumstances are perturbed. Not only does the average appearance change, but it also becomes much more variable.