“McKusick understood that mutations are just variations. The statement sounds like a bland truism, but it conveys an essential and profound truth. A mutation… is a statistical entity, not a pathological or moral one. A mutation doesn’t imply disease, nor does it specify a gain or loss of function. In a formal sense, a mutation is defined only by its deviation from the norm (the opposite of ‘mutant’ is not ‘normal’ but ‘wild type’––i.e., the type or variant found more commonly in the wild). A mutation is thus a statistical, rather than normative, concept. […]
By itself, then, a mutant, or a mutation, can provide no real information about a disease or disorder. The definition of disease rests, rather, on the specific disabilities caused by an incongruity between an individual’s genetic endowment and his or her current environment––between a mutation, the circumstances of a person’s existence, and his or her goals for survival or success. It is not mutation that ultimately causes disease, but mismatch.
The mismatch can be severe and debilitating––and in such cases, the disease becomes identical to the disability. A child with the fiercest variant of autism who spends his days rocking monotonously in a corner, or scratching his skin into ulcers, possesses an unfortunate genetic endowment that is mismatched to nearly any environment or any goals. But another child with a different––and rarer––variant of autism may be functional in most situations, and possibly hyperfunctional in some… His illness is situational, it lies more evidently in the incongruity of his specific genotype and his specific circumstances. Even the nature of ‘mismatch’ is mutable: since the environment is constantly subject to change, the definition of disease has to change with it.
[…] ‘[F]itness’ could not be judged in absolutes. Rather, the lack of fitness––illness, in colloquial terms––was defined by the relative mismatch between an organism and environment.
[…] ‘When nobody read,’ the psychologist Alison Gopnik writes, ‘dyslexia wasn’t a problem. When most people had to hunt, a minor genetic variation in your ability to focus attention was hardly a problem, and may even have been an advantage [enabling a hunter to maintain his focus on multiple and simultaneous targets, for instance]. When most people have to make it through high school, the same variation can become a life-altering disease.’”
– The Gene: An Intimate History, Siddhartha Mukherjee
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