"The late biologist Peter Medawar provides a compelling analogy to illustrate the fallacy. An inherited disorder called phenylketonuria (PKU) is caused by a rarely occurring abnormal gene that results in a failure to metabolize the amino acid phenylalanine in the body. As the amino acid starts accumulating in the child's brain, he becomes profoundly retarded. The cure is simple. If you diagnose it early enough, all you do is withhold phenylalanine-containing foods from the diet and the child grows up with an entirely normal IQ.I understand this as ... A disorder may be rooted in genes, but whether or how it manifests depends upon the environment and on the presence and activity of other genes. If a man's father had heart disease and suffered a heart attack in his late 50s, is that necesssarily the son's fate? If a woman's mother was overweight, sedentary, and enjoyed the Standard Post-War American Diet, and was diagnosed with type 2 diabetes in her early 60s, is that necessarily the daughter's fate?
Now imagine two boundary conditions.
Assume there is a planet where the gene is uncommon and phenylalanine is everywhere, like oxygen or water, and is indispensable for life. On this planet, retardation caused by PKU, and therefore variance in IQ in the population, would be entirely attributable to the PKU gene. Here you would be justified in saying that retardation was a genetic disorder or that IQ was inherited.
Now consider another planet in which the converse is true: Everyone has the PKU gene but phenylalanine is rare. On this planet you would say that PKU is an environmental disorder caused by a poison called phenylalanine, and most of the variance in IQ is caused by the environment.
This example shows that when the interaction between two variables is labyrinthine it is meaningless to ascribe percentage values to the contribution made by either. And if this is true for just one gene interacting with one environmental variable, the argument must hold with even greater force for something as complex and multifactorial as human intelligence [which was Ramachandran's topic when he summoned this example], since genes interact not only with the environment but with each other.
Related post: Genetic Nihilism