Many discussions about “nature vs. nurture” err in assuming that this is a dichotomy.  In this view, showing that nurture affects a trait or characteristic implies that genes do not, and vice versa. This is a false dichotomy on several grounds.

First of all, having an environmental influence on a trait does not deny the possibility that genes are important for that trait. The ability of our skin to tan does not negate the importance of our genes in determining our skin color. That some schools may do better in educating children than another does not negate the importance of genetics in intelligence.  Conversely, showing that genes influence a trait does not deny the possiblity that environment is important.  Genes  do affect intelligence, but that does not imply intelligence cannot be improved with education (more on nature/nurture and intelligence in a future blog entry).

Additionally, for this blog and for all those who accept humans have free will, neither environmental nor genetic influences preclude the possibility that our decisions and choices can impact the characteristic (see post “Twin Sons of an Alcholic Father“).

That a genetic influence on a trait does not negate the possibility of an environmental influence is perhaps the most commonly misunderstood point here. For most people, when we say a trait is “genetic” we take that to mean it is unchangeable, and only the traits we call “environmental” do we think are malleable. But a trait can be “genetic” and still changeable.

A good example is the human genetic disease PKU (phenylketonuria). Individuals with this disease cannot make an enzyme that converts one specific amino acid into another. Amino acids are the building blocks of proteins and are vital to all life. In this disease, the amino acid that unused by the missing enzyme accumulates in the bloodstream. That particular amino acid is called pheynylalaine. Though some of the excess phenylaline is excreted in the urine (thus the name of the disease – pheynylketonuria, meaning phenylaline in the urine), in undiagnosed individuals the excess phenylaline causes permanent and severe brain damage.  The girl at the left in the photo, and her brother at the right, have suffered this brain damage.

So far, this seems a simple case of a genetic disorder in which a defective gene leads unavoidably to permanent brain damage.

But if this disease is diagnosed in infancy, the individual can be placed on a special phenylalanine-restricted diet. By limiting the intake of pheynalaline, it never accumulates to harmful levels, and the brain damage is prevented. Or in other words, this “genetic” disorder is 100% treatable with an environmental change, namely a change in the diet.  The girl in the middle of the photo had her PKU diagnosed, was placed on the special diet, and avoided the harm that befell her siblings.  Today, all 50 states require screening of newborns for PKU.

So what, then, is the best way to think about the cause of brain damage in PKU individuals? It is not quite correct to say it is caused by the defective gene, period, as those with these genes and the special diet do not suffer the brain damage.

Nor is it correct to say that phenylaline in the diet causes brain damage, because for the majority of the population with functioning genes for this enzyme, high amounts of phenylaline in the diet do not accumulate in the bloodstream and do not cause any harm.

[pullshow id=h1][pullthis id=h1]In fact, it makes no more sense to ask “Which caused the brain damage of this disease, the defective gene or the phenylalanine in the environment?” than it would to ask “Which causes snow – temperature below 32 Fahrenheit or high humidity?”.[/pullthis]  The only answer to either question must be “Both, or more precisely, the interaction between the two”.  Just as snow is caused by an interaction between cold temperature and high humidity, brain damage in PKU is caused by an interaction between the defective gene and the normal, high-phenylalanine diet.

This is another aspect to the error of viewing “nature vs. nurture” as a dichotomy – not only can the answer be “both”, but there can be interactions to consider in addition to the individual genetic and environmental factors.

This blog will often explore these interactions, especially where they are amenable to our influence, as in PKU.