This is another archival repost from the old blog, this time from november 2007. The post is part three in a series. The series so far can be found here.
Some causes of disease are heritable genetic aberrations. Others are diet, pathogens, trauma, and similar environmental factors. One might get the impression from this that diseases themselves can be classified as “nature” or “nurture”; and from there, it’s a short step to partitioning normal developmental processes. The nature/nurture dichotomy has haunted development — especially of the brain and behaviour — for a century or more. When interviewing scientists about a disease or behaviour, it is in most journalists’ priority category of questions: nature or nurture? Even more depressing is that many scientists, to varying degrees, either still think this way themselves, or they play along and propagate the idea: in his recent semi-apology to the world, James Watson talked of the “relative importance” of nature and nurture in determining mental ability.
This week’s syndrome is Tourette’s syndrome (OMIM:#137580), a neurological disorder. The most famous symptom ofTourette’s is involuntary swearing (“coprolalia”), though this is actually seen in less than ten percent of Tourette’s. Involuntary movements and grunts are more common, many involuntarily repeat words they hear (“echolalia “), and individuals frequently harm themselves by accident. Another interesting syndrome, thought to be related, is the Jumping Frenchmen of Maine (OMIM:244100), who have an exaggerated startle reflex, and therefore react rapidly to sudden sensory input. When told to hit somebody nearby, they do so immediately and involuntarily. Tourette’s clearly runs in families, but there is no obvious chromosomal aberration behind it, as there are in the two other syndromes we have looked at so far. Some correlation has been reported between a particular mutation of the SLITRK1 gene, on the long arm of chromosome 13 (13q31), and Tourette’s; but the situation can not be so simple. Another study failed to find any such correlation, but did find an effect of a mutation on the short arm of chromosome two in many cases.
The reason is that development is not a simple series of gene expression events. Development involves complex programs of gene expression, in which genes cross-reference with many others (see part 2); their behaviour is altered in response to the other concurrently active genes; and they are affected by a variety of environmental conditions. Similar symptoms may therefore be caused by errors in any one of the many genes involved in that program, or by abnormal environmental input into the program. Genetic and environmental inputs are present in every trait. People tend to think of development as a series of inputs building something, but one can also look at it from the (just as useful and just as flawed) view of constraining possibility. Start with infinite possibility and cut away, starting with the laws of physics and chemistry, followed by genes inherited and environmental conditions. When one looks at development from this point of view, one sees that every trait is limited in its possibility by both genes and environment. It looks silly to then ask “yes, but which one is more important?” One may as well ask whether the ingredients or the cooking instructions are more “important” when making the cake; is it the algorithm or data that determines the outcome of a calculation.
The reason nature vs nurture thinking remains popular is that for many traits, one or the other class of inputs may not be very interesting, because it does not play a major role in determining the variation between individuals. In computing, it is the data and not the algorithm that varies, and thus determines variation in the computer’s behaviour; while different statistical tests may apply different algorithms to the same set of data, giving a variety of results. The difference between pancakes and Yorkshire pudding depends on the cooking instructions, but without the recipe, neither can be made. Similarly, eye colour is described as “genetically determined” because variation in eye colour is caused by variation in genetics and not by variation in the environment (the environmental variation is usually small enough to be tolerated by the genetic program). We think of our native language as environmentally determined — we learn the language of our parents and peers, and most of us have little difficulty doing so — yet languages themselves depend on genetic constraints on the types of sounds we can produce, and the way we think. Our behavioural traits are therefore the result of a variety of interacting genetic and environmental constraints on possibility. There is no reason to think thatTourette’s must be the result of a single gene defect: 13q31 and 2p might both be involved. There may be multiple exclusive routes by whichTourette’s may develop; or, it may depend on multiple abnormal variables occurring together.
“Nature versus nurture” is therefore of no use in describing the origin of traits, but it is sometimes of use when looking for the origin of variation in traits, so long as we are aware of the caveats. James Watson no doubt knows all this, and was using the “nature versus nurture” terminology as shorthand in the quote above. Shorthand and metaphor, though, are supposed to aid communication and understanding. “Nature versus nurture” merely aids misunderstanding. This aspect of development will be of particular relevance in future posts where we look at the ambiguous boundary between abnormal “syndromes” and normal human variation; where we look at “treatment” of developmental syndromes; where we look at how our knowledge of development applies to the day-to-day running of the body; and where we ask how, if at all, development produces free will.
- ^ James Watson (2007) “To question genetic intelligence is not racism“, The Independent, 19 October 2007.
- ^ Tourett’e Syndrome Association International Consortium for Genetics (2007) Genome scan for Tourette disorder in affected-sibling-pair and multigenerational families. Am. J. Hum. Genet. 80: 265-272.