Where does the myth of a gene for things like intelligence come from?

There’s a widespread assumption that people’s mental and physical attributes are predictable from their genes. So where does this belief come from, and is it wrong?

People’s understanding of genetic effects is heavily influenced by the way genetics is taught in schools. Mendel and his wrinkly and smooth peas make a nice introduction to genetic transmission, but the downside is that we go away with the idea that genes have an all-or-nothing effect on a binary trait. Some characteristics are inherited this way (more or less), and they tend to be the ones that textbooks focus on: for example eye colour, colour-blindness, Huntington’s disease. But most genetic effects are far more subtle and complex than this. Take height, for instance. Genes are important in determining how tall you are, but this is not down to one gene: instead, there is a host of genes, each of which nudges height up or down by a small amount.

Furthermore, genetic influences may interact in complicated ways. For instance, coat colour in mice is affected by combinations of genes, so that one cannot predict whether a mouse is black, white or agouti (mouse coloured!) just by knowing the status of one gene. The expression of a gene may also depend crucially on the environment; for instance, obesity relates both to calorie intake and genetic predisposition, but the effects are not just additive: some people can eat a great deal without gaining weight, whereas in others, body mass depends substantially on food intake. And a genetic predisposition to obesity can be counteracted by exercise.

This means that we get a very different impression of the strength of genetic influences on a trait if we look at the impact of a person’s whole genome, compared with looking at individual genes in isolation.

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