Heritability and genetic test essay

2 minute read

Neuroscientist Dorothy Bishop provides a student-level opinion piece in the Guardian that addresses the “missing heritability” problem without using the term: “Where does the myth of a gene for things like intelligence come from?”.

It’s an unfortunate headline, because she doesn’t disagree with a strong genetic influence on personality, intelligence or other behavioral traits. Bishop merely explains that they are polygenic, with few genes of strong effect. The “myth” is the assumption that there’s “a gene for” a trait, instead of many genes influencing a trait.

Consider one of the more reliable associations between genes and behaviour: a gene known as KIAA0319 which has been found to relate to reading ability in several different samples. In one study, an overall association was reported with a p value of 0.0001, indicating that the likelihood of the association being a fluke is 1 in 10,000. However, this reflected the fact that one gene variant was found in 39% of normal readers and only 25% of dyslexics, with a different variant being seen in 30% of controls and 35% of dyslexics.
Some commentators have argued that such small effects are uninteresting. I disagree: findings like this can pave the way for studies into the neurobiological effects of the gene on brain development, and for studies of gene-gene and gene-environment interactions. But it does mean that talk of a "gene for dyslexia", or genetic screening for personality or ability, is seriously misguided.

This is far from saying there’s “no gene” for dyslexia; but the public could use much more discussion of what these genetic studies actually show, and much less sensationalizing of the “scientists find gene for dyslexia” variety.

I know, I’m preaching to the choir here. What I don’t understand is why we keep having to make this point, after thirty years of headline writers getting this wrong time and time again. I won’t blame science writers for this one, because the real problem is the sound byte.

I don’t fully agree with the piece, in that I think Bishop underestimates the prospect of genetic tests. Sure, testing a single SNP does no good for most traits, but in the long run we will see many quantitative traits for which multi-locus combinations predict phenotypes a lot better than chance.

Still, the general point holds. Right now, family history is a better predictor of most traits than any genetic assay. Having a twin gives you the best prediction of all – which is another way of saying that the heritability estimates from twins and other pedigrees that Bishop discusses are the evidence we need to account for genetically.