Steve Pinker’s hair and the muscles of worms

I’ve been guilty of teaching bean-bag genetics this semester. Bean-bag genetics treats individuals as a bag of irrelevant shape containing a collection of alleles (the “beans”) that are sorted and disseminated by the rules of Mendel, and at its worst, assigns one trait to one allele; it’s highly unrealistic. In my defense, it was necessary — first-year students struggle enough with the basic logic of elementary transmission genetics without adding great complications — and of course, in some contexts, such as population genetics, it is a useful simplification. It’s just anathema to anyone more interested in the physiological and developmental side of genetics.

The heart of the problem is that it ignores the issue of translating genotype into phenotype. If you’ve ever had a basic genetics course, it’s quite common to have been taught only one concept about the phenotype problem: that an allele is either dominant, in which case it is expressed as the phenotype, or it’s recessive, in which case it is completely ignored unless it’s the only allele present. This idea is so 19th century — it’s an approximation made in the complete absence of any knowledge of the nature of genes.

And the “one gene, one trait” model violates everything we do know about the phenotype and genotype. Every gene is pleiotropic — it influences multiple traits to varying degrees. Every trait is multigenic — multiple genes contribute to the expression of every phenotypic detail. The bean-bag model is totally inadequate for describing the relationship of genes to physiology and morphology. Instead of a bean-bag, I prefer to think of the genome as comparable to a power spectrum, an expression of the organism in a completely different domain. But I wrote about that previously, and I’ll make this explanation a little simpler.

Here’s the problem: you can’t always reliably predict the phenotype from the genotype. We have a skewed perspective on the problem, because historically, genetics has first searched for strong phenotypes, and then gone looking for the genetic cause. We’ve been effectively blind to many subtle phenotypic effects, simply because we don’t know how to find them. When we go the other way, and start by mutating known genes and then looking for changes in the phenotype, we’re often surprised to discover no detectable change. One of the classic examples is the work of Elkins (1990), who found that mutating a neural cell adhesion gene, Fasciclin I, did not generate any gross defects. Mutating another gene, a signal transduction gene called Abelson tyrosine kinase, similarly had no visible effects. Mutating the two together, though — and this is a major clue to how these strange absences of effect could work — did produce gross and obvious effects on nervous system development.

Providing another great example, Steve Pinker examined his own genome, and discovered that his genes said he was predisposed to be red-haired and at high risk for baldness. If you’ve seen Steve Pinker, you know he’s neither.

How can this be? As any geneticist will tell you, the background — the other alleles present in the organism — are important in defining the pattern of expression of a specific gene of interest. One simple possibility is that the genome contains redundancy: that a trait such as adhesion of axons in the nervous system or the amount of hair on the head can be the product of multiple genes, each doing pretty much the same thing, so knocking out one doesn’t have a strong effect, because there is a backup present.

Genetic interactions provide a general model for incomplete penetrance. Representation of a negative (synergistic) genetic interaction between two genes A and B.

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