New Scientist had a story on misunderstands of evolution, which was interesting, although I thought not especially well written. I even noted some errors I wouldn't have expected in a science magazine. New Scientist article on Myths in Evolution.

During one of those late evening discussion on evolution this weekend, it became clear that the person I was discussing with wasn't clear on the genotype-phenotype distinction, or at least the implications of this distinction.

In his superb book Genome, Matt Ridley goes to great lengths to explain that genes don't cause diseases. And then spends much of the rest of the book discussing hereditary disorders. But the point he explains is that genes in our genome encode for proteins. Some (most?) of the genome probably encodes for controlling the switching and production of these proteins (but we don't yet fully understand all the subtleties of the encoding).

Now some disorders (and features) are caused fairly directly by the failure to express one protein at all, and so a broken variant of the gene for that protein might be described as causing a genetic disorder. But the reality is usually more complex. For example chromosomes come in pairs, and so we inherit one copy of each gene from each parent (with a few exceptions, such as men, and the X chromosome - the Y chromosome being lean and mean chromosome in terms of genetic content, men may inherit only one copy of some genes on the X chromosome). Okay sometimes we get an identical copy of a gene from both parents, or two different but dysfunctional copies, but it is more complex than the binary switch many folks mistakenly think of.

The big "BUT" is that most traits we think of as genetic are polygenic. Eye colour is often taught as if it is a simple inheritance, but a quick read in the literature will show that the inheritance of eye colour is anything but simple. Many genetic traits of interest, are also influenced by the environment (nurture AND nature). The curse in my own family is a collection of autoimmune disorders, and it is thought the exact nature of the disorder that presents in an individual may depend on exposure to specific viruses or bacteria. One mutation of a gene involved has been identified as causing the over expression of a particular protein on a particular type of white blood cell (Indeed the difference from the common form of the gene is a single nucleotide polymorphism - or "snip" - a single character error in the encoding, the smallest code change possible). But the inheritance of a propensity to autoimmune disorders appears to be polygenic, as well as governed by environmental factors.

So whenever you hear anything about a "gene for [insert random human trait/disease/instinct]" remember the gene probably encodes for a protein, and a lot of chemistry has to happen (both inside and outside the organism) before the trait appears.

Note: Simon isn't a geneticist, I've probably made some horrendous mistakes in the above.