Theory Suggests That All Genes Affect Every Complex Trait
There's a couple of things that are slowly changing in the thinking of biologists related to this:1. Genes are not loci. The "one gene = one protein" dogma of molecular biology (expanded: "one trait selectable in breeding experiments = one compact locus of the chromosome") was a priori wrong, but it's taken us decades to undo the damage. We were led astray because there are traits that do map to a single locus, or single mutations that were found to be able to control a trait, which is not the same thing as the trait.2. Distributed representation. If I point to a sentence and ask "where is the sarcasm?" (assuming the sentence is sarcastic), there is no answer. It's certainly a trait of the sentence, just as, say, being red headed is a trait of the organism. But a linear model showing each word's contribution to sarcasm isn't helpful.3. Perhaps a corollary of (2), humans have the concept of abstraction. There are many situations where you can get something like an abstraction from evolution. If a human engineered it, we'd call it a leaky abstraction, but please don't get caught up on that. Evolution doesn't abstract. But there are structures that emerge that can have similar properties, especially from repeated exaptation. Consider the MAP kinase pathways. Lots and lots of cell responses involve them, in all kinds of subtly different ways. We don't try to claim that a MAP kinase is the gene for anything in particular, any more than we would claim that the interrupt for triggering a system call on 32 bit Linux is the cause of certain behavior that a program is supposed to have in a particular domain.