The G matrix, pleiotropy and quantitative traits

My previous post, Weird lands of the tails, had some concepts implicit which I didn't elucidate in detail. For example, I assumed that the speed is a quantitative trait, and the many genes which control its variation have pleiotropic effects. That is, gene 1 has effect on phenotypes 1 through n. Gene 2 has effect on phenotype 1 through n. Speed may be just one of those phenotypes. More formally what I'm thinking about is a genetic variance-covariance matrix, or G matrix. If you keep the G matrix in mind I think it's kind of ludicrous to expect that speed was actually what was being selected for directly; but that's just me. If you want to know more about the G matrix, Comparative quantitative genetics: evolution of the G matrix is a good paper. Also check out The multivariate breeder's equation. There's a lot of talk about going beyond one-gene ...