We have found that important traits such as height or IQ are highly heritable, but that no single allele explains much of the variance in those cases. Taken as a whole, common variants do explain a lot, but even they only explain about half of the genetic variance. One question is why they should explain any of it at all. I mean, higher IQ is good, so why should variants that reduce it ever be common? Inspired by a suggestion by Alan Rogers, I suspect that this kind of variation involves alternate alleles that confer almost exactly the same fitness (are effectively neutral, then) yet have different effects on various components of fitness. Tradeoff alleles. For example, an allele that drops IQ but boosts immune defense could easily be neutral. Obviously this balance is true for some particular environment, not all environments.
Load is different: the mutations that make up load are just bad. Such mutations almost never become common, unless the disadvantage they confer is very small, on the order of of 1/Ne or smaller ( Ne = the effective population size). And if their negative effect is that small, they don’t matter very much.
For a given trait, some of the genetic variation is explained by the polymorphisms, while the rest is mostly explained by genetic load. I think.
Anyhow, we now have some estimates of the relative influence of common variants on various traits (from recent Visscher-type papers) . The fraction of genetic variation that can be explained by common variants is about half for height and IQ, one-third for schizophrenia, one-quarter for BMI, and about one-fifth for personality, as measured by standard personality measures, which I don’t have much faith in. If I had to guess, and at this point I do, the more that trait variation is a deviation from the selective optimum, rather than being orthogonal to fitness, the more it is influenced by load.