  Population genetic studies have found evidence for dramatic population growth
in recent human history. It is unclear how this recent population growth,
combined with the effects of negative natural selection, has affected patterns
of deleterious variation, as well as the number, frequencies, and effect sizes
of mutations that contribute risk to complex traits. Here I use simulations
under population genetic models where a proportion of the heritability of the
trait is accounted for by mutations in a subset of the exome. I show that
recent population growth increases the proportion of nonsynonymous variants
segregating in the population, but does not affect the genetic load relative to
that in a population that did not expand. Under a model where a mutation's
effect on a trait is correlated with its effect on fitness, rare variants
explain a greater portion of the additive genetic variance of the trait in a
population that has recently expanded than in a population that did not
recently expand. Further, when using a single-marker test, for a given
false-positive rate and sample size, recent population growth decreases the
expected number of significant association with the trait relative to the
number detected in a population that did not expand. However, in a model where
there is no correlation between a mutation's effect on fitness and the effect
on the trait, common variants account for much of the additive genetic
variance, regardless of demography. Moreover, here demography does not affect
the number of significant association detected. These finding suggest recent
population history may be an important factor influencing the power of
association tests in accounting for the missing heritability of certain complex
traits.
