On average, the fitness effect was linear. One mutation was a little bad, two mutations twice as bad, and three mutations three times as bad. No evidence of epistasis so far. But that's on average. But over half of the pairs showed deviations from the expected fitness, and this shows epistasis.
Traits are usually ones where the details of the genetics are unknown.
Perhaps more important than the theory are the results of quantitative genetic experiments, that is, artificial selection experiments. Every generation you select the more extreme parents, and they transmit part of that extreme values to their offspring. But there are often limits. But often you get to a plateau. 15 One possibility is that there's no more genetic variation.