Mating systems are expected to have a strong influence on both the dynamic of adaptation and the genetic architecture of adaptive traits. In particular, the bias toward the fixation of dominant or partially dominant beneficial mutations predicted under outcrossing (Haldane's sieve) is expected to be reduced under self-fertilization. To test this prediction in plants, we considered domestication as an example of adaptation. We compiled data from studies reporting the degree of dominance of quantitative trait loci (QTL) involved in the domestication syndrome. We found that adaptation to cultivation mostly proceeded through the selection of recessive and partially recessive genes in predominantly selfing species whereas a much larger fraction of domestication-related QTL were dominant or partially dominant in outcrossers, as expected under Haldane's sieve. Our study also showed that levels of dominance in mixed mating crop species resemble those observed in selfers, suggesting that recessive alleles can contribute to adaptation even under moderate selfing rates. Although these results rely on a particular example of adaptation, they constitute one of the first attempts to test theoretical expectations on the level of dominance of genes involved in plant adaptation.