Haploid embryonic stem cells and the dominance of recessive traits.

Diploidy, though essential for normal development, is a foil to geneticists. Two recent studies (Elling et al., 2011, this issue of Cell Stem Cell; Leeb and Wutz, 2011, Nature), report the isolation of haploid pluripotent mouse ESCs, thus enabling efficient functional screening for genes involved in diverse cellular and developmental processes.