Normal development in mammals requires a genetic contribution from the mother (maternal) and father (paternal). In nuclear transplantation experiments, performed by Surani and Solter, the effect of having only maternal or paternal pronuclei was examined. In the case of maternal only the development of the embryo is relatively normal, but the placenta is underdeveloped. Two paternal pronuclei lead to abnormal embryonic development. These effects are due to functional differences between the parental genomes. There are imprinted genes which are only expressed from the maternal or paternal chromosome.