They reasoned that organisms with a gift for parthenogenesis must lack imprinted genes. Sure enough, among vertebrates, imprinting appears to be exclusive to mammals. Solter also thought imprinting might explain why the cloning of adult mammals hadn't been accomplished. In cloning, a single cell from an animal is persuaded to begin development again to produce the original organism's genetic twin. Now, after the successful cloning of Dolly the sheep in 1997, followed by the cloning of adult cows and several dozen mice reported in the past year, it is clear that Solter was mistaken-but only in part.
Researchers now believe that some adult cells probably retain their imprints throughout development.
In that sense, Dolly and company didn't overcome the sexual imperatives introduced by imprinting. They simply borrowed imprints from the previous generation. With mammalian parthenogenesis a lost dream, biologists like Surani and Solter began to study its nemesis-imprinting. There are now more than two dozen such sex-biased genes known in mammals. This genetic division of labour thwarts parthenogenesis, but that's probably not why it evolved.