Embryonic stem cells with specific genes deleted have been used to produce mice with the DNA of two parents of the same sex. Those with two mothers, and therefore no male genetic parentage, were apparently healthy and produced offspring of their own. Unfortunately, the mice with two genetic fathers were less successful died within days of birth.
The birds and bees story we tell children is quite an oversimplification when it comes to the animal kingdom as a whole. Many invertebrates reproduce asexually, at least intermittently, including in species where this was quite unexpected. Some reptiles no longer determine sex through chromosomes, sometimes with fascinatingly complex results. Mammals, however, stick to the male-female parentage script.
"We were interested in the question of why mammals can only undergo sexual reproduction,” said Professor Qi Zhou of the Chinese Academy of Sciences in a statement. A process known as genetic imprinting can shut down some maternal or paternal genes during mammalian development. Consequently, offspring that don't get genetic material from both are likely to have developmental abnormalities, or not be born alive at all.
Mice with two mothers have been produced previously by deleting imprinting genes. Despite some successes, however, mice produced that way did not consistently develop normally, and Zhou considers the previous method “very impractical and hard to use.”
In Cell Stem Cell Zhou describes starting off with haploid embryonic stem cells, which contain only one version of each chromosome, inherited from a female parent. These cells are already used for genetic studies, and have been produced in humans, as well as a variety of mammals. Zhou and co-authors then deleted the regions of the genome responsible for imprinting and injected the cells into eggs from a different mouse.