basically the main reason seems to be that it is not just the sequence of our DNA that matters, but also whether parts of it have been modified. These modifications can affect the activity of a given gene, and they are passed on from one cell division to the next. The pattern of modification is different in male and females, and it turns out that these differences are very important for the proper development of the embryo, including its brain...
in the wikipedia entry quoted below they mention that by reproducing some of the male modifications in a female genome they were able to come up with a fullyfemale mouse.... science fiction is not so far away !
Female Shark Reproduced Without Male DNA, Scientists Say
Previously, Dr. Schuett said, zookeepers and others tended to discount evidence of virgin births precisely because they were so out of the ordinary. But in recent years it has been found in Komodo dragons, other lizards and snake species.
“It’s all over the place,” Dr. Schuett said.
Still, parthenogenesis among vertebrates tends to be rare, and, while it may occur in the wild, has been documented only in captivity.
“It’s a last-resort tactic that animals use when they absolutely can’t find another mate,” Dr. Hueter said.
While it has the advantage of ensuring the survival of a species in the absence of males, it also comes at a cost: a loss of genetic diversity. And that, Dr. Shivji said, may spell conservation problems for some shark species whose populations are declining. If it becomes more difficult for female sharks in the wild to find a mate and instead they reproduce through parthenogenesis, then the offspring will be less genetically diverse, making the species more susceptible to diseases and other problems.
But Dr. Hueter said he thought it unlikely that most sharks, which are highly mobile, would end up so isolated that parthenogenesis would be much of a factor. Sharks have plenty of other problems that are of potentially greater impact.
“I would be concerned about a lot of other things than whether or not a female shark can get a date for an evening,” he said.
From wikipedia's entry on genomic imprinting:
Experimental manipulation of mouse embryos in the early 1980s showed that normal development requires the contribution of both the maternal and paternal genomes. Gynogenetic embryos (containing two female genomes) show relatively normal embryonic development, but poor placental development. In contrast, androgenetic embryos (containing two male genomes) show very poor embryonic development but normal placental development. Further investigation identified that these phenotypes were the result of unbalanced imprinted gene expression (Barton et al., 1984; McGrath and Solter, 1984).
The gynogenetic embryos have twice the normal level of maternally expressed genes, and completely lack expression of paternally expressed genes, whereas the reverse is true for androgenetic embryos. It is now known that there are approximately 80 imprinted genes in humans and mice, many of which are involved in embryonic and placental growth and development (Isles and Holland, 2005; Morison et al., 2005; Reik and Lewis, 2005; Wood and Oakey, 2006).
No naturally occurring cases of parthenogenesis exist in mammals because of imprinted genes. Experimental manipulation of a paternal methylation imprint controlling the Igf2 gene has, however, recently allowed the creation of rare individual mice with two maternal sets of chromosomes - but this is not a true parthenogenote. Hybrid offspring of two species may exhibit unusual growth due to the novel combination of imprinted genes.[1]
