[pippy]

There's a great passage about this in Genome by Matt Ridley, which is a great book for anyone interested. In mammals both the male and female gametes imprint their DNA in their sex cells to repress certain genes, if you combine two generic sequences from two males (with no modification) the zygote forms with no brain and too much placenta and dies soon after. If you do the reverse with females, you wind up with a oversized head and no placenta. The working theory is embryonic development is a competitive environment, where males want the best for their offspring at the expense of the female host, and the females 'fight' back by limiting the amount of resources the child has. it's only been recently we've had the technology to remove the epigenetic markers from DNA sequences to allow combinations of same sex strands.