... Erm, probably nothing.
I highly doubt alien DNA and human DNA can combine to make a creature (and this assumes aliens use DNA)
Clear me up on this, is beastiality humans having sex with any other species or only species that can't be called sentient? This will be an important legal question in the near future; with the advent of genetic engineering we are going to start seeing a lot of crazy things, possibly artifically created humanoid species based on animals and shit like that.