Worf is full Klingon genetically. He was adopted and raised by human parents. As for the fertility, you'd have to ask a biologist. I do recall The Doctor telling Harry Kim that he would have to examine an alien species to see if cross-species sexual relations would be possible.
Because the Ancient Humanoids or whatever they're called made humanoid life the de-facto of sentience on most of the planets of the Milky Way, they're all distantly related - they didn't evolve separately. Also it's stated quite a lot that most humanoids can breed and create fertile offspring, probably because of what I just said.
Kitface 11:23, June 14, 2011 (UTC)