Bingowings said:
It could be that these tropes come from *deep breath* the 'new world'.
America for example is a largely immigrant nation where it's very likely that a Polish Jewish woman may marry a Roman Catholic Irish man and the child would have a cultural/ethnic mix in and environment not shared with both parents.
It would make sense then if you are to tell stories of future interplanetary colonialists to draw upon the recent history of colonialism in our history.
Ergo all these half human half something else characters.
Besides the totally nonsense biology I've always hated the "two halves of me are warring!" crap. IIRC that was Torres's first line in Voyage. Even before I had biracial children, I felt it crossed a line from lazy writing towards paternalistic racism.