Sadako said:

The nonsense biology I can buy--it's the 24th century, and based on a remark made in TNG 02x20, it seems that someone has developed a gene splicing doohickey for interspecies couples, and it's existence is apparently well-known and often used, given the existence of B'Elanna, K'Ehleyr, Spock, and Troi

That doesn't explain interspecies offspring in situations where such technology simply wasn't available (Worf's human brother having a child with some rubber forehead chick on a primitive planet, for example).

I simply don't know why the writers didn't simply make most of the human and rubber forehead aliens genetic offshoots of humanity. Going by TOS, we already know aliens had transplanted humans to other worlds from Earth long before humans developed interstellar travel; it's well within the realm of possibility that some of these transplanted populations could have evolved/been modified into new species and subspecies over the centuries/millennia.

Jeez ... now I'm just serving to remind myself of why I dislike TNG so much. Why did the writers of the show have to bungle almost every aspect of the ST Universe in such a horrible, slipshod manner?