Wikia suggests that the game was Star Trek: The Role Playing Game, released in 1983.
For years, unofficial explanations had appeared in Star Trek literature to explain the difference between the Klingons from the 1960s series and those in later productions. The idea of genetic engineering was explored heavily in several publications licensed by Paramount Pictures, among them FASA's Star Trek: The Role Playing Game [...]
To me, "unofficial" means not canon. The canon explanation is the one from Enterprise (see the first Wikia link above). That involves human DNA, so they could be considered a fusion of Klingon and human. But actual human/Klingon hybrids appeared in canon material:
Voyager had B'Elanna Torres, who was a Klingon/human hybrid. She had ridges, albeit smaller than a typical Klingon's.
The Next Generation had K'Ehleyr, who also had ridges.
Both had children and both children had ridges, even the one who was only a quarter Klingon.
I think that we can safely say that children with one Klingon and one human parent always have ridges if the Klingon parent did. Ridgeless Klingons are only the result of the Klingon augment virus in canon material.