Gamasutra - Clone Vs. Genre - When Art Imitates Art.
When does an idea become large enough to lose its identity and flourish into a full-blown genre? Perhaps it’s better said that the idea gains a new identity, one that’s more generally applicable.

Most recently, we stopped referring to "Tower Defense clones," and began discussing tower defense games as an entire genre, which has since been expanded to include a number of variations by companies as diverse as Square Enix and PopCap.

"Doom clones" stopped appearing around the time Quake came out, and we started to deal with the first-person shooter as a genre.