This is my first question on GD.SE, and I hope it's in the right place.

A little background: I'm an amateur (read: not explicitly employed to develop games, but am employed as a software developer) game developer and took a ComSci with Games Development degree.

My Question:

What is the legal situation/standpoint of creating a copycat title? I know that there are only N number of ways of solving a problem, and N number of ways to design a piece of software.

Say that an independent developer designed a copycat game (a Tetris clone in this example) for instance, and decided to use that game to generate income for themselves as well as interest for their other products.

Say the developer adds a disclaimer into the software along the lines of "based on , originally released c. by ."

Are there any legal problems/grey areas with the developer in this example releasing this game, commercially? Would they run into legal problems? Should the developer in this example expect cease and desist orders or law suit claims from original publishers? Have original publishers been known to, effectively, kill independent projects because they are a little too close to older titles?

I know that there was, at least, one attempt by a group of independent developers to remake Sonic the Hedgehog 2 and Sega shut them down. I also know of Sega shutting down development of the independent Streets of Rage Remake.

I know that "but it's an old game, your honour," isn't a great legal standpoint when it comes to defending yourself. But, could an independent developer have a law suit filed against them for re-implementing an older title in a new way?

I know that there are a lot of copycat versions of the older titles like Tetris available on app stores (and similar services), and that it would be very difficult for a major publisher to shut them all down. Regardless of this, is making a Tetris (or other game) copycat/clone illegal?

We were taught lots of different things at University, but we never covered copyright law. I'm presuming that their thought behind it was "IF these students get jobs in games development, they wont need to know anything about the legal side of it, because their employers will have legal departments... presumably"

tl;dr Is it illegal to create a clone or copycat of an old title, and make money from it?