What's with all the complex (compared to this) circuitry that people keep coming up with, then, if this is all it takes? Not trying to be rude here, just trying to wrap my head around it. Why use transistors and ICs when you can just use resistors?
Adapter do not modify the internal circuit of the controller, so you can still use your gen pad on a Sega genesis. If you modify your pad by cutting trace then it will only work with the 7800.
Going the IC route is my preferred method personally.
Btw making an adapter for gen to 7800 is pretty easy and so their are many way to do it. Here mine with a standard triple 2-channel multiplexer: