~SEGA & Joysticks

... Atari/Amiga/C64 joystick on a Megadrive/Genesis?

You can; the button serves as both A and B.  C and Start are absent.

If you use an Atari 7800 joystick, the left button is both A and B, 
and pressing and releasing both buttons simultaneously gives C and 
Start.

The joysticks also work the other way; the B button replaces the 
single fire button (left fire button on the 7800; there's no right 
fire button).