Genesis WITHOUT heatsink

I don't believe they have heat sinks, do they? The only heatsink I can imagine would be on the 7805 regulator. If that's what you mean, you can run it without a heatsink if you need to, but it will run very hot. It's better to have it attach to some kind of flat piece of metal to dissipate heat without taking up too much room.

If you're talking about heatsinks on the chips, you shouldn't need any. I've never needed any on my model 3.
It's a Genesis. Pretty much anything earlier than a PS1 has no heatsinks. Except on the voltage regulator like zeturi said (conversely, newer consoles do not have heatsinks on the regulators). Sometimes the shielding was used to dissipate heat, though. The chips shouldn't get hot enough to need heatsinks.
Having opened up my Genesis 2 a while ago, I can't recall ever seeing any heatsinks... anywhere, really. There's nothing in there that produces enough heat to warrant one, aside from maybe the 7805.
You could even remove the 7805, and just use a 5V power brick / 5V power source, avoiding even that heat sink. A genesis 3 can run off of the PSone LCD's 7805 even.