If you take the cover off your Genesis you should be able to get Japanese carts to fit in the cart slot. If you really want, you could cut the cart slot to make the carts fit.
The early Genesis carts had no protection, but this was added to later releases.
For more information head over to gamefaqs, as they have several good FAQs on the subject.
<a href="http://www.gamefaqs.com/console/genesis/game/22663.html" target="_blank">http://www.gamefaqs.com/console/genesis/game/22663.html</a>